Simple redox reactions

0:00- [Instructor] You've probably heard about this word, oxidation, oxidizing, or antioxidants and stuff like that. But what exactly does it mean and what can you do knowing about it? Well, let's find out. Oxidation has the word oxygen in it. You can see that, right?

0:12So we can guess that it's probably got something to do with the oxygen, right? And that's how we defined it earlier. Not currently, but that's how it started. So let's start with that. Oxidation can be thought of as adding oxygen atoms. For example, take a look at these reactions.

0:28What do you notice? We see that, like for example, in this first reaction, oxygen is getting added to magnesium, right? Therefore we say magnesium is being oxidized or magnesium is undergoing oxidation. Remember, oxygen is diatomic in nature. That's why there's an O2 over here, okay?

0:45But the same thing's happening over here. Look, oxygen is being added to iron. You can see that over here. And therefore we say iron is being oxidized or iron is undergoing oxidation. Same thing over here. Oxygen look, gets attached to carbon. So carbon is being oxidized.

1:01Carbon is undergoing oxidation. But a question I always had was, well, why do we give a name to these kinds of reactions? What's so special about oxygen? Well, I don't think there's anything special about oxygen, but I think these reactions are important. For example, if you look at iron oxide, which we call it iron three oxide, because the charge on the iron over here will be plus three, we will get it to that.

1:21But this is the molecule that you find in rust. This is rust. I mean rust also involves water. So the actual reaction also involves water molecules over here. But since rusting involves oxygen, that's an important reaction for us because we want to prevent rusting.

1:35And when you're burning stuff, well guess what? Oxygen is involved. Combustion involves oxygen. That's also another important reaction for us. And so since these reactions are important and they involve oxygen, that's probably the reason why we gave a name to it and we call it oxidation.

1:50But anyways, this was the earlier definition of oxidation. Later on we thought, okay, let's just think about what's really going on with the electrons over here. So for example, over here we realized when magnesium is a metal, it loses electrons. In fact, over here it's going to lose a couple of electrons, and magnesium will now become magnesium two plus, because it has lost two electrons.

2:11And oxygen, well, it gains two electrons and therefore oxygen becomes two minus. And that's why they stick to each other, okay? This is how that reaction is formed. So what we notice is that when magnesium is undergoing oxidation, it's losing electrons. Well, let's see if the same thing is happening over here.

2:30Well, yeah, iron is also a metal and it's losing electrons over here. In fact, iron loses three electrons to from iron three plus, and that's why this is called iron three oxide because it is charge of three plus, it loses three electrons over here. And oxygen gains those electrons.

2:47I mean each atom of oxygen gains two electrons making it oxygen two minus. And so because oxygen has a two minus charge and iron has a three plus charge. Now you can notice three atoms of oxygen has total of minus six. And two atoms of iron has a total of plus six, plus six, minus six, very nicely becomes neutral.

3:08But again, what's important is that iron, which is undergoing oxidation, is losing electrons. And what about this one? Well, this is actually a slightly more interesting case, so we'll get back to it towards the end of the video. But this means, in most cases when you add oxygen to something, that something loses electrons.

3:24So we said, "Hey, hey, hey, oxidation is involving loss of electrons. So let's call that as a definition of oxidation." And that is our new definition because it's generalizing the definition of oxidation. We now say, forget about oxygen atoms. Anything that loses electrons, we will call it oxidation.

3:43So magnesium is being oxidized because it's losing electrons. Iron is being oxidized because it is losing electrons. That is the current definition of oxidation, okay? Now, if something loses electrons, something else should gain electrons. And in this case, the oxygen is the one that gains electrons.

3:57So we thought, okay, let's give a name to gaining electrons as well. Gaining electrons is what we call reduction. And now immediately looking at this, it feels weird, right? You're gaining electrons. Why do we call it reduction, okay? This sounds weird, and there might be some history behind it.

4:16We'll not worry too much about the naming and where it came from, but here's how I think about it. When you gain electrons, since electrons are negative, you are gaining negative charge, okay? And that means, look at your charge, your charge with zero to begin with over here.

4:30Oxygen at zero charge to begin with. But now it has a negative charge. That is in some sense reduction, right? It has reduced. So gaining negative reduces your overall charge because you you gain more negative charge. And so that kind of, sort of makes sense to me in calling that reduction.

4:48And so oxygen over here is getting reduced, okay? But of course, what if we forget this? How do we remember that oxidation is losing electrons, reduction is gaining electrons? Well, there are a couple of mnemonics you can use. For example, one mnemonic that I like is, "LEO the lion goes GER."

5:06Here, losing electrons is oxidation, gaining electrons is reduction. Another mnemonic you can use is "OIL RIG," oxidation is losing, reduction is gaining. My friends keep debating about which one is better. But I personally like to think in terms of charge. When I gain electrons, I gain negative charge, and therefore it's a reduction.

5:30And that's how I like to remember that gaining electrons is a reduction, therefore losing electrons must be oxidation. But now we have multiple ways to remember, whichever one works for you. Anyways, now think about this. If you have a reaction in which something is getting oxidized, meaning something is losing electrons and something else must be gaining electrons.

5:48So something else must be reduced. And therefore such reactions in which something gets oxidized and something else gets reduced, we give a name to it. We call them redox reactions because both reduction and oxygenation must be happening. And now here's my question.

6:03Do redox reactions need to involve oxygen? The answer is no, because our definition has become more general. Sorry oxygen, but we don't need oxygen anymore. Any reaction in which something is losing electrons and something else is gaining electrons becomes a redox reaction.

