Basics of Redox

Argh I hate redox. I'm not too bad at it, but it annoys me for some reason. Anyway, rant over, it's time for me to try and explain it.

Now, redox is all about the transfer of electrons. When an atom gains electrons and thus has its charge reduced, we say that it has undergone reduction. When an atom loses electrons, we say that it has undergone oxidation. The word "oxidation" comes from the fact that there are lots of cases in which the addition of oxygen makes another atom lose electrons.

When do redox reactions occur? Well, you know very well that the transfer of electrons happens when atoms become ions. (If you didn't already know this, go here to find out more.) But there are other times when electrons are transferred too. It's easier to explain this by talking about oxidation numbers, so that's what I'm going to talk about first.

You can assign each element in a compound an oxidation number. If it's just a single element you're looking at, like Fe or O2 or Na, the oxidation number is 0. Hydrogen usually has an oxidation number of +1, but if it's a metal hydride, then it has an oxidation number of -1. (And, as said before, if it's on its own, like in H2, then it has an oxidation number of 0.) Similarly, oxygen usually has an oxidation number of -2, but there are a few exceptions where this is different (I'll have to check what the exceptions are). In ionically-bonded compounds, the oxidation number of each monoatomic ion (ion with only one atom (ion?), like Na+) is simply the charge on the ion (for example, the oxidation number of Na in NaCl is +1 because Na has a +1 charge). Assign these elements their oxidation numbers first.

Now, in a neutrally-charged compound, all of the oxidation numbers have to add to 0. Similarly, in a charged compound or polyatomic ion, all of the oxidation numbers have to add to the charge on the ion or compound. Simple, isn't it?

Now I'm going to talk about carbonic acid, H2CO3. Let's start off by assigning it some oxidation numbers.

Since this isn't a metal hydride, the oxidation number of H is +1. That was easy, wasn't it?

Now, CO₃<sup>2- has a -2 charge. Therefore, the oxidation numbers of C and O need to add up to -2. Each O has an oxidation number of -2. Three Os have a combined oxidation number of -6. C must have an oxidation number of 4 because 4-6 = -2, the overall charge on the ion.

That wasn't too hard, was it? (If it was, don't feel stupid, just ask me to clarify.) Now for two more examples: carbon dioxide and water.

Water's easy. Each H has an oxidation number of +1 (the two Hs combined have an oxidation number of +2) and O has an oxidation number of -2. The overall charge is 0.

Carbon dioxide isn't much harder. Each O has an oxidation number of -2, and because carbon dioxide is neutral, the oxidation number of C has to balance out the oxidation number of the Os. The oxidation number of C is therefore 4.

Now, the decomposition reaction of carbonic acid to carbon dioxide and water is not a redox reaction. Why? Well, it's because none of the elements has changed oxidation number! If any of the elements had changed oxidation number, then it would be a redox reaction. (Sorry, I thought this decomposition reaction would be a redox reaction as well, but soon realised that it wasn't. One day I'll put a better example up.)

In a redox reaction...

The element that loses electrons, thereby obtaining a higher oxidation number, has been oxidised and is known as the reducing agent or reductant (as it reduces the other element).

The element that gains electrons, thereby having its oxidation number reduced, has been reduced and is known as the oxidising agent or oxidant (as it oxidises the other element as it is being reduced).

Remember that if electrons are being lost then they have to have somewhere to go to, and vice versa (if electrons are being gained the atom needs to gain them from somewhere).</sup>

By the way, there's a nice little mnemonic to help you remember which is the oxidant and which is the reductant. It's OIL RIG. It stands for Oxidation Is Loss of electrons, Reduction Is Gain of electrons.

I'm too tired to do any more for now. At least I've done the first four dot points here (explain oxidation and reduction as electron transfer, calculate oxidation numbers, identify and name oxidants and reductants and identify redox reactions using oxidation numbers). I think I'm good for now. Only 5 dot points left and I'll have gone through pretty much the whole Chemistry 2AB course- unless you count all of the Applied Chemistry stuff, of course.