Hi, guys. Just introducing my IGCSE EDEXCEL chemistry video. It’s all-in-one. Hope you find it helpful. The timings for all the topics are in the description box below, so if you’re not keen on watching the whole thing in one, you can skip ahead there. As always, I have written perfect answers all the way through. If you can’t be bothered to write them and want to get hold of one of my PDFs for three pounds, just email me: [email protected] And just so you know, this video is for people sitting exams in 2018. It’s old- spec, A*-U, so make sure it’s applicable to you. And I hope you find it helpful. Please like it. It does take me ages to make these sorts of videos. Now we want to look at states and matter, so we’re talking about solids, liquids, and gases. If they ask you to describe the properties of each of those, you’re gonna talk about solid as having particles held tightly in fixed positions, they vibrate very little, and they have strong forces between the particles. Liquids, the particles are slightly further apart, they have middling-sized forces holding those particles together, and they vibrate a bit more. Gases, the particles are far apart, the forces between them are very weak, and they move around freely. (no audio) So, first of all, let’s start by defining an element. Now, an element is a substance which contains only one type of atom and cannot be split by any chemical means. And remember, those elements are found inside the periodic table. When we’re talking about a compound, we need to know that it’s two or more elements chemically combined, so the key word here is chemically combined. If it’s not chemically combined, then it’s a mixture. So the next definition is: what is a mixture? Well, it’s two or more elements not chemically combined. So as you can see, those definitions are very, very similar. Now we’re going to dip straight into the periodicity topic, so we’re going to look at the structure of an atom more closely. So, as I’ve already said, the nucleus is made up of protons, and you also find neutrons in there. Now, they both have a mass of one, but they have different charges. A neutron, being neutral, has zero charge; proton has a one plus charge. That just leaves the electrons which are in this shell surrounding the nucleus. They have a negative charge, and they have a very, very small mass, and you can write that mass as 1/2000. So it’s a fraction; it is absolutely tiny. (no audio) If they ask you what an atom is, you want to say it’s the smallest particle of an element that can exist, and a molecule is two or more atoms bonded together. Don’t get it confused with a compound. In a molecule, the two atoms can be the same element, like a hydrogen molecule, but again, water is a molecule, but that has different elements, so it’s hydrogen and oxygen. So just make sure you’ve got those very clear. Molecule is to do with how many atoms are bonded together. Compound is to do with how many elements are bonded together. If we look more closely at the periodic table now – because the periodic table is the most important bit of chemistry – you’ll look, and you’ll see the key. Make sure you use the key. And you’ll see that the mass number tends to be the top number. Make sure you use the key in order to back up what I’m saying. Now, the mass number is made up of the total of the protons and neutrons. So if you add their numbers together, you’ll get the mass number. The electrons have nothing to do with that. The bottom number is the atomic number, and this is the proton number. And that is also the same as the electron number. Why can we say that? Because atoms are neutral. So the bottom number is the atomic number, which is the proton number. Now we need to look at isotopes. Now, you need to learn a very specific definition of an isotope. An isotope is atoms of the same element with the same number of protons but different number of neutrons, and this means they have a different mass number. So you will see something like chlorine exists in an isotope because you’ll see that its atomic number, its proton number stays the same, but its mass number and neutral number changes. If you’re asked to calculate the relative abundance of each isotope, you simply need to multiply the percentage of that isotope by its mass number, add it to the second percentage, times by its mass number, and then divide by 100. And I’ll show you that equation now. It’s just a matter of plugging in the numbers. Looking again at the periodic table, remember that the columns are called group numbers, and the group number, which you’ll see at the top, corresponds to the number of electrons in the outer shell. So lithium is in group 1, which means it has one electron in its outer shell. Fluorine, group 7: seven electrons in its outer shell. The horizontal rows, we call those the periods, and their number corresponds to the number of shells of electrons. So something like oxygen is in period 2. Its electron configuration is 2.6, so it has two shells of electrons. It’s in period 2. Remember when you’re filling up electron shells that the first shell contains two electrons, and then after that you fill by eight. And if you’re asked to draw the shells, you need to draw a circle, two crosses in the first shell to represent the first two electrons, and then make sure you’re filling up to eight. Don’t go into a third shell until the second shell has eight electrons in it. They often ask you why the noble gases are so unreactive. Remember, noble gases are in group 8 or group 0. You can interchange those numbers. The reason why they’re so unreactive is because they have a full outer shell. Why is Group 1 so reactive? Because it only has one electron in its outer shell, so it wants to very quickly lose that electron in order to gain a full outer shell because that’s all the elements really want to do, is they want to gain a full outer shell, so that’s why they’ll enter into reactions. Now we’re on to ionic and covalent bonding. Summary of ionic bonding: it occurs between metals and nonmetals. Make sure that you’re aware of that. And you’re going to draw brackets in order to demonstrate those ions. So if we’re going to bond lithium chloride, for example, what’s going to happen is lithium will donate an electron to chlorine, so that lithium now has a full outer shell. Chlorine has gained an electron, so it has now got a full outer shell, and you need to add charges to show that. And I’ll add a picture of lithium chloride now. With covalent bonding, you’re looking at nonmetals only, and this time you’re sharing electrons, which is when you need to do the flower diagrams – so no brackets, and you need to show them overlapping where you’ll be sharing electrons. So I’m going to draw you water now so you’ve got a good idea of what covalent bonding looks like. If they ask you for a definition of covalent bonding, you’re going to say it’s a shared pair of electrons. If they ask you what an ionic bond is, you need to say that it’s the strong forces of attraction between oppositely charged ions. Now we’re going into the chemical structures part of chemistry. So, I’ve already mentioned what an ionic and covalent bond is. Now we’re going to describe the reason why ionic compounds have high melting and boiling points, and that’s because they have strong forces of attraction between oppositely charged ions which require a lot of energy to break. Why are ionic compounds brittle? And what brittle means is when you hit them, they fall apart. The reason that they’re brittle is when a force is applied, ions with the same charge end up next to each other. The charges repel, causing the whole structure to fall apart. Why don’t ionic substances conduct when solid? Well, that’s because the ions are not free to move. Why do ionic substances conduct when molten or in solution? And that’s because the ions are free to move to carry the current. Now we’re going to look at just straightforward metals. So, describe the structure of a metal. Well, it’s made up of positive ions surrounded by a sea of delocalised electrons. What is an alloy? Well, an alloy is a mixture of two or more metals. Why do alloys tend to be harder than pure metals? And that’s because the layers are distorted, and that’s due to the fact that ions have different sizes, which means the layers can’t slide over each other so easily. So that’s why alloys are harder than pure metals. What does the word malleable mean? It means it can be hammered into shape. What does the word ductile mean? It means that it can be drawn into a wire. Now we’re going to look at giant covalent structures. Now, examples of this is things like graphite and diamond or silicon dioxide. Now, why does diamond have such a high melting point? So, you can say it has a giant tetrahedral structure, which means that each carbon atom is bonded to four others. Then you talk about the fact it has many strong covalent bonds which require lots of energy to break. If they ask you why graphite has a high melting point, say the same thing, but just say that each carbon atom is bonded to three others. If they ask you why graphite conducts electricity but diamond doesn’t, you’re going to focus in on those electrons. So you’re going to say in graphite, each carbon atom is only bonded to three others, meaning that there’s a free electron to carry the current. In diamond, there’s no such free electron. Why can graphite being used as a lubricant, or why is it slippery? And that’s because the carbon atoms are arranged in layers with weak intermolecular forces between them, so the layers can slide. Now, with this particular topic, please use the exact wording I’m using because people often get confused and end up arguing with themselves, so do try and use the exact wording that I’m using now. The last type of structure we need to look at is simple molecular substances, so something like methane or carbon dioxide. If they ask you why simple molecular substances have a low melting point, you’re going to say because they have weakened molecular forces which do not require a lot of energy to break. I’m just going to show you how to work out the formulae of common compounds. I’m going to show you the quickest, easiest way, and it is called swap and drop. So you’ll be asked for something like, work out the formula of aluminium oxide, but obviously you don’t know because that’s the final answer there. So what you do is you grab the ion for aluminium, the ion for oxygen, and then you swap and drop, which means you take the three from aluminium and drop it down to oxygen, and you take the two and drop it down to aluminium, so your final formula is Al2O3. The reason why that works is because you have on both sides six plus and six minus charge, which means they balance out. Check out my common ions video if you’re not following. But just remember, those little numbers must go at the bottom. We take the second example, calcium chloride, We swap them over, so the two lends itself to chlorine down here. The one, which – because although you can’t see the one, that’s what that minus means: it means it’s one minus. You take that down to here, and because it’s one you don’t need to write it. So the final answer here is calcium chloride, CaCl2 Slightly more tricky example here, which is ammonium sulfate. You’re taking the two down from sulfate and dropping it here. You’ve got the one at the top for ammonium and taking it down to here. Now, luckily, that’s actually all