Transcription
[Music] Hello and welcome to this video. This is the OCR Redux revision video. My name is Chris Harris and I'm from allert tutors.com. And basically, we're just going to go through, um, the topic as an overview, um, for OCR, um, and make sure that you've kind of got a good grasp of, um, of what's required in terms of content for Redux.
Now, the PowerPoints that I'm using here, they are available, um, for you to use and you can purchase them, um, if you just click on the link in the description box. Take to the place where you can buy them, um, and use them for revision, print them out, do whatever you want with them, really. Um, but it's probably a good way of, of supplementing the revision, uh, notes and books, etcetera, that you've got already. Okay.
Right, so like I say, this is very specific to OCR and it addresses the specification points for, um, the OCR A syllabus. So let's make a start. So we're going to look at reduction and oxidation first, okay? So, um, electrons are transferred when reduction and oxidation occurs. Okay? So you can use the acronym OIL RIG, um, and OIL RIG stands for oxidation is the loss of electrons and reduction is the gain.
Now, this is really important. You've got to know, um, obviously the difference between the electrons, um, in terms, you've got to know, sorry, you've got to know the difference, uh, between oxidation and reduction in terms of electrons, um, because all the stuff that we're going to do here is pretty much dependent on you knowing that bit. Okay?
So let's have a look at this reaction here. This is one where calcium completely burns in oxygen, um, and, um, this reaction actually involves a reduction and an oxidation process. So this reaction is a Redux reaction, um, and let's split it up. A way to kind of identify this is by splitting up into two halves. So this bit, let's say, look at the calcium bit first. See, calcium is going to form calcium 2+ because it's form, uh, it's part of a, of a compound, and two electrons are also produced. So this is calcium being oxidized. It is losing electrons, as you can see here. So this is an oxidation process.
We look at the other half, this one, um, the oxygen is being reduced, which is here. So because the oxygen is being reduced, um, it is accepting electrons and we form O2 minus, as you can see here. It's gaining electrons. The oxygen is being reduced. So if we combine these two, we get that reaction there, and we call it a Redux reaction. Okay?
Also, it's really important to note about reducing agents. Okay? So reducing agents, what these do is they lose electrons but themselves are oxidized. Okay? So if you look at the calcium here, calcium has been oxidized because it's losing electrons. However, we describe it as a reducing agent. Make sure you don't get these two confused. It's the difference between a reducing agent and reduction. So reduction is the gain of electrons, but reducing agents lose electrons. Okay? There is a difference.
Obviously, oxidizing agents are, um, they gain electrons. So remember, oxidation as a process is the loss of electrons, but an oxidizing agent gains electrons. So they'll reduce themselves. So in this case, this is your oxidizing agent, which is O2. Okay?
Right, let's look at some oxidation numbers. So each element can be assigned an oxidation number, um, or we'll call it an oxidation state, and basically this number depends on a set of rules, and we're going to have a look at these rules here as well. Okay? You've got to know these rules, very important. Okay?
So the first one, question one, question one, part one, is an uncombined elements. Uncombined elements always have an oxidation state of zero. Um, so that could be something like chlorine, Cl2, FeO2, anything like that. They, um, these uncombined elements always have an oxidation state of zero.
Ions, oxidation number is the same as the charge in the ion. So Cl minus has got an oxidation state of minus one. Ca2+ has got an oxidation state of plus two, etcetera. Okay?
Group ones are always plus one. They always have an oxidation state, um, of plus one. So for example, KCl. Potassium in here has an oxidation state of plus one because that's in group one.
Group twos have an oxidation state of plus two. Calcium is obviously in group two, so it has an oxidation state of plus two. Uh, aluminium is always plus three. So for example, in aluminium oxide, again, it's in group three.
Hydrogen is plus one, except in hydrides where it's minus one. So for example, it's plus one in HF. But hydrides is basically just where you've got a metal bonded to a hydrogen. And so this is called a hydride. Hydrogen in this case would be minus one, and sodium the value of plus one. Remember, group ones are always plus one, so the hydrogen must be minus one. Okay?
