Redox Reactions

Hi. It's Mr. Andersen and this chemistry essentials video 31. It's on redox reactions. And the word redox is a combination of both reduction and oxidation. And so these are reactions where we're actually transferring high energy electrons to more low energy electrons. And so that's providing us with energy. And so when you're burning gasoline that is a redox reaction. Likewise inside our bodies, cellular respiration, when we breakdown our food and get energy from it, is also a redox reaction. And so the first thing you should understand that in a redox reaction we have two parts to it. We've got reduction and oxidation. Now the oxidation portion is going to be the losing of high energy electrons. And the reduction is going to be the gaining of those electrons. But when you're looking at a chemical equation, we don't put electrons in the equation. So often times it's hard to figure out where these electrons are actually going. And so we use something called oxidation numbers to figure out where the electrons are going. We can then break it down into what are called half reactions that show the electrons. And then we can finally combine that back into a balanced chemical equation. Now these are important in the chemistry lab because we can use them as a different type of titration. We call that a redox titration. And again these are really important because they allow us to produce energy. Not only the energy that drives your car but the energy that drives you. And so in a redox reaction we're moving high energy electrons from the oxidized substance to the reduced substance. And it's really common to get these two terms messed up. And so the mnemonic that I use is called oil rig. And what does that mean? Well the O in oil rig stands for oxidation. And oxidation is losing electrons. So what is reduction? The R stands for reduction is gaining electrons. And so just like you maybe learned SOH CAH TOA in your geometry class you should learn oil rig in your chemistry class. It's a good way to remember where the electrons are flowing. And so let me give you a real simple example of a redox reaction. Let's say we have hydrogen gas and fluorine gas. And they combine together to create hydrogen fluoride. That would be a redox reaction. And so did you see what was being oxidized? So where were the electrons being lost from? It was from the hydrogen. And so we would say in this redox reaction of hydrogen plus fluorine to make hydrogen fluoride, the hydrogen is being oxidized. And the fluorine is being reduced. It's gaining these high energy electrons. And so we could write this out as half reactions. In other words we've got the hydrogen making hydrogen ions plus these electrons that it is losing. And then we have the fluorine which is gaining those electrons. And so this would be the oxidation portion of the redox. And this would be the reduction portion of that. But you should have been watching carefully. How did I decide that the hydrogen was losing the electrons and the fluorine was gaining the electrons? And so how would you figure that out? Well we use something called oxidation numbers to figure out where they're going. And there's some simple rules that allow you to type, or excuse me, allow you to figure out the oxidation numbers. One thing you should know is that when writing the charge, let's say a charge of negative 2 charge, a charge generally we write the number first 2 and then we would write the minus. But when you're writing oxidation numbers you're going to write either the plus or the minus before the number. And so the first rule is really really simple. If we ever have elements that are free or free elements, the oxidation number is always going to be zero. So if we have hydrogen gas by itself, what's going to be our oxidation number? It's simply going to be 0. Let's say we have magnesium solid. What's going to be the oxidation number of the magnesium? It's going to be 0. Let's say we have sulfur. Remember which will form into these big 8 atom molecules. What's going to be the oxidation number? Again, it's going to be 0. So rule numbers really simple. If they're by themselves it's always going to be 0. If we've got ions, then it's going to match the charge. And so if we're looking at sodium chloride, you can see this is an aqueous solution, what's going to be the charge? Well the sodium is an alkali metal so it's going to have a plus 1 charge. And the chlorine is a halogen so it's going to be a minus 1 charge. And so those are going to match. One thing you'll start to notice is that these are going to add up to 0. Let's say we're looking at potassium, what's going to be the charge? It's going to be plus 1. Let's say we would look at magnesium chloride. Well the chlorine is going to have a minus 1 charge. And since we have 2 of those, what's my magnesium charge going to be? It's going to be a plus 2. Alright. Let's move to the next one. These deal with oxygen and hydrogen. Oxygen is always going to be a negative 2 oxidation state. Unless it's in a peroxide. And it would be a negative 1. And hydrogen is always going to be a plus 1 charge if it's bonded with a nonmetal. And it's going to be a minus charge if it's bonded with a metal. Last thing that I mentioned this just a second ago, that in a neutral compound, all of them are going to sum up to 0. But if it was an ion they would sum up to the charge of the ion. So let's start applying that. If we look at water right here, what's going to be the charge of the hydrogen? It's going to be plus 1. And we could see that in a couple of ways. Number 1 since it's bonded to a nonmetal it's going to be plus 1. What's going to be the charge of the oxygen? It's going to be minus 2. And since we have two of these they're going to sum up to 0. Let's look at this next one. How do we work through this? Well if we look at the sodium it's going to be a plus 1 charge. Again that's going to be in the first column of the periodic table. Where could we go next? Let's look at the oxygen. The oxygen remember is always going to be a negative 2. So we could write that in as a negative 2. Okay so what's our charge at this point? We've got 2 of these