you have to do. Don’t get freaked out when it looks confusing because I promise it’s simple as long as you follow these rules. This is how you do balancing equations. I always do the same way. So draw a line separating both sides of the equation, and then list the elements. So we’ve got hydrogen, chlorine, zinc, carbon, and oxygen. And then just copy it across exactly as you’ve done it. Keep them all lined up. That’s crucial. And then you’re going to use a tally chart to work out how many atoms you have of each element. So we’ve got one hydrogen, one chlorine, one zinc, one carbon, and three oxygens. Let’s do the same for this side. Hydrogens, two. Chlorine, two. Zinc, one. You’ve got one carbon. And you’ve got three oxygens. And now it’s just a matter of comparing them. Okay, well, we don’t have enough hydrogen and chlorine on this side, so you’ll put a two here. Remember, you have to add a big number at the front. That’s the only thing you’re allowed to do when balancing equations. Make an adjustment because we now have two hydrogens, two chlorines. Step back, and yes, you can see they’re all balanced, so that is done. Here’s another example. So, line down the middle, list your elements. Iron, oxygen, and hydrogen. Copy them across. And then tally chart. We’ve got one iron. The three applies to everything inside the brackets, so that’s three oxygens, three hydrogens. Two irons. Three plus four, so that’s four oxygens, and then two hydrogens. The obvious thing here to do is match up those irons, first of all, so we’re going to put a two there. Adjust. And the two applies to everything, so you now have six oxygens. And you have six hydrogens. And then, we don’t have enough on this side, so I’m going to pick a sensible number, which would be three here because it means that I now have six hydrogens. And then readjust your oxygen. So you’ve got three oxygens plus three, so that’s six. And yay, both sides match. So that’s how you do the more tricky ones. Now we’re moving on to rates of reaction, which is to do with collision theory. So, you need to be very specific with your wording here. What is activation energy? Remember, that’s the minimum amount of energy required for a reaction to occur. So, why does increasing the temperature increase the rate of reaction? Well, remember the particles have more kinetic energy, they collide more frequently, and they have more successful collisions. Why does increasing concentration increase rate of reaction? Because there are more particles in the same volume, collisions occur more frequently. Why does increasing the surface area increase the rate of reaction? Well, it’s because you have more frequent collisions. So do get the term “frequency of collisions” in your answer, and that will really help. Obviously, if you decrease pressure, concentration, temperature, surface area, the opposite will be true. What is a catalyst? This is asked in both biology and chemistry. Catalyst is a substance which speeds up a reaction without being used up. And in terms of chemistry, you want to say it offers an alternative reaction pathway with lower activation energy. I’m going to discuss the group 1 elements in more detail. These are the alkali metals, so they’re the first column in the periodic table. So why does the reactivity increase as you descend the group? So, as you go down the group, you will find that something like potassium is more reactive than lithium. The reason it is is because potassium is larger. It has more shells of electrons. The outer shell electron is farther from the nucleus. It’s more shielded, and therefore, it’s easier to lose that outer electron. And you need to use those exact words in order to score full marks. They like to ask you some observations with group 1 metals in terms of what happens when you add them to water. So things you can say which is true for all group 1 metals is they fizz, releasing hydrogen gas. That’s what fizzing actually means. They move around. They float, and they eventually sink, and they turn the water purple when you add universal indicator to it, which means that they’re alkali. Now, you’ll get slightly different reactions depending on which element you’re talking about. With sodium, you’ll actually an orange flame, and with potassium, you’ll get a lilac flame. We need to talk about the reactions now, when those metals are added to water. So if you add a metal to cold water, what you find is you get a metal hydroxide plus hydrogen. So in the case of lithium, lithium plus water makes lithium hydroxide plus hydrogen. If you add them to steam rather than cold water, you get an oxide made. So lithium plus steam will produce lithium oxide plus hydrogen. And the same is true of all metals. Now we need to talk about displacement reactions to do with the halogens. Now, remember, halogens are group 7, so that’s fluorine, chlorine, iodine, bromine. And remember, what you do is you react them with a compound, and the more reactive halogen will displace the element from its compound. So you do find that as you descend the group, the halogens become less reactive. We can actually explain why using the argument as with the group 1 elements. The reason it becomes less reactive as you go down the group is because the elements are larger. They have more shells of electrons. The outer shell electrons are more shielded, and therefore, that means it is harder to gain that eighth electron because remember, with group 7, they’ll choose to gain an electron to become full rather than losing seven because it’s much easier to gain one than lose seven. So, in terms of their reactivity, you’ll see that chlorine will displace potassium iodide, potassium bromide. You’ll see that bromine will displace potassium iodide, but it won’t displace potassium chloride. That’s because iodine is less reactive than chlorine. so just make sure you’re comparing and you know that the top of the group is more reactive. And I’m going to add a summary table to show you because I think that’ll be easier for you to see. If they ask you some properties of the halogens, you want to say they have low melting and boiling points, and they are poor conductors of heat and electricity. Now we’re going to look at the transition metals. What are the physical properties of transition metals? Well, they have high melting and boiling points. They’re good conductors of heat and electricity. They’re hard, they’re strong, and they have high densities. They may ask you about the properties of transition metal compounds. Remember that they form coloured compounds. They have variable valencies, such as iron (II), iron (III). And they form coloured compounds. Now we’re going to talk about the reactivity series. Remember that this is the list which places the metals in order of their reactivity, and it also includes carbon and hydrogen often because although they’re nonmetals, they provide good reference points. Now, in terms of working out where an unknown metal goes in the reactivity series, the first thing you do is react that unknown metal with water. If it reacts nice and strongly, you know it’s going to be very reactive because remember, things like potassium, as I’ve already said, burn with a lilac flame. If it doesn’t react – something like gold won’t react if you put it in water – that tells you it’s very unreactive. So first of all, you start by testing with cold water. If there’s no reaction, you’ll then test with steam, which is obviously boiling water. And then, after that, if there’s still no reaction, you’re going to test with acid. And if it really doesn’t react with acid, then you know it’s a very unreactive metal. What is the test for hydrogen? Now, with these chemical tests, you’re going to have to be very specific. Remember, it is the lighted splint pops. Ow. Claws. Oy. Can you calm down, please? Now we’re looking at neutralisation equations. So, when you add an acid to a base, you get a salt and water. So, for example, let’s take an acid, such as hydrochloric acid, and we’re going to react it with a base, such as sodium hydroxide. So what you’ll find is happening, is you’re going to produce the salt, which you name by picking the metal name out, which is sodium, plus the ending on the acid. Next, we’re going to talk about what happens if you just add a metal by itself to an acid. So let’s take magnesium. We’re going to react magnesium with nitric acid this time, and you’re going to produce a salt, which would be magnesium nitrate due to the nitric acid. But instead of water, you’ll get hydrogen. Now we’re going to look at a metal oxide. So if you add a metal oxide, you get the same result as a metal hydroxide, so you get a salt plus water. So for example, we’ll take lithium oxide plus hydrochloric acid, so this time you’re going to get lithium chloride plus water. Lastly, take the carbonates. So any metal carbonate plus an acid will produce, yet again, a salt. This time, the byproducts will be water and carbon dioxide because of the carbonate. Make sure that the elements coming in come out the other side. It’s really odd when people write carbon dioxide coming out on the right-hand side if there was nothing that contained carbon on the reactant side. So taking a metal carbonate: We’ll take calcium carbonate, we’ll react it with nitric acid, and we’re going to produce calcium nitrate plus carbon dioxide plus water. Now we’re going to look a bit more at acids and alkalis. So remember, the pH scale is just a way of measuring how acidic or alkaline something is. It ranges from nought to 14, 0 being very acidic, 14 being very alkali. Neutral is pH 7. And remember, when you add universal indicator, that will turn green. At the alkali end of the scale, you’ll see blue. At the very acidic end, you’ll see red. If they ask you what the ion is that’s responsible for making something acidic, you want to say H+. If they ask you for the ion making something alkali, the answer is OH-. What is the definition of a strong acid? You have got to learn this enormous grid and work out basically which salts are soluble and which ones are insoluble. Now, I teach lots of different people, and some people like to learn the grid, which I just couldn’t do because there’s so many things in there, and other people just like to learn the rules, which is much more what I would do. And then you can deduce afterwards which salts are soluble or insoluble. So I’m going to, obviously, show you the grid, which is here. Now, don’t panic. I know there’s a lot on there, but it is easy if you know how, and there’s just a few rules you need to learn. Firstly, all ammonium, sodium, potassium compounds are soluble, so it doesn’t matter what else is in the name of the salt. If you see one of those three words – ammonium, potassium, sodium – it’s soluble. Secondly, all nitrates are soluble. Same thing: if you see the word nitrate, it’s soluble. Then it gets slightly tricky. And we look at the chlorides. Now, most of the chlorides are soluble, but you’re going to have to learn some exceptions. and those are silver chloride and lead(II) chloride, so if you see either of those compounds, it’s insoluble. Then we have the sulfates. Again, these are all soluble, with a couple of exceptions. And again, it’s lead(II) sulfate, and this time we’ve got barium sulfate and calcium sulfate. Both of these are insoluble. Now we flip, and we say that the next set of compounds are insoluble, and they are the carbonates. So, all carbonates are