Chlorine is always minus one, except if it's in a compound with F and O, um, where it would hold a positive value. So for example, it's minus one in KCl. However, chlorine is a positive value, um, which is plus three, for example, in ClF3, because it's bonded with fluorine, so it holds a positive value. Um, this is pretty much, you can kind of work this out because fluorine and oxygen are more electronegative than chlorine. So basically, the more electronegative element takes priority in terms of their oxidation number.
Fluorine is always minus one. That's dead easy. Um, so doesn't matter what it's bonded to, unless obviously it's an element, uh, it's uncombined, but it's always minus one. So for example, in KF.
And finally, oxygen's always minus two, except, um, it's minus one in peroxides, um, and plus two in OF2. Again, fluorine is more electronegative, so it takes the priority in terms of its oxidation number. Lithium dioxide is an example where oxygen's minus two, but oxygen has the value of minus one in hydrogen peroxide. So this is H2O2. So look out for your peroxides because oxygen's minus one. Okay? Make sure you know the rules.
Let's have a look and, um, look at some examples here. So we're going to work out the oxidation, uh, states of the elements in Red. So if we look at this one here, this is NH3. Now, the oxidation state of, um, nitrogen here, hydrogen is plus one. Individually, you've got three of them, so nitrogen must mean it's minus three for nitrogen. Okay? So it's rule number six that we're looking at. Hydrogen's plus one.
H2S. So we're looking at the oxidation state of sulfur in this one. Again, hydrogen's plus one. We've got two of them. The whole molecule is neutral, so sulfur must be minus two to make sure it balances. Again, we're looking at rule six, um, because hydrogen's plus one, so sulfur, um, to make sure it balances, is going to be minus two.
Oxygen in O2. Now, because this is an uncombined element, it's always zero because that's rule number one. It's an uncombined element.
H2O2. So we're looking for oxygen in H2O2. Now, remember, this is one of the exceptions. We're looking down here. Oxygen is minus two, except it's minus one in peroxide. So individual oxygen is minus one here. So that is the answer to that one. And also hydrogen is plus one, except in hydrides, but this isn't a hydride, so it's plus one. So you've got two rules really to kind of work this out.
H2SO4. Okay, this is a bit more complex. Oxygen, if we look at the rule here, number nine is minus two. So we've got four of them, so that's minus eight. Hydrogen, which is rule six, is plus one all the time, except in hydrides, but this isn't a hydride. So we've got minus eight here, plus two there. So that means the sulfur, to make sure it balances to zero because it's neutral, has to be plus six in this case. Okay? So we're looking at rule six and nine for this one.
Uh, and the last one is sulfate. Now, notice the charge here, that's going to have an impact. Oxygen's minus two. Got four of them, so that's minus eight in total. The sulfur has got to be positive, but you've got to basically make this positive so that the total, the sum adds up to minus two. This means that sulfur in sulfate is going to be plus six. Okay? So all the oxidation states must add to the overall charge of the molecule, and that's why it is plus six. Make sure it adds to that charge. Okay? So we're looking at rules six and nine for this one.
Okay, right, so we're going to work out some more oxidation states. Uh, so transition metals have variable oxidation states. You've got to be careful with transition metals because they are slightly different. They don't have, they don't belong to a group, as you may, as you may work out. So Fe2O3, which is iron oxide. Again, look at oxygen. Oxygen is minus two. We've got three of them, so that's going to be minus six in total. So two ions must be plus six, so individually iron must be plus three, which is fine. Okay? So we've worked that out, but, um, and there's the formula there as well. So we've got iron three. We represent the three bit, the plus three bit, with a three in Roman numerals in brackets. We say iron three oxide to say that iron's got a plus three charge. That's what that represents.
But if you look at another one here, this is iron oxide as well, um, however, this is a different type of iron oxide. Oxygen's minus two, so that means iron must be plus two in this case. So this is the same element, but it's a different oxidation state. We represent it as iron two oxide with the Roman numerals in two again. It represents plus two oxidation state. Okay?
