In this lesson we’re going to continue talking about oxidation reduction reactions and these are often abbreviated as “redox” for short and specifically in this lesson we’re going to learn how we identify the substances involved in oxidation reduction in a reaction, as well as identifying the oxidizing and reducing agents in a redox reaction. So the first thing we have to do is we have to assign oxidation numbers. So according to the rules we’ve learned, we’re going to assign oxidation numbers to every species in our reaction. So we see that iron is a free element, so it has an oxidation number of zero. Copper is a monatomic ion, so it’s oxidation number is equal to its charge. Same thing for iron: it’s a monatomic ion, so it’s oxidation number is going to be equal to its charge. And then copper is a free element and so we’re going to have an oxidation number of zero. Remember that by free element, we mean an atom by itself, or an element by itself, with no charge shown. So let’s look at this reaction. I start with one of my reactants. I’m going to start with iron, simply because it’s written first and I see that it has an oxidation number of zero. The reactant side is what we’re starting with and so I want to go from that 0 over here to the iron on the product side, where I see it has an oxidation number of +2. I see that I’ve increased my oxidation number. Therefore I have lost electrons. This is undergoing oxidation. Now we can look at the copper. And what we see is copper goes from a +2 to a 0 and so we see it’s a reduction in the oxidation number- a decrease in the oxidation number-and so we have the gain of electrons because we went from +2 to 0, so I had to gain some negative charge to reduce that number down. And so this shows our decrease in oxidation number and as a result the reaction from Cu2+ to Cu is known as a reduction. Now we need to look at two more terms: reducing agent and oxidizing agent. So these terms are indicating who is causing either reduction to happen in the case of reducing agent or who is causing oxidation to happen in the case of the oxidizing agent. So the species that is oxidized is the reducing agent. So in this case, iron is undergoing oxidation because it goes from an oxidation number of 0 to +2. So it’s undergoing oxidation. And as a result, it is known as the reducing agent because by being oxidized it’s causing another species to be reduced. Remember that oxidation and reduction have to happen in pairs. Now we can look at the oxidizing agent. And so the oxidizing agent is causing another species to be oxidized because itself is being reduced. So we go from Cu2+ to Cu 0, and we see the gain of electrons. We see a reduction happening, so copper two plus is the oxidizing agent. Notice that both the reducing agent and the oxidizing agent are on the reactant side and they are causing the thing to happen. So an oxidizing agent causes oxidation to happen. And the way that it causes oxidation to happen is by being reduced. So this slide summarizes about the agents. The reducing agent is the substance that causes something else to be reduced and is itself oxidized. The oxidizing agent causes something else to be oxidized, but itself is reduced. Now remember, we said that oxidation and reduction is not always so easy to identify based on the oxidation number and so sometimes it’s easier to look at this in terms of oxygens and hydrogens. So let’s look at our example here on the left of an alcohol. Notice that this carbon has one bond to oxygen and there’s also a hydrogen attached. When I look over here at the next substance, which is an aldehyde, I see that now I have a carbonyl. So now have two bonds to oxygen, so I’m gaining oxygen. I do in fact lose a hydrogen from up here attached to the oxygen and down here attached to the carbon, but my main thing I see here is that I’m gaining a bond to oxygen, therefore this is an oxidation process going from our alcohol to our aldehyde. I don’t actually have to see both of these things happening, although I often will. If I see one or the other, I can still say that oxidation is occurring. So an aldehyde can also be further oxidized by the addition of an oxygen or the loss of a hydrogen, and in this case we see our aldehyde forms a carboxylic acid. We have our C double bond O and our OH. We’ve gained an oxygen. So here we don’t readily see our change in hydrogen, but we know that we see the gain in oxygen so all is well. The reduction is just the opposite. We go from a carboxylic acid to an aldehyde, and the easiest way to see that is the gain or the reduction in the number of oxygen bonds, and then we go from an aldehyde to an alcohol. So we’ve gained a hydrogen and lost a bond to oxygen.