insoluble, but there are exceptions to that, and they’re the first things we talked about. That’s ammonium, sodium, potassium carbonates are soluble. Okay. So, disregard the carbonate when you see it. If it has ammonium, potassium, sodium in it, then it is a soluble carbonate. Everything else, insoluble. And then lastly, we’ve got the hydroxides, and these are all insoluble, with exceptions, and you’ve guessed what those will be. They’ll be the ammonium, sodium, and potassium hydroxides. They will all be soluble. Anyway, next up, you need to know how to make these salts, and that is really unpleasant. It’s probably my least favorite thing that I teach to my tutees or my students. However, they have to do it, you have to do it; therefore, I’m going to have to do it. First of all, we need to work out if our salt is soluble or insoluble based on what I was just talking about. Now, there are three methods you need to know about. Firstly, the insoluble salts method, and this is the most straightforward method, and it’s the precipitation method. And this precipitate is just a solid which forms in a solution. So what you have to do here, because you’re producing an insoluble salt – actually, I think I’m just going to run through some crucial key words to make sure you’re happy with those. First of all, soluble means something dissolved. Insoluble means something doesn’t dissolve. A solute is the solid which gets dissolved. The solvent is the liquid in which the solute dissolves. The solution is a mixture of the solute and the solvent. And a saturated solution is just something which can’t dissolve any more solute in it. Anyway, that was a little side note to just help you with any keywords that you’re struggling with. So going back to our first method, which is the method we use to produce insoluble salts, and we’re going to use the precipitation method. I’m going to run through the overview on how you do this, and then I’m going to provide a brief summary, which is actually the only bit that you’ll need to include in your exam. And luckily, it’s really short and easy to remember. So because you’re making an insoluble salt, one which doesn’t dissolve, what you’re going to do is you’re going to react your two solutions together, and you’re going to produce an insoluble salt. Now, you need to get that insoluble salt out of the solution, so what do you use there? You use the simple filtration method. So you’re going to get a filter funnel, you’re going to add filter paper, you’re going to place it over a beaker, and you’re going to pour your solution through that contains your salt. Because the salt’s insoluble, it will stick to the filter paper while the rest of the solution will drain through into the beaker, and you’ll be left with your wet crystals – your wet salt, basically, sat in your filter paper. So what do you need to do then? You need to allow it to – first of all, you need to wash it to remove the excess solution, and then you need to allow the whole thing to dry, and therefore, you’ll be left with your insoluble salt. So the summary – so this is the only thing you need to write in the exam even though the question will be worth four or five marks – is: react, filter, wash, dry, and I promise you only need to mention those few words. You can add a few extra details to make your answer look more sensible, but really in essence what you need to say is that you need to react your solutions together, you need to filter, you need to wash off the excess solution, and you need to dry the crystals. And that’s it. Done. So remember, those are the insoluble salts. That will be most of the carbonates, most of the hydroxides, the few exceptions like the silver chlorides and the lead(II) chlorides, and the lead(II) sulfates, et cetera. Okay. Now we get more complicated, and we start looking at the soluble salts. And there are two methods you need to know, and which method you choose depends on whether you’re talking about the ammonium, potassium, or sodium salts, or the rest. So we’re going to talk about the rest. So this is any soluble salt which does not contain ammonium, sodium, or potassium. So, what you need to do this time is really similar. So you need to react to your solutions together. You need to filter, but crucially because the salts produced are soluble, you’re not going to get that salt left behind in the filter paper. It’s going to drain through, so you’re going to use the filtering method to remove the excess solid, but then you’re going to be left with your solution – your salt solution – in the beaker. So at this point, you need to evaporate off the excess liquid. So what you’re going to do is pop that solution in an evaporating basin, over a gauze, on top of a tripod, with a Bunsen burner sitting on a heatproof mat underneath, and you’re going to use the heat from the Bunsen burner to evaporate off that excess liquid, and you’re going to be left with the salt. So your summary here is: react, filter, evaporate, cool, dry. So remember, you’re going to react, filter, evaporate, cool, and dry. You’re evaporating to remove that excess liquid. You’re cooling because you need to let the whole thing cool down. And it’s going to dry and remove all that excess liquid, the last bits of liquid left on your salt. And that’s it. Done. Really similar to the first method, the precipitation method, but just with that added step of using the evaporating basin to remove that excess water. Your third and final method is for soluble salts which do contain potassium, ammonium, and sodium, and this is the most complicated method. You can’t just react the metal with the acid and expect the salt to form because remember, things like potassium and sodium are unbelievably reactive. Unbelievably reactive. They’re in group 1. They react with the moisture in your hands. You can’t go adding them to acid because you’ll end up with an explosion, so you’re not allowed to use that method. Equally, you can’t just react them together because what happens is the product produced is so soluble that it just dissolves away again, and you’re left with nothing, so you just can’t use that method. What you need to do is use a method whereby you know the exact amount of acid and alkali that you need to add in order to make the salt. And you don’t want to add any more – you don’t want to add any excess of either because, like I said, the product will continue to dissolve away. So what is the method we use to decide the exact amounts? Well, it’s the titration method. Now we’re going to look at: how do you separate an insoluble solid or a solute from a solvent? So the easiest way to do this is by filtration. So you’re going to set up your filter funnel, your filter paper. You’re going to pour through your solution. The solid will stay behind in the filter, and you’ll be left with the solvent in the beaker below. So that’s nice and straightforward. How about if you’ve got a soluble solute or soluble solid? Well, obviously, it’s going to dissolve in the water or the solvent, and it’s going to pass straight through, so you can’t use filtration in this case. Instead, you’re going to use evaporation. So you’re going to place your evaporating basin on top of the Bunsen burner. You’re going to heat it to get rid of the excess liquid, and you’re going to allow crystals to form, and that’s how you separate a soluble solid from a liquid. Now we’re looking at how you separate liquids of different boiling points, so that could be a mixture of ethanol and water, for example. This time, you’re going to use distillation. And what happens in this case is you heat both the liquids, they’re mixed together, and the liquid which has the lower boiling point – in this case, it will be ethanol because its boiling point is around 78 degrees – will evaporate first of all, and it will rise, cool, and condense, so you’ll be able to take away the ethanol. And then later at 100 degrees, the water will evaporate, so you’ll be able to tap that off separately. If you’ve got lots of liquids at different boiling points, you’re going to use fractional distillation, and that’s what is used in crude oil. So remember, crude oil contains lots of different hydrocarbons of different lengths, so you need fractional distillation in order to separate that. I’ll talk about crude oil a bit later on. Now we’re going to talk about chromatography. So remember, chromatography is used to separate different inks or dyes or food colourings. You do need to know a little bit, how it’s set up. So remember, what you do is you get a piece of filter paper or blotting paper. Use a pencil line to draw a reference point. Then you dot your inks along that reference point, and then you dip the very end into water, and as the water soaks up, it will draw the dyes up, and they’ll separate, and you can see what’s in there. Now, why is it drawn in pencil, that reference line? Because obviously, if it was drawn in ink, that would rise too and totally screw up your findings. They like you to calculate the Rf value. So that’s quite a simple equation. You just need to do Rf=distance moved by dye divided by distance moved by solvent. And in this case it would be water. So you’d use your ruler to measure those lines, and then you pop it into the equation. Next up, we’re talking about tests, so we’re looking at tests for positive and negative ions and more generic tests. So we’ll start with the generic test for gases. Think I’ve already mentioned that the test for hydrogen is lighted splint pops. Now, do you use the exact wording that I’m using because I’m using the most concise, succinct wording possible to make sure you have to learn as little as possible. So to that end, we’re going to look at how oxygen is tested for. So this time, you want a glowing splint, so one that you’ve blown out and is now – you’ve kind of blown out, but it’s still smoking and glowing at the end. And that’s going to relight in the presence of oxygen. Carbon dioxide: you’re going to bubble that through lime water, and if it’s carbon dioxide, it will turn that lime water cloudy. The test for chlorine gas is that it bleaches damp litmus paper. Now we’re going to look at flame test. Remember, flame tests are used to test for positive ions. So you need a clean Nichrome wire, which you’re going to hold in a Bunsen to make sure it’s perfectly clean. You’re going to dip it in your sample of your positive ion that you don’t know what it is. You’re going to hold it in a roaring blue flame of a Bunsen. That is essential. And you’re going to look at the colours formed. If you have potassium, you’ll see a lilac flame. If you have sodium, you’ll see a yellow flame. Lithium, crimson. It’s a very specific shade of red: crimson. Calcium, you’ll see brick red. Another way you can test for positive ions is using precipitation reactions, and you’re going to add sodium hydroxide as your reagent. Now, if you have an aluminium ion, magnesium ion, or calcium iron, you’ll see a white precipitate formed. If you add excess sodium hydroxide, that white precipitate will dissolve in the presence of aluminium, so you know that that particular ion was aluminium. You get a lovely colour if you add it to copper this time. So ignoring everything we just said, this time you’ll get a blue precipitate. With iron (II), you’ll get a green precipitate, and with iron (III), you’ll get a brown precipitate. Now, we’re going to look at detecting negative ions. So we’re going to start with the