6:19So let's look at a few. So here are a few reactions. Our goal now is to analyze each reactions and think about which one is undergoing oxidation and which one is undergoing reduction. So lemme start with this one. So we have iron that reacts with sulfur to give you iron sulfide.

6:37So how do I know which is undergoing oxidation, which is undergoing reduction? Well, I know iron is a metal, so it loses electrons. So I just keep track of charge. Over here iron is neutral, single atom of iron, zero charge. Sulfur is also neutral. Zero charge.

6:51Sulfide, usually sulfur gets a charge of negative two. So iron must have gotten a charge of plus two, meaning iron must have lost two electrons, sulfur must have gained two electrons, giving you iron sulfide, or we say iron two sulfide. But what's important for us is that look, iron is the one that has lost electrons, which mean iron got oxidized.

7:14Sulfur is the one that gained electrons, and therefore sulfur is reduced. Again, sulfur's charge has reduced, sulfur is the one that underwent reduction. Now it'd be a great time for you to pause the video and try to analyze the remaining three reactions. All right, let's look at this one.

7:31So sodium has a charge zero. Chlorine also has charge zero over here. I know sodium is a metal, so it's gonna lose an electron. In fact, it loses one electron, giving a plus one charge. And chlorine gains an electron giving it negative one charge. So look, it's the sodium that lost electrons.

7:49Sodium got oxidized. Chlorine that gained electrons, so chlorine got reduced. All right, here's the next one. This looks slightly more complicated, but we'll work it out. Silver being a metal over here loses electrons. So it loses one electron, so it actually has plus one charge.

8:05And sulfur gains two electrons means it has minus two charge. In fact, that's the reason why we have two Ag's, so giving you plus two and minus two, that balances it out. Okay, anyways. That's the charge over here. Aluminum look, it's not bonded to anything. So it has zero charge over here.

8:24But what happens over here? Hey, now aluminum gets bonded to sulfur and aluminum is a metal. It's the one that loses electrons, and it usually loses three electrons. So it gets plus three and sulfur gets minus two. You can just look at the the molecular formula and also guess what the charge they must have had, how many electrons they must have lost and gained.

8:44And finally, now look, silver has is not bonded anymore, so it has a zero charge. So if you look at silver, it went from plus one to zero. Ooh, ooh, silver got reduced because its number has reduced, it has gained electrons. Ah, so silver underwent reduction. What about aluminum? Aluminum, look!

9:04It underwent oxidation because look, it must have lost electrons, so it went from zero to plus three. So aluminum underwent oxidation. Silver underwent reduction. What about sulfur? Hey, sulfur's charge has not reduced at all. Sorry, it has not changed at all.

9:20So, sulfur neither went oxidation nor reduction. So this was an interesting one, okay. That brings us to the last one. Again, if you haven't tried it, feel free to try it now because it's an interesting one. All right, here it goes. So sodium chloride, we already saw, plus one minus one charge.

9:38Silver fluoride, silver plus one, and fluorine minus one. Sodium fluoride, well, sodium is again plus one, fluorine minus one. Silver chloride. So silver is plus one, chlorine is minus one. What do you notice? Which elements underwent oxidation? Which elements underwent reduction?

10:02None of them, because none of their charges changed. That means none of them have lost electrons or gained electrons. So this is not a redox reaction because nothing underwent oxidation, nothing underwent reduction. When I first came across such reactions, I was surprised because in my mind any chemical reaction involves transfer of electrons.

10:23So in my mind it was like every reaction must be redox reactions. But right in front of your eyes, you can see not every reaction is a redox reaction. Okay, finally, this brings us back to our carbon dioxide example. Why was this interesting, you know? Well, because in all these cases something was losing electrons and something else was gaining electrons, right?

10:40But over here carbon does not lose electrons to oxygen. Instead, carbon and oxygen share their electrons forming a covalent bond. So how does it make sense according to this new definition, according to the current definition, how does it make sense to talk about oxidation reduction over here?

10:56Well, it is slightly more complex and interesting, but here's the gist of it. The electrons are not shared equally. Oxygen has a tendency to pull those electrons more towards itself compared to carbon. We say oxygen is more electronegative. Electronegativity is the tendency of pulling the shared pair of electrons towards yourself.

11:16So oxygen, since it's more electronegative compared to carbon, it pulls the shared pair of electrons more towards itself, kind of hogs on those electrons. And as a result, oxygen also ends up getting a negative charge, but not a complete negative charge, because it's not a complete transfer.

11:30We say it gains a partial negative charge. I know it sounds complicated, but at the end of the day it's still kind of, sort of getting a partial negative charge. So in that sense it still makes sense to say oxygen is reduced and therefore carbon gets a partial positive charge, meaning carbon is oxidized.

11:50So even here we can think in terms of oxidation and reduction. Anyways, our last question could be, what can we do now that we understand what oxidation reduction really is? Well, here's an example. Now that we know that rusting is basically iron losing electrons to oxygen, one way to prevent rusting is to coat it with a metal which loses electrons more readily compared to iron.

12:12And one such example is zinc. And that's why if you take an iron nail and you don't want it to rust, we usually coat it with zinc. It kind of gives you double protection. First of all, it kind of forms a barrier, so it doesn't even allow oxygen to reach iron. It's the zinc that gets oxidized.

12:30But even if there's a breach somewhere over here and iron is exposed, even at that site of exposure, oxygen is much more likely to react with zinc because zinc much more readily gives out its electrons compared to iron. We say zinc is more reactive compared to iron.

12:45So using this knowledge we can protect our precious iron from getting rusted. That's awesome, right?