M.Ed., Columbia Teachers College
Kendal founded an academic coaching company in Washington D.C. and teaches in local area schools. In her spare time she loves to explore new places.
M.Ed., Columbia Teachers College
So let’s talk about redox reactions, what the heck are they and what’s the purpose? So redox reaction is a shorten way of saying oxidation-reduction reactions.
So basically, these are reactions where there is an exchange of electrons. Electrons are floating from one place to another. So we have to show how many electrons are going from the reactants to products and where are they going to and from. So basically this is an exchange of electrons, I’m going to just write that down.
So I’m going to remember which ones is being oxidized, and which one is being reduced. I use the terms OIL RIG or, LEO the lion says GER. They actually both go off my head at the same time when I’m figuring this out.
OIL RIG is oxidation is loss of electrons. And RIG is reduction is gain of electrons. So if something is being oxidized it’s losing its electrons. If something is being reduced it’s gaining its electrons. And that’s kind of a little of intuitive, but it is what it is.
LEO the lion says GER is loss. Lose Electrons Oxidized, Gain Electrons Reduced. So again the same thing, just different ways of remembering them. So what the heck am I talking about? So let’s a look at a single replacement reaction. All single replacement of reactions are actually redox reactions as well. Redox reaction is covering number of different types, but single replacement reactions are all redox reactions.
Also, if your teacher asks you about double replacement reactions, they are never redox reactions. Double replacement reactants are never redox, but single replacements are. Let me write that down. SR is redox, DR is never redox. just a little side view.
So let’s look at this reaction, potassium bromide what comes across in chlorine gas, and it exchanges. There is single replacement reaction to potassium chloride and bromine gas. So let’s separate this up. What is actually happening? I’m going to do my complete ionic equation. This is an aqueous solution, so I’m going to break it up. So I’m going to say 2K+ plus 2BR- plus Cl2, yields, this also soluble, KCl is. So I’m going to say 2K+ plus Cl- plus Br gas. Notice my potassiums are not doing anything to the reaction, everything else is. So I’m going to eliminate those. I’m sorry they should have a 2 here.
So the bromide is a -1 charge on the reactant side. And a zero oxidation number on the product side. This is -1, I’m going to put this up here. This is how I do it. And so it is coming from a -1 to zero. So it is losing its electrons. When we said it’s losing electrons it’s being oxidized. So I’m going to say Br- bromide is being oxidized. It's losing electrons. Chlorine is going from 0 to -1. It is gaining electrons. So therefore it’s being reduced.
So a question might ask you, what is bromide? And you would say bromide is oxidizing in this reaction. What is chlorine doing? Chlorine is being reduced in this reaction.
So then we also can go further and do some half reactions. So half reactions. So we have our oxidation reaction, let’s do that first. Well let’s say our reduction reaction because it’s redox, just doesn’t really matter which is which. So my reduction reaction, this is chlorine being reduced. We have Cl2 yields 2Cl-. So we are reacting with two electrons. Because this is a -1 charge, but there is two of them, so two electrons are being added to chlorine to produce chloride. This is my reduction half reaction. My oxidation half reaction is Br-, 2 of them, are going to Br2. So we’re losing two electrons, we’re producing two electrons when we’re doing this.
So notice, that electrons actually should be the same number of sides. Whatever the reduction reaction is requiring you to react the oxidation number is out. You add this two half reactions up to get your whole reaction which makes sense, and then I’ll bring this back to our original reaction which is here, if we add these up.
So this tells you how many electrons are being exchanged. It shows where are the electrons going. I’m going to write that, shows where the electrons are going, and how many of them are going. This also is another way to balance equations, a little bit more complicated equations which will show, I’ll talk about in just a second.
So I’m going to write all this out for you, 2BR- plus Cl2 yields Br2 plus 2Cl-. Bromide, in this case it’s being oxidized, which means it’s a reducing agent. Let’s think about this. If you think about a sports agent, does a sports agent plays sport himself? At least professionally? No he doesn’t, or he or she does not. See he is an agent, he helps somebody else play sports. Reducing agents helps something else be reduced. It itself is not reducing.
Cl2 is the one that’s reducing, so it is the oxidation agent, or oxidizing agent. It is not the one that’s being oxidized, it is the one that’s helping something else be oxidized. If you have a redox reaction, they come in pairs, something is being reduced and something is being oxidized. You do not have a reduction without an oxidation, they come hand in hand. You cannot have two reduction reactions within one whole reaction. You cannot have two oxidations. It is called redox for a reason. They have reduction and then oxidation together.