halogens, group 7, which we call the halides. So this time, you’re going to add nitric acid in order to remove any carbonate ions which might interfere with your test. Then you add silver nitrate, and then you’ll get the following colour changes: with chlorine – chloride ions – you’ll get a white precipitate; bromine, you’ll get a cream precipitate; and iodine, you’ll get a yellow precipitate. And it gets darker the lower down in the group you get. And what are those things called? Well, they’re called silver chloride, silver bromide, silver iodide. Testing sulphates now, you’re gonna add hydrochloric acid followed by barium chloride, and if you have sulphates present, it will be barium sulphate, and you will see a white precipitate. Lastly, carbonates, this is nice and straightforward. So something like calcium carbonate, add any acid to it, something like nitric acid would work well. It will start to fizz. Capture some of that gas. Test it with lime water, and because it’s a carbonate, you expect that gas to be carbon dioxide, so if it turns milky or cloudy, yes, the original substance you had was a carbonate. Now we’re moving on to electrolysis, which is a pretty major topic. So, let’s, first of all, start with the word electrolysis. What does it mean? Well, remember, electro- means to do with electricity, -lysis means splitting, so it’s splitting apart a substance using [*electricity]. What sorts of substances undergo electrolysis? Well, that’s going to be ionic substances because you have to have both a positive and negative ion because remember with electrolysis, what you’re doing is you’re running a current around a circuit with two electrodes that dip into a solution, and what happens is the positive ion present goes to the negative electrode, the negative ion present goes to the positive electrode. And in terms of what you’re expected to know, you need to know which ion goes where and why. You also need to be able to draw your half equations. So we need to look at the terms oxidation and reduction. They are key here. So remember, because it’s an ion, we’re going to want to either gain or lose electrons. So oxidation in terms of electrons is – oxidation is loss of electrons; reduction is gain of electrons. Why does the ionic compound need to be molten or dissolved in solution? It’s so that the ions are actually free to move, so they can actually move over to the electrode that they’re interested in. So now we’re going to take an example. We’re going to take aqueous sodium chloride, and we’re going to work out what’s going to happen at each electrode. So if we take aqueous sodium chloride, we know that there’s sodium and chloride ions, and there’s also H+ ions and OH- because aqueous means to do with water. Now we need to understand which of those ions will go to the negative electrode. First of all, with the negative electrode, clearly, a positive ion will go there because opposites attract. It’s now a question of working out whether it’s going to be sodium or hydrogen. Your rule here is, it’s the least reactive element will move to the negative electrode, which is why hydrogen discharges. In terms of on the positive electrode, we’ve got two options: OH- or Cl-. Now, the halogen – group 7 – always wins, which means it will be the chlorine which discharges, leaving behind in the solution sodium and hydroxide. So that’s going to form sodium hydroxide. They do like to ask you the uses of all these different products. So sodium hydroxide is used in paper making. It’s used in making bleach. The chlorine that discharges is used in disinfecting swimming pools because it kills bacteria. It’s also used to make bleach. And the hydrogen is used as a hardening agent in vegetable oils to make margarine and also used as a fuel. Let’s take a look at the half equations now. So you want to get your hydrogen ion, and you’re going to work out how that will become neutral. And because it’s positively charged, you therefore need to add negative charge to it, which is why you add electrons to the hydrogen iron. Because hydrogen is diatomic – meaning that it’s H2 – you’re going to have to add two electrons. Let’s take chlorine. Now, chlorine is a negative ion, Cl-, so actually what you want to do is take away that negativity, so you’re literally going to take away electrons from it. Again, it’s diatomic, so you’re going to want to take away two electrons. If they ask you which is oxidation and which one’s reduction, you can literally see the plus and the minus signs. So in hydrogen, you added electrons, so it’s reduction. And in the chlorine, you’re taking away electrons, which is oxidation. And maybe your teachers have taught you OIL RIG: Oxidation Is Loss (of electrons); Reduction Is Gain (of electrons). Let’s talk about the electrolysis of aluminium. Remember that aluminium comes from the ore bauxite. And this time, it’s not aqueous. You’re just going to have molten aluminium. Now, what happens, therefore, is because it’s Al3+, it’s going to move to the negative electrode, and oxygen is going to form at the positive electrode. And you need to know both of those half equations. Why is it such an expensive process? Well, first of all, because aluminium has a very high melting point. So what they do is they add molten cryolite to lower their operating temperatures, so that’s the role of the molten cryolite. But you will find that it’s still very expensive, due to high electricity costs. And the second reason is because the oxygen which forms at the carbon electrode, or the carbon anode because an anode is a posh word for a positive electrode – what it will do is it will burn away that electrode, so it will need replacing very often. First of all, it’s important that you understand the two graphs for endothermic and exothermic reactions. Remember that an exothermic reaction is one that gives out heat energy. That’s your definition. An endothermic reaction is one which takes in heat energy. And if you actually look at the energy change, if you have an exothermic reaction, you’ll have a negative sign, and if you have an endothermic reaction, you’ll have a positive sign. And it’s really important that you know that. I mentioned the graphs. So if you have an exothermic reaction, you can see that the reactants have more energy than the products because they’re at a higher level on the graph compared with the products, which means that the energy change is negative, which is why you’ll see a negative sign in front of an exothermic reaction. For an endothermic reaction, you’ll see the graph is slightly different. You’ll see that the reactants are lower in energy than the products, so you see a positive sign there. The reason why this is so important is when we look at chemical equilibria and reversible reactions. Remember that a reversible reaction is one where the forward reaction and the reverse reaction are taking place simultaneously. And the problem with reversible reactions is in manufacturing processes, you really don’t want the reaction going to completion and then going back again because you’ll end up with less of the valuable product that you’re after. So it’s really important that we improve the conditions in order to encourage that forward reaction as much as possible. And what we’re saying here is that we need to increase the yield, i.e. increase the amount that we make. Don’t get yield confused with rate of reaction because rate of reaction is simply how fast the reaction is occurring, but you don’t know whether it’s happening faster in the forward reaction or faster in the reverse reaction. So basically, what I’m saying is that fast rate of reaction is not indicative of having a high yield. But let’s go back to our reversible reaction. So the first thing we need to look at is the temperature. If your forward reaction is exothermic, that means it’s giving out heat energy, so it’s getting hot. So basically, to encourage it to give out that heat energy, you need to oppose that heat, so you want to lower the temperature. So if the forward reaction has a negative sign, it’s a negative exothermic reaction, what you want to do is you want to lower the temperature to increase the yield. So if you see, again, that it’s an exothermic reaction, they might say, “We’ve lowered the temperature; which way will the equilibrium move?” You’re going to say it moves towards the right, in favour of the forward reaction, and that’s because the forward reaction is exothermic. Now, if there’d been a positive sign, that would have told you that the reaction is endothermic, which means it takes in heat, it gets cooler. So what you need to do is talk about the fact you want it to be higher, the temperature, in order to increase the yield. If this is sounding confusing, you have to check out my video on the Haber process because this is supposed to be a summary, not really teaching you the whole thing . Now we need to look at pressure. So if you increase the pressure, what you’re going to be doing is you’re favouring the side with fewer moles of gas, so you literally count the number moles in front of each of the formulae, and you compare it on both sides of the equation, and if there are few moles on the right-hand side, then increasing the pressure will increase the yield and favour the forward reaction. If there are more moles on right-hand side, then what you want to do is lower the pressure. And if we take ammonia as our example – because they like to do that. So the Haber process – remember that nitrogen comes from the air. That’s the raw material. Also, hydrogen is the second raw material, and that comes from natural gas, which they like to ask you a lot. Because what you’re doing is you’re making ammonia, and you have a look – it is an exothermic reaction – what you want to do is lower the temperature in order to increase the amount of ammonia. But the problem is, with lower temperatures, you have very little kinetic energy between the particles, so the rate of reaction is very low. So you need to increase that temperature in order to make sure that those particles are colliding, which is why they actually use a temperature of 450°C because that’s low enough to provide a high yield but high enough to increase the rate of reaction. Now if we look at the pressure, you can see that there are fewer moles of gas on the right-hand side. So you want to increase the pressure to increase the yield of ammonia. But what’s the issue with high pressures? They are dangerous and very expensive. So again we compromise with our conditions, and we lower the pressure to 200 atmospheres so it’s high enough to provide a high yield, high enough to provide a high rate of reaction, but not so high that it’s too expensive or dangerous. Remember also in the Haber process that we add an ion catalyst. What a catalyst is is something which speeds up the rate of reaction without being used up. But remember, if you’re being really technical, it does this by offering an alternative reaction pathway with lower activation energy. So if you add the catalyst – the ion catalyst – to the mix, it’s increasing the rate of reaction of both the forward and reverse reaction equally, so it has no effect on yield at all. It just makes sure that the reactions are occurring simultaneously and much faster. I hope you followed that. That is so hard. And just remember, if they ask you what equilibrium is, just talk about the fact that the reaction – the forward and the reverse reaction – are occurring at the same time and that the overall concentrations of the reactants and products don’t change. And again, if they ask you what a closed system is, you’re going to say that neither the reactants nor the products are allowed to leave. First of all, we need to talk about the ore from which iron comes from. Now, ore is just a rock which contains a large amount of metal, which makes it worthwhile to extract that metal from the rock. So iron ore’s name is haematite. Because iron is combined as an oxide in haematite, we need to remove that oxygen, and we’re going to use carbon to do that. It’s really important that we look at the reactivity series here. Because carbon appears higher than iron in the reactivity series, what you’ll find is that when you react the two together, carbon will effectively bully the oxygen out of the way, leaving iron by itself, which is what we’re after. The three raw materials which go into the blast furnace are: first of all, obviously, iron in the form of iron oxide; then we add some coke which contains carbon; and then finally, we add limestone – calcium carbonate – CaCO3, and what that does is it removes any acidic impurities. So we start burning our coke then. We reach really high temperatures. And it’s at that point that carbon reacts with oxygen to form carbon dioxide. It’s important that you learn these equations, the balanced simple equations especially, because you might be asked that very easily in the exam Then the carbon dioxide reacts with more carbon in order to form carbon monoxide. And carbon monoxide is our reducing agent. So let’s quickly take these words and understand what they mean. So oxidation is when something gains oxygen. Reduction is when a substance loses oxygen. So therefore, reducing agent will be something which causes another substance to lose oxygen, and an oxidizing agent will be a substance which causes another substance to gain oxygen. So carbon monoxide is our reducing agent, and what it’s going to do is it’s going to force the iron to give up its oxygen. So the third equation you need to know is that iron oxide reacts with carbon monoxide to form iron by itself and then carbon dioxide as a by-product. Sometimes at the very hot parts of the blast furnace, you’ll find that iron oxide reacts just with the carbon, and this time you’ll form iron, but instead you’ll form carbon monoxide as the by-product. And I hope you’ve seen all the equations flashing up. Make sure you know how to balance them. So either learn the balanced equation, or if you’re good at balancing equations, just work it out in the exams, so it’s not extra stuff to learn now. Now we need to talk about our limestone. Now as I said before, limestone is calcium carbonate, CaCO3. First of all, this limestone needs to undergo thermal decomposition reaction. Thermal decomposition means breaking apart something using heat: thermal, heat; decomposition, breaking apart. So we’re going to break apart that calcium carbonate, and we’re going to break it into calcium oxide and carbon dioxide. So the calcium oxide is the bit we’re interested in now because that will be the bit that removes the acidic impurities found within the iron. We’re going to take that calcium oxide from the thermal decomposition reaction, and we’re going to react it with the acidic impurities, which in this case is silicon dioxide. So you’re going to have calcium oxide plus silicon dioxide, and it’s going to form calcium silicate, which – its ordinary name is slag. I don’t know why they call it that; it’s such a strange word. But that is slag. A tutee, Jasmine – hi, Jasmine – told me a good way of remembering this. It’s Casio3. So, you know your calculator that you use in maths, Casio? And then add a three, and that’ll help you remember the formula of slag. And its other name is calcium silicate. So at this point, now, we have our iron, and it’s fairly pure, and at this point, we can start adding back – ridiculously – various amounts of carbon in order to form the alloy steel because that’s far more useful because pure iron is actually very soft and hard to work with. So, remember, rusting is about metals breaking apart, lots of flakes falling off, them losing their structural integrity. Now, just so you know, rusting only refers to iron. Zinc can’t rust, neither can titanium. So, yeah, rusting applies to iron. What conditions are needed for rusting to occur? You very much need two conditions: you need oxygen and you need water. Now, there are lots of ways in which we can help prevent rusting from occurring. There are the cheaper ways, like painting or adding oil or grease to something – something like a bicycle chain – and that provides a barrier method, so it actually prevents the water getting in. If you’re feeling fancy, you can do something called galvanizing, which is when you cover the iron with a more reactive metal – so it tends to be zinc – and this is called sacrificial protection because what will happen is the zinc, being more reactive, will react away before the iron, meaning that the iron can never actually rust. Already mentioned it, but I’m going to mention it again. What is an alloy? It’s a mixture of two or more metals. Why are alloys harder than pure metals? Because they have ions of different sizes. The layers are distorted, which means the layers can’t slide over each other, and therefore, it’s a much stronger substance. What is stainless steel? Well, it’s an alloy made up of chromium and nickel as well as, obviously, steel. Lyra, what are you doing? So, what are the useful properties of stainless steel? Why do we do it? Well, because it’s very hard. It resists corrosion. It’s very strong And we can make use of these properties in using stainless steel to make cutlery. Lots of cutlery is made out of stainless steel. Check it out next time you’re eating. Okay, crude oil, then. So hydrocarbon chemistry. This is a massive topic. We’re going to be looking at the the alkanes, the alkenes, complete and incomplete combustion. But let’s dive straight into it with the alkanes. So, remember, the general formula of the alkanes is CnH2n+2, and you can use that formula in order to work out the molecular formula of any alkane. So, for example, what is the molecular formula of the third arcane? Subbing in three to that equation gives you an answer of C3H8. You must be able to name the first four alkanes, and you want to use Monkeys Eat Peanut Butter in order to help you with this. So methane, Monkeys; Eat, ethane; Peanut, propane; Butter, butane; So methane, ethane, propane, butane. What is the formula of butane? C4H10. Now, remember, alkanes and alkenes are both hydrocarbons. So, you need to provide a perfect definition of a hydrocarbon, which is that it is a compound containing hydrogen and carbon atoms only. If you don’t say “only,” you don’t get the second mark. Well, someone’s much happier now, with her head buried in her fur. With the alkenes, now, what is the general formula of the alkenes? It’s CnH2n. List the first four alkenes. Well, remember with an alkene, they’re very special because they have a C double bond C (C=C), which means methene doesn’t exist because you need a minimum of two carbon atoms to have an alkene. So therefore, technically, the first alkene is ethene, followed by propane, butene, pentene. What is the formula propene? C3H6. You must be able to draw all the displayed formulae. Now, key terms you need to know – is the difference between saturated and unsaturated. We say that alkanes are saturated because all the carbon bonds are single, whereas alkenes are unsaturated because they contain the double carbon bond. How do we actually get hold of the alkenes and the alkanes? Well, we get them from crude oil, and fractional distillation has to occur in order to separate that crude oil into all the different length hydrocarbons. So if they ask you a four-marker on how to carry out fractional distillation, you’re going to say that you heat the hydrocarbons until they evaporate. They rise, cool, and condense, and because there’s a temperature gradient within that fractionating tower, you find that the short chains condense at the top where it is coolest; the long chains condense at the bottom where it is hottest. You need to know the order of fractions in the fractionating tower, starting from the top. So those are refinery gases – remember that they’re used in central heating – followed by petroleum, which is used as petrol in cars or fuel in cars. Then you have kerosene, which is airplane fuel; diesel, used in lorries and buses; fuel oil, which is used as ship fuel; and lastly bitumen, which is used for road surfacing. Other words you’ll need to know are things like viscosity. Remember, that’s how runny a substance is. So the more thick a substance is, the more viscous it is. Volatility, that’s how easily something turns into a gas. So if they ask you for a comparison of the volatility, viscosity, and colour of bitumen and petrol, you’d say that bitumen has a darker colour. It is more viscous, and it is less volatile than petrol. Now, why are we bothering with all this? Well, it’s obvious: because we are trying to get fuel that can be burnt to supply our cars, et cetera, with energy so they can run. So what is a fuel? Well, it’s a substance which releases energy when burnt. Now, depending on – how much oxygen is available will dictate if you’ve got complete or incomplete combustion. So with complete combustion, you have a plentiful supply of oxygen, which means the by-products produced are carbon dioxide and water, whereas with an incomplete combustion, you have insufficient oxygen, and carbon monoxide and water are produced instead. What’s the issue with carbon monoxide? Well, first of all, it’s poisonous, and what that means is – how it actually works is that it combines irreversibly with the haemoglobin in your red blood cells and therefore prevents oxygen being transported. Obviously, there are still issues with carbon dioxide. Carbon dioxide is a greenhouse gas leading to global warming. Sea levels rise, et cetera, so neither of these situations is ideal. There are other pollutants that you get from burning these hydrocarbons. In car engines, for example, the high temperatures found will cause the nitrogen and the oxygen to react, and what happens is nitrogen oxides are formed. They dissolve in water, and that can fall as nitric acid, which is acid rain. Again, you’ll get acid rain from sulfur, and this time, sulfur impurities in the original crude oil cause sulfur dioxide to be produced. That falls as acid rain. What are the issues relating to acid rain? Well, first of all, it damages limestone buildings. You must say limestone. Secondly, it makes lakes and rivers too acidic. And lastly, it damages trees. They often like to asking you how you test for a saturated or unsaturated hydrocarbon. So what you’re going to do here is add bromine water. And with an unsaturated hydrocarbon, you’ll see orange colour turning to colourless, and that’s because the bromine’s added itself to the original hydrocarbon, so it doesn’t exist anymore. And here’s the equation showing that. With saturated hydrocarbons, you won’t see any reaction. Getting more technical again. So, what is a homologous series? Remember, it’s a family of compounds with the same functional group. So something like the alkenes, they all have a C double bond C (C=C). The alcohols will all have an OH group. Today, I’m going to