Look at the next one here. This is another one. This is vanadium oxide. Again, minus two for oxygen, but the whole charge is plus two. So that must mean vanadium has to be plus four to give it that overall charge. So this is vanadium four oxide, as you can see. Uh, this one, another vanadium compound. Again, oxygen minus two. You've got two of them, so that's minus four in total. Overall charge positive. So vanadium in this case must be, uh, plus five. So we call this vanadium five oxide. Now, you can see both vanadium oxide, but this has got a different oxidation state to that, and obviously the formulas are different too. So make sure you can obviously identify these and you know the Roman numerals as well. Okay?
So also, still looking at some odd examples, is looking at the systematic name of ClO2 minus. So here we've got ClO2 minus. Let's look at rule nine first. Oxygen's minus two. We've got two of them, so that's two lots of minus two is minus four. For rule seven, chlorine must be plus three. Is the overall charge must add to give minus one because we've got that negative charge there? So as chlorine is bonded to oxygen, we add eight to the end of it. So this is called a chlorate. Okay? That's quite common. So when you get bonded with oxygen, you add eight to it. So chlorine has an oxidation state of plus three, as we've worked out here. So you can see the systematic name is chlorate three. Look, three Roman numerals, and we put eight on the end. That basically tells we've got chlorine bonded to oxygen, and that's got a three, a charge of plus three. Okay?
So we need to be able to identify something as oxidation or reduction. So basically, reduction is a decrease in oxidation number. So you're looking for a difference there, and oxidation is an increase in oxidation number. So let's have a look at this example. We've got 2 Na plus chlorine will form 2 NaCl. So this is basically sodium plus chlorine forms sodium chloride. So we're going to try and decide whether we've got reduction and oxidation and where we've got it.
So let's have a look at the oxidation numbers first. Sodium has got an oxidation state of zero, and so is chlorine because these are both elements. So remember them rules. Sodium here though is plus one because we've got, um, this is sodium combined, so it's plus one. Chlorine is minus one in this case. So we put minus one there. And all we're doing is we're looking for the change in oxidation number.
So here we've got sodium being oxidized. It's going from zero to plus one. Oxidation number is increasing. So sodium's being oxidized, going from zero to plus one. And chlorine, as you probably guess, is being reduced. It's going from zero to minus one. So we say chlorine is being reduced. Um, so that's really important. A decrease in oxidation number.
And in terms of reducing and oxidizing agents, remember sodium is a reducing agent here. If sodium's been oxidized, it is a reducing agent. Okay? Remember that important point. This can get confusing, so you've got to make sure you're really thinking about this. Chlorine is an oxidation, is an oxidizing agent because itself has been reduced. So make sure you kind of distinguish between the two. Very common for them to ask these, so make sure you know it. Okay?
Right, metals and Redux. Okay? So when metals react with acids, they're oxidized, and a salt and hydrogen gas is formed. So we know that from the acids topic in the same module, module two. So reaction with with HCl. Okay? Let's have a look. We've got calcium and, uh, so metal plus acid gives salt plus hydrogen. So this is the typical reaction. If we look at the oxidation states here, calcium zero, that's plus two. Uh, hydrogen here is plus one, that's zero. So let's have a look, see what's going on. This one shows oxidation, because we've gone from zero to plus two. However, your hydrogen is being reduced. It's going from one plus to zero. So basically, we have, um, the acid. So when metals react with acids, they're oxidized, and you can see that your metal is being oxidized from zero to plus two.
Okay, let's look at the reaction with sulfuric acid. So we got sodium plus sulfuric acid forms sodium sulfate and hydrogen. Let's look at the same thing. Sodium's gone from zero, it's gone to plus one. Hydrogen's gone from plus one individually to zero because it's an uncombined element. So again, that's showing oxidation. The sodium, the metal has been oxidized, and obviously your hydrogen has been reduced. Okay?
And that's it. That's a bit of a summary of Redux reactions for OCR. Um, if you would like to, uh, purchase these PowerPoints again, you just click on the link below, um, and in the description box, and you can get a hold of them there. Uh, please subscribe to the channel as well. If you just click on the, the middle button there, what you can see now, and you'll be able to get all the latest updates of all the videos that's been uploaded. But that's it now. Bye-bye.