sodiums. So we're at plus 2 on this left side. Our oxygen, since it's minus 2 and we have 3 of those, it's going to be minus 6 on the right side. So what is the sulfur going to be? It has to be plus 4 because we're going to sum up to 0. Let's look at this ion over here. In this ion here, let's just label what the oxygen is going to be. The oxygen is going to be minus 2. We know that oxygen is always minus 2. And so what's going to be this sulfur in this case? Well since this is minus 2 and we have 4 of them that would be minus 8. Since the ion has a charge of 2 minus we know that the sulfur has to be plus 6. And so you can see that the sulfur is actually going to change depending on what compound it's in. So do you think you have those rules? I know it seems complex but it's really powerful when we're looking at redox reactions. So let's look at a redox reaction. Let's say we take some magnesium solid and we just put it in hydrochloric acid. What's going to happen? Well you're going to start to see bubbles show up. And so those bubbles are going to be hydrogen gas. And this is a redox reaction. And so we're producing hydrogen gas on the right side of the equation. And so what we can do is we can go through this reaction and we can start writing down the oxidation numbers. And so let's start with the magnesium. The magnesium is all by itself. So what's its charge going to be? It's going to be 0. Now if we look at the hydrochloric acid, we could start with the chlorine ion since it's in aqueous solution. And so that's going to be a minus 1. What's our hydrogen going to be? It's going to be a plus 1. So now we've got our oxidation states here. Let's go over to magnesium chloride. So this is also in aqueous solution. So we could start with the chloride. That's going to be minus 1. Since we have 2 of those what's our magnesium going to be? It has to be plus 2. Now let's go over to hydrogen on this side. Hydrogen on this side is simply going to be 0. Because it's all by itself. Okay. So now what we can see is if we look through chlorine for example on the left side is minus 1. On the right side is minus 1. And so that's neither going to be oxidized or reduced. But if we look at magnesium here it has an oxidation state of 0 on the left side. What is it on the right side? Plus 2. So since it's gotten more positive, it's lost 2 electrons. And so if you're losing electrons what is that? That is simply oxidation. So magnesium is being oxidized. In other words it's losing 2 electrons. Where did those electrons go? Well let's look at hydrogen. It's plus 1 on the left side. And it's 0 on the right side. And so since it's gone down in value, that means that it's gained electrons. But you might say well, it's only gained 1 electron. But again you could look at the coefficient out here since there's 2 of these, we've actually gained 2 electrons. And so that would be the reduction portion of this equation. So now we could write the half equations for this redox reaction. The first one is going to be the oxidation. We have magnesium as a solid. And it's losing those 2 electrons. And then if we look at the hydrogen, the reduction portion is going to be this. The hydrochloric acid is being reduced. In other words it's gaining those electrons that are lost from the magnesium itself. And so let me give you a practice problem. Right here we've got manganese plus lead nitrate is going to make manganese nitrate plus lead solid. And so what you should do is write out the equation. Figure out the oxidation numbers of all of the atoms in this equation. Then figure out what is being oxidized. And what is being reduced. And then try to write out those half reactions. And if you think you have it right you could try to post some of your answers in the comments down below. And so what are some applications of this? The first one would be titrations. Remember titrations are always used in stoichiometry. We're trying to figure the amount of an unknown. And so a great example of a redox titration would be the Winkler Method. And the Winkler Method is used to figure out how much oxygen is in water. And in biology that's really important. The amount of oxygen in water tells us how healthy the water is. In other words what organisms can live in the water. How much of that oxygen is dissolved and they can actually use. And so what you can do is a series of reactions. So a series of reactions where we're converting that oxygen into a precipitate, manganese hydroxide. We then convert it into iodine. And then we can use thiosulfate to actually do a titration. And what we're doing is we're reducing the iodine. And so remember in a typical acid-base titration what we're doing is changing the pH. Here what we're doing is changing or doing a redox reaction when we're actually transferring electrons. And we'll use starch in this as well. And when we get a change in those electrons what we get is a color change. And so we can figure out the amount of the iodine and then we can work backwards to figure out the amount of oxygen. What would be another application of redox? Remember it's in energy production. And so this big molecule right here is going to be a triglyceride. That's a fat. That's going to be the fat that you find in your food. It has a high amount of energy because there's going to be energy in the electrons of the hydrogen around the carbon. But if we were to look at something like this. This is isooctane that's going to be found in gasoline. It also has a high amount of energy. And that energy is going to be found in the hydrogen atoms that are around the carbon. And so what happens in a redox reaction is we can actually transfer those electrons to oxygen and we can release energy in that transfer. And so did you learn the following? Could you identify what's the oxidized substance and what's the reduced substance? Remember the oxidized substance is the one that's losing the electrons. And the reduced substance is gaining it. And then finally do you have some kind of an application? A great example would be a titration like the Winkler Method we said in determining the amount of oxygen. That's redox reactions and I hope that was helpful.