be talking you through the topic of structural isomers, but before we get onto that, we need to look at some key definitions to make sure you’re clear on those first. Our first definition is for empirical formulae. Remember that this is the simplest ratio of atoms of each element present in a compound. Now, molecular formulas are similar, but the difference here is that they are the actual number of atoms of each element present in a compound. And I’m going to use an alkene as an example, and the alkene I’m going to choose is butene, C4H8, and I’m going to highlight the difference between an empirical formula and a molecular formula So, as I just said … butene is C4H8 … which I’m going to write here. And that is the molecular formula because that is the actual number of atoms of each element: four carbons and eight hydrogens. However, if we look back at the empirical formula definition, it’s the simplest ratio of atoms of each element, so, as it’s a ratio, that means if we look back on C4H8, we’re going to cancel down those numbers to make them as small as possible. Now, four goes into both carbon and hydrogen, so I’m going to cancel that down, and therefore, the empirical formula of butene is CH2. I hope you see what I’ve done there. What I’ve done is I’ve literally canceled that down into its simplest form, and by definition, I now have the simplest ratio of atoms of each element present in a compound. Now we can move on and look at the displayed/structural formula, and in this instance, what you’re doing is you’re looking at the actual bonds between the atoms present in a compound. So you’re actually going to draw a diagram this time. So if the question says draw the displayed formula of methane, for example, this is what you need to draw … CH4, but with all the bonds showing. It’s useful to remember that carbon always forms four bonds and hydrogen always forms one. And there’s your displayed formula. And then finally, the structural isomer. All this means is compounds with the same molecular formulae but different structural formulae – so, for example, C4H10. Now, there are lots of different ways in which you can draw that, and I’m going to show you those now. So C4H10. I’m going to draw the first isomer, and that would look like this. Okay, so that’s finished. I’m going to double-check my bonds. Each carbon has four bonds. Each hydroden has one. So I know I’ve drawn it accurately. What is the name of this? Well, it’s four carbons in a line, so the name of it is butane. However, there is another way in which I can draw this, and that is, therefore, the structural isomer. So this time, we’re going to draw three carbons … and then I’m gonna add another group down here to make the fourth carbon, and I’m just gonna finish the diagram by adding my hydrogens. Just checking that I’m still recording because it’s so annoying when I forget to press record. I need to double-check that this is indeed C4H10, so I’m counting up my carbons. One two, three four. And then hydrogens: one, two, three, four, five, six, seven, eight, nine, ten. Great. So that’s right. So now I need to work out how I’m gonna name this. Now the best way of doing this is naming the longest carbon chain, which I can see is this line here – one, two, three. Because it’s three carbons, I need to name it after three carbons, which remember is prop-. So the last part of my name is going to be propane. Now, when you add a CH3 group somewhere in the molecule, what you’re doing is you’re adding what’s called a methyl group, so the name of this compound is going to be methylpropane. However, you have to specify where the … methyl group has been added. So, it will actually be on the second carbon, so it’s 2-methylpropane. So, just remember that when you’re doing addition polymerization, what you’re doing is you’re forming polymers, and these are effectively plastics. Now, you need to learn these definitions. Now, poly(propene) is a stronger plastic, and it’s used for things like crates or carpets or rope. Now, let’s have a look at chloroethene. The -ethene, obviously, are two carbons, but the chloro- tells us we’ve just got a chlorine rather than four hydrogens. Then I’m gonna draw the arrow, break the double bond. Elongate. Gosh, this is so weird, the way I’m saying it. And then fill in. There you go. Write your ends, and that is done. And we call that poly(chloroethene) or PVC. PVC is used in underground pipes for carrying water, gas, and sewage. Condensation means that, in this process, a small molecule is lost, and it tends to be water. So, the example I’m using today is nylon, and it’s quite complicated because you need to learn that there are two different monomers. The first monomer is hexanedioic acid. The second monomer is called 1,6-diaminohexane. And I’m going to show you how to draw both of those now. So hexanedioic acid. Right. The hexane bit tells us here … So treat it like a normal alkane. This bit here – hexane -tells us that there are six carbons in a line. This bit here tells this we’re talking about an ethanoic – sorry, we’re talking about a carboxylic acid which has a group of COOH. The fact that there’s di- means that there’s two of those, so together we can form the exact displayed formula. So let’s start with the simplest bit, which is hexane, so that’s six carbons. Now, I just told you that the carboxylic acid group is COOH. That C has to be part of the long-chain, so it’s included in the hexane, so don’t go adding another C. Just add the endings, which looks like this, and this is how carboxylic acid always looks. There’s double bond O. And then fill in the remaining hydrogens. And make sure that each carbon has four bonds. Cool. And that’s the first monomer done. The second monomer is this beast called 1,6-diaminohexane, but we can break it down again. That bit there – again, -hexane, meaning six carbons, so we’re looking for a six-carbon alkane. The -amino bit means that there’s going to be an NH2 group. There’s “di-” there, which means there’s going to be two NH2 groups. And this bit here tells us where those NH2 groups are going. They’re going on the first and sixth carbon. So let’s draw it. So six carbons in a row. And now we just need to add the NH2 group … which looks like this. And remember that goes on the – oh, sorry – the first and last carbon. And then fill in the hydrogens. Gosh. What a flipping mess. Now I’m going to draw the simplified version of each, and all that means is drawing a rectangular box to represent that central chunk of carbon-hydrogen repeats. So that will look like this for hexanedioic acid. We need the COO double bond thing here because that’s the important bit, the -dioic bit, but then I’m just going to draw a box to represent the hexane bit, and then double it up here. So there’s one of my monomers done. And then for the second one, 1,6-diaminohexane, remember, that’s an NH2 on the first and last carbon. So that will look like that. Draw another rectangle to show that central chunk, and then join it to the last NH2 group. This is when we actually do the condensation polymerization, and what that means is we’re going to lose a water group, and that’s going to come from here. And then simply join those bonds together to form one really large molecule. So that will be like this. Start here. Obviously, draw it way neater than me. I always say this, but it’s true. We tend to draw it this way round. Finish off the molecule. And voilà. Obviously, this reaction keeps happening at each end to join lots and lots of these very large molecules together, and at that point, you will have nylon. Jeff and Leslie are here, watching me do the the mole calculations part of this video. So I hope you know they were there with you suffering as well. So Jeff and Leslie are Martin’s parents. (Laughter) Going to show you how to do an empirical formulae calculation, and you want to lay it out in exactly the same way. So, first of all, pick out your two elements, which we have iron and chlorine, and you’re going to list them at the top: Fe and Cl. Then draw a nice table, and you want to put the following subheadings: mass, Mr – so that’s relative formula mass – and number of moles. Remember your triangle, which is mass at the top, Mr, number of moles. And then pop into that table the information you’ve been given. So you’ve been told it’s 2.8 grams of iron. And 5.325 grams of chlorine. Use your periodic table to work out the Mr. Iron is 56. Chlorine is 35.5. Then you can use your formula triangle to work out the number of moles by covering up the number of moles and doing mass divided by Mr. So 2.8 divided by 56 is 0.05. 5.325 divided by 35.5 is 0.15. Then identify the smallest number here, which is obviously this one. You want to divide both sides by the smallest number. And that will give you your ratio, which is 1:3. Perfect. So your formula is FeCl3. Make sure you write it out. And that’s how you do it. Now I’m going to show you how to do titration calculations, and it will look something like this. So we’ve got the volume of acid, which is 25 centimetres cubed, the concentration, and then the volume of lithium hydroxide. And now they want us to find the moles. Right. I always let lay it out like this, regardless of what the question’s asking, just to help you sort it out. So you want to write out the balanced symbol equation, first of all, again. And we know I’m a big fan of tables, so draw it as a table, and this time you’re going to do n, C, and V: number of moles, concentration, and volume. So number of moles, concentration, and volume. And then let’s look at what we’ve been given. So we’ve been told the volume of sulfuric acid is 25 centimetres cubed. So I’m going to pop that in here at the V under sulfuric acid, but divide by 1000 because you need to convert it to decimetres cubed. Don’t forget to do that. You’ve been told the concentration of sulfuric acid: 0.107. And then the volume of lithium hydroxide is 22.85. Again, that needs converting to decimetres cubed, so divide by 1000. Then it’s quite straightforward. You can see there’s a nice gap here, so you’re going to work out the number of moles. By covering up the moles, so you can see you do concentration times volume. So do 25 divided by 1000, times it by 0.107 … to get 2.675 times 10 to the -3. Now it’s important that you compare the big numbers in front of the formula. There’s a big two here, so you need to take that number that you’ve just worked out and multiply it by two. 5.35 times 10 to the -3. And then it’s ideal because you’ve got a missing concentration here, so you’re going to do number of moles divided by the volume … which is 0.2341. Now, I’ve done an awful lot of working. Let’s actually look what the question was asking, which is calculate the number of moles of sulfuric acid. So that is this number here because that’s the number of moles, and that’s sulfuric acid. So that’s 2.68 times 10 to the -3. Calculate the number of moles of lithium hydroxide. We’ve already done that. So that’s 5.35 times 10 to the -3. And lastly, calculate the concentration of lithium hydroxide. I’ve already done that. And by doing this, you’ll always make sure you get the right answer. So, now we’re looking at a reacting masses type of question. So I’ve just made this one up out of my head, but it’s the same sort of thing you’ll be asked. So, “3.75 g of CaCO3 reacted. Find the mass of HCl.” So, again, table. Make sure your equation is balanced. Mass, Mr, number of moles. So we’ve been told the mass is 3.75 grams. We’re looking for the mass of hydrochloric acid, so we’re going to put an x. Use your periodic table to work out the Mr of calcium carbonate. So you’re going to do – actually, I already know the answer; it’s 100. And then hydrochloric acid, it’s 35.5 plus 1, so that’s 36.5. Now we can work out the number of moles using that triangle again, which is mass, Mr, moles. So it’s mass divided by Mr. So 3.75 divided by 100. I should have done that in my head. 0.0375. Then it’s a matter of looking at the big numbers. There’s a two here, so you need to double that number that you’ve just worked out and write it in here. And then we’ve now got the Mr and the number of moles, so the mass is worked out by multiplying those numbers together. So your mass of hydrochloric acid is 2.74 grams. Now we’re looking at how to work out volumes. So, “What volume of H2 would be produced when 0.8 g of Ca is added to HCl?” Tricky this, so you’re going to have to start by writing out the balanced symbol equation. So we’ve got calcium reacting with hydrochloric acid, and we know it’s going to produce a salt, which will be calcium chloride … plus hydrogen. Then we need to balance it. If you don’t like what I’m doing, just look at the next bit of my video, and I’ll show you how to write formulae in that bit. So that’s balanced. So we’re going to go to lay it out, as per usual, as a table. Mass, Mr, moles. We’ve been told we’ve got 0.8 grams of calcium. We want the Mr of calcium, which is 40, so the number of moles is 0.8 by 40, which is 0.02. Let’s look at that number. Right. It’s a two, so we need to double this, so we’ve got 0.04 moles of hydrochloric acid. Why am I doing this? This is why you shouldn’t do this: because I’ve gotten so carried away that I’ve forgotten what the question’s about. I’m looking for the volume of hydrogen. Oh, my goodness. So, we look at the big number in front of hydrogen. It’s a one, so it’s the same as calcium, so we just take that 0.02 across. Now, to work out the volume, you simply have to times it by 24 because remember, one mole of a gas occupies 24 decimetres cubed. So just times it by 24, and you get an answer which is 0.48 decimetres cubed. If you want that in centimetres cubed, times that number by 1000. Don’t get caught out. And don’t waste your time like I just did then. Put you through the energetics part of the spec using past exam question because that’s the easiest way to do it. But remember, effectively, you’re going to be doing an endothermic or exothermic reaction, so something like a metal might be added to acid, or you might use a fuel to heat up some water. It’s all the same sort of thing. But you’re going to measure the temperature change, and then based on whether the temperature goes up or down, you’ll be able to work out if it’s endothermic or exothermic. Remember, if the temperature goes up, the reaction is exothermic; if the temperature goes down, it’s endothermic. And they always like asking you about sources of error, and often that’s heat to the surroundings. So you’ll just write that down. One way you can minimize that is by using something like a polystyrene cup, which is a good insulator, and adding a lid. Why do you need to stir the contents? Well, that’s to ensure that the reactants are fully mixed. That’s just me blurting out things I’ve seen on past papers and helping you get perfect answers. But let’s have a look at this sort of question. So, “Butane (C4H10) is a gas at room temperature and pressure. The equation for the complete combustion of butane is” as follows. So complete combustion, so obviously that means in plentiful oxygen. “Butane is used in an experiment to determine its ΔH [enthalpy change] of combustion.” So we’ve got the experiment all set up. “(a) State what the symbol ΔH represents.” And that’s the enthalpy change of the reaction, or you could say the heat energy change. “The table shows the results of the experiment. Use this equation to calculate the heat produced when 0.725 g of butane is burned.” So, look. They’ve been really kind because they’ve actually told you that it’s the mass of water they’re after. Sometimes they just write mass. Make sure you use the number that is the stuff being heated up, so the water. People get confused and put the mass of butane in there. Don’t do that. It’s the mass of water. So you’re going to be putting in 200, timesing it by the specific heat capacity of water, which is 4.2, times the temperature rise. So you simply need to do 43.7, take 20.2, which is 23.5. So you plug into your calculator 200 times 4.2, times 23.5, and you’ll get an answer which is 20000 joules. Having a nightmare with my iPad, which is why I’m not writing it. (reading visual aid) “What is the significance of the negative sign for ΔH?” Okay, that literally means that the reaction is exothermic. That’s all you have to say. “The student notices at the end of the experiment the bottom of the beaker is covered in […] soot. Suggest how this soot is formed.” And the crucial clue at the top was that it mentioned complete combustion, so this soot must have come from incomplete combustion due to a lack of oxygen. “Explain how the formation of soot may account for the difference between the […] ΔH from the experiment and the […] ΔH in the data book.” Well, clearly if soot’s being made, we think that less heat energy is actually being made to heat the water. “Suggest one other reason why the two ΔH […] are different.” I actually already alluded that to that at the beginning of the question, and that’s due to heat loss to the surroundings. So yeah, I know that’s a weird way of showing you those questions, but they’re all the same, so I thought that’d be the most helpful. Just going to show you how to do bond energy calculations using a past question, so … (reading visual aid) The crucial thing here, then, is this bit. If they haven’t given it to you like this, youv’e got to draw it out because this will be the best way of doing it so you don’t make any mistakes. And then it’s just a matter of adding up the individual bonds, so that value plus that value plus that value plus that value plus two lots of that value. So, let’s see what that actually looks like. So we have – I like to write it out. We have four C-H bonds. One, two, three, four. Therefore, I need to do – using this table here and that value there, 412 – so, therefore, I need to do 412 … times 4. And then on the reactant side, we’ve got two lots of this. Using the table again, here, 496. So we’re going to do 2 times 496. Plug those into your calculator, add them up, and you’ll get 2640. Part (ii), we’re using the “values from the table to calculate the energy given out when the bonds in the products are formed.” So, this time, we’re looking at these products here. So, let’s list the bonds again. So we’ve got O-C. And we’ve got two of them. Using the information in the table, therefore, we need to do 2 times 743. And then, don’t get confused here. We’ve got O-H bonds, but there’s actually four of them. One, two, and then there’s the big two at the front, so that’s why there’s four of them. So you’re going to do 4 times 463. Plug that into your calculator, and you’ll get a value which is [*3338]. And then we’re using our “answers to (i) and (ii) to calculate the molar enthalpy change.” This is the only vaguely confusing bit. So what you have to do is take away the energy given out in the products away from your value for your energy in the reactants, and if it’s negative, it tells you the reaction was exothermic; if it’s positive, it tells you the reaction was endothermic. So for this particular reaction, you’re going to do 2640, take away 3338, and you’ll get a value which is (-698) kJ per mole. So I was trying to work out the best way of teaching you electrolysis calculations, and they are hard, but I’ve got these equations which I kind of made up. They probably do exist, but I never saw them in the textbook. And I wanted to share you with them now because if you use them, basically, you’ll get the answers right. So, first of all, the crucial equation is that charge equals current times time, so that’s Q equals IT, given here. So that’s going to help you work out the number of coulombs that have been created. So that value that you get here will help you here and here. And then it’s a matter of working out the number of moles because what some of the questions want you to do is work out mass, so remember, mass is number of moles times Mr. So this whole chunk here will help you work out the number of moles. If you’re asked to calculate volume, remember, volume is calculated using number of moles times 24 because one mole of any gas occupies 24 decimetres cubed. So this chunk here will give you the number of moles. And if the question gives it in faradays, this equation here will give you number of moles. So I really do recommend learning these. If this is sounding confusing as hell, I’m just going to show you some questions, and you’ll see that it does indeed work every time. So here’s a basic question to kick us off. “Calculate the amount of charge transferred when a 5 A current is used for 2 minutes during electrolysis.” So Q equals IT. I is current, so that’s 5 amps, times time. The only thing here to remember is to convert that to seconds, so we get an answer which is 600 coulombs. Now we’re looking at bromine being produced during the electrolysis of molten lead(II) bromide. So 2Br- makes Br2 + 2e-. “A current of 13.4 A was used for 0.5 hours. Calculate the mass of bromine produced.” So, we’re going to use both sets of equations. So Q=IT, first of all. And as always, I’m laying out nicely so that the person can see my thinking. So, the current was 13.4. We’re going to times it by the time, which is 0.5 hours, not forgetting to convert that to seconds. So that’s that conversion there, which gives us a value of … 24120 coulombs. Then we’re just going to use that equation that I was bandying about earlier to work out the mass, which was number of coulombs, which we’ve just calculated, 24120, divided by 96500, times by the number of moles of electrons, which we can – There’s two. There’s two moles of electrons, so that’s why I’m going to times this by two. And then we’re timesing it by the Mr of Br2, which they’ve given us here … which is 160. So you’re going to pop that all into your calculator for me, and you’ll get an answer, which is 20 grams. Now we’re looking at a volume one, and this time they’ve given us the number of moles, so this step here and here is redundant. We’ve already been given it. So it’s 0.125. And then we just have to times it by 24. And then plug that into your calculator, and you’ll get a value which is three decimetres cubed. If they ask for it in centimetres cubed, just times it by 1000, to get 3000 centimetres cubed. So last question we’re going to answer is: (reading visual aid) This is hard. They’ve given us barely any information. So we’re, first of all, going to have to write out our ionic equation, which is Cu2+ plus 2e – forms Cu. Cool. Q=IT, again. So the current was 0.15 … times 10, times 60 … which is 90 coulombs. Do the big long equation thing. So that’s 90 divided by 96500 … times it by the number of moles of electrons, which I’ve worked out by drawing out the equation which was two, and then times it by the Mr of copper, which is 64, which we get from the periodic table. Pop that into your calculator. 0.03 grams. (music)



  2. Can anyone please help me
    I didnt understand the part at 13:53
    When magnesium is reacted with oxygen, MgO turns UI blue
    What does UI mean?

  3. Hey Hazel thanks but i think i found a mistake in the equation at 13.53 . The sulfur reaction with oxygen is 2S+02->SO2 but it should 2SO as the product right?

  4. Hiii…. I don’t know if you will get this in time, but
    Your videos are absolutely brilliant! I was watching the biology video whereby you made perfect answers for the long questions. Please could you do the same for chemistry and physics… I have chemistry exam in 11 days and physics in around 2 weeks… your biology video literally saved my life!
    Please please get back to me soon
    Thanks soooo much x

  5. you are actually amazing please come teach at my school i live in Dubai and i adore uyour videos, truly a life saver

  6. this video SAVED ME gosh… i have my igcse in chemistry tomorrow and i feel like im ready now because of thisssss!!! thank youuuuu

  7. I have my exams tomorrow, and i didn't want to go through my revision papers. This has refreshed my mind . Thank you for this. I already feel like i'm going to do well.

  8. hazel what's your address, me and like 10 of my friends wanna send you a gift for literally being the only reason we are passing

  9. hi hazel.
    your videos are incredibly amazing. I just had my bio exam yesterday and without your biology video i would have been completely lost. however in your chemistry video i did realize that there was a mistake at 13:53 with the last one about sulphur burning in oxygen. the equation was not balanced but i did it here. so its 2S+2O2=2SO2. But anyways your videos are amazing. You are such a kind person in the video and any student would be very lucky to have you as their teacher(I wish I did). Thank you!!!!!!!

  10. Your biology video was literally the only reason I passed the exam so I hope this video helps me just as much 🙂

  11. Exam is tomorrow! I suck at chemistry but some of this is going in!! Making notes today and will watch the video tomorrow morning for revision 😩
    If I pass science it’s honestly all thanks to you and myfreesciencelessons haha:)

  12. Hi Hazel! Do you mind making a "perfect answers" video for chemistry? I used the biology one yesterday and it was a lifesaver!
    Thank you so much for all your hard work!

  13. in the final electrolysis calculations of the video where you show your equations that you've created what does 'number of C' stand for. the charge?
    great video btw, really helped me

  14. tbf I wouldnt be surprised if they just turned this into a food tech practical ater the AQA biology exam

  15. Why did I leave revision til the night before the exam?? There's still hope for me because of this video tho!

  16. Solids, liquids and gases 0:36

    Elements, compounds and mixtures 1:11

    The structure of the Atom 1:43

    Ionic and Covalent Bonding 5:25

    Chemical Structures 6:29

    How to Draw Formulae 9:22

    Balancing Equations 10:45

    Rates of Reaction 12:41

    Oxygen and Oxides 13:47

    Group 1 metals 13:55

    The Halogens 15:35

    Transition Metals 17:02

    Reactivity Series and Salt Equations 17:31

    Titrations 20:46

    Making Soluble and Insoluble Salts 20:52

    Separating Mixtures 27:58

    Positive and Negative Ion Tests 30:17

    Collecting Gases 33:13

    Electrolysis 33:18

    Endothermic and Exothermic Reactions and THE HABER PROCESS 37:37

    Blast Furnace 42:58

    Rusting of Iron 46:20

    Alkanes and Alkenes 47:52

    Isomers 53:30

    Manufacture of Alcohol 57:08

    Addition Polymerisation 57:12

    Condensation Polymerisation 58:02

    Moles 1:01:36

    Enthalpy Calculations – bond making and breaking 1:08:38

    Energetics 1:11:56

    Electrolysis Calculations 1:14:40

  17. Hazel, thank you so much for ALL of your hard work~ We really appreciate it! Just wanted to know whether you'll also be making a video for the Edexcel International GCSE / IGCSE for the three sciences, new spec (9-1)? THANK YOU!! <3

  18. wait im confused…
    do we not need the cotactg process, or have i skipped over it?
    i'm doing the chemistry paper 2 tomorrow.

  19. I'm quite confused about the titration one.she wrote different points like in Titration she wrote "use a pippette to add acid to the Conical flask and another one in Making Salts she wrote "An acid from a burrette with an alkali in a conical flask".which one do I believe?

  20. Can someone tell me why she multiplied the number of moles of sulfuric acid ×2 in the titration Calculations? What does the number of moles in the sulfuric acid have to do with the moles in lithium Hydroxide?
    Ps. Sorry for bothering all of u with my questions but I'm weak in chemistry

  21. 1:08:01 XD the struggle is real, I can relate. I get carried away because I FINALLY understood, then I find out that isn't what they ask for. Then I run out of space. Sad life.

  22. i just purchased the perfect answer guide but i was wondering if i could use them for my igcse cie paper? cause i realised that the one i purchased from you is edexcel

  23. Hey, this video looks useful but in description it says that it is suitable for 2018 students and i am going to appear in 2019 so can you make a video for 2019 syllabus.

  24. Hi hazel,
    I got my IGCSE results and I was really amazed. You had really helped me with you bio and chem video. I was failing both but then I did really well in my GCSE and just wanted to thank you. Anyone doing GCSEs this year GOOD LUCK GUYS!!

  25. is this video suitable for students sitting for the exam on january 2019 PLZ i really want to know cuz my exam is in 2 weeks and 4 days

Leave a Reply

Your email address will not be published. Required fields are marked *