Chemguide: Core Chemistry 14 - 16

An introduction to redox reactions

This page introduces redox (reduction and oxidation) reactions in terms of the transfer of oxygen and electrons. This is a key part of chemistry, and you should take your time over it to make sure you understand it.

I am assuming that you have read the page about reactions between metals and metal oxides because this page builds on that one.

Oxidation and reduction in terms of oxygen transfer

This is the easy bit!

If something gains oxygen, it is said to have been oxidised. So in the equation . . .

C + 2CuO     CO₂ + 2Cu

. . . the carbon has been oxidised to carbon dioxide.

The process of adding oxygen to something is known as oxidation.

The opposite of oxidation is called reduction.

In the equation, the copper(II) oxide has lost its oxygen. We say that it has been reduced to copper.

Oxidation and reduction always go hand-in-hand. If something is being oxidised, something else must be being reduced. A reaction in which oxidation and reduction is occurring is called a redox reaction.

Oxidation and reduction in terms of electron transfer

The commonest reaction to discuss here is the one between magnesium and copper(II) oxide. You will remember that this is a violent reaction in which the magnesium takes the oxygen away from the copper.

Mg + CuO     MgO + Cu

Clearly the magnesium has been oxidised, and the copper(II) oxide has been reduced - you can work that out from what we have said above.

But it is interesting to take this reaction apart and see what is really going on in terms of the structures of the various things involved.

Obviously, the magnesium and copper are metals, and consist of giant structures of atoms. But the magnesium oxide and copper(II) oxide are both ionic compounds - each consists of an array of metal ions and oxide ions.

So let's rewrite the equation with that in mind:

Mg(s) + Cu²⁺(s) + O^2-(s)     Mg²⁺(s) + O^2-(s) + Cu(s)

The oxide ions aren't changed in any way in the process. They may have got a new partner, but they are still exactly the same particles. Ions which aren't changed during a reaction are known as spectator ions - they are just looking on, watching other things being changed!

You don't write things that aren't changed into equations, and so we rewrite the equation as:

Mg(s) + Cu²⁺(s)     Mg²⁺(s) + Cu(s)

This is known as an ionic equation, and just shows the things that are actually changed.

Now let's take this apart still further to see what is happening to the magnesium and the copper in the ionic equation.

What is happening to the magnesium?

It starts as magnesium atoms and ends up as magnesium ions. In the process it loses 2 electrons per atom.

Mg(s)     Mg²⁺(s) + 2e^-

What is happening to the copper?

It starts as copper(II) ions and ends up as copper atoms. In the process it gains 2 electrons per ion.

Cu²⁺(s) + 2e^-    Cu(s)

Overall

Magnesium atoms have given electrons to copper(II) ions, and you end up with magnesium ions and copper atoms.

But we have called this a redox reaction, and there is no sign of oxygen being involved! We need to extend the definitions of oxidation and reduction to take this into account.

An extended definition of oxidation and reduction

Oxidation Is Loss of electrons; Reduction Is Gain of electrons.

O I L     R I G

This is one of the most important ways of remembering something in the whole of chemistry. Learn it!

So, in the example we are exploring,

• magnesium atoms are being oxidised because they are losing electrons;

• copper(II) ions are being are being reduced because they are gaining electrons.

So does this mean that the definition in terms of oxygen transfer is wrong?

No! - definitely not! It is just less generally useful.

When you are starting chemistry, it is a lot easier to see that the reaction between, for example, aluminium and iron(III) oxide is a redox reaction if you just think about oxygen transfer - and so that's what you do.

But we will soon come across reactions where the oxygen transfer definition just can't work because there isn't any oxygen involved at all, but the processes involved are just like the ones above.

We will look at cases like this in the next page in this sequence.

Oxidising agents and reducing agents

An oxidising agent is something that oxidises something else.

So, in the reaction between magnesium and copper(II) oxide . . .

Mg + CuO     MgO + Cu

. . . the copper(II) oxide is the oxidising agent because it oxidises magnesium to magnesium oxide.

A reducing agent is something that reduces something else.

So in this case, the magnesium is the reducing agent because it reduces copper(II) oxide to copper.

Working it out where electron transfer is involved

Does it make any difference which definition of oxidation and reduction you use? In essence, no - although you have to rephrase things a bit.

So let's work from the ionic equation for this reaction:

Mg(s) + Cu²⁺(s)     Mg²⁺(s) + Cu(s)

Here are the processes actually going on in terms of electrons:

Mg(s)     Mg²⁺(s) + 2e^-

Cu²⁺(s) + 2e^-    Cu(s)

Remember OIL RIG.

Mg is losing electrons and so being oxidised. The oxidising agent is Cu²⁺.

Cu²⁺ is gaining electrons and so being reduced. The reducing agent is Mg.

Oxidation and reduction in terms of hydrogen transfer

This is an old definition and isn't much used nowadays, but I just want to mention it in case you come across it.

Loss of hydrogen is oxidation; gain of hydrogen is reduction.

It is most commonly used in organic chemistry. For example, converting ethanol, C₂H₆O, into ethanal, C₂H₄O involves loss of two atoms of hydrogen. It has been oxidised.

If you converted ethanal into ethanol, you would have to add two atoms of hydrogen - that's reduction.

A simple way of remembering all these assorted definitions

It all seems a bit of a mess, but there is an easy way to remember it all.

Remember that oxidation is gain of oxygen - that just sounds obvious. Everything else is the opposite of that.

If oxidation is gain of oxygen, oxidation is also loss of electrons and loss of hydrogen.

And reduction is alway the opposite of oxidation.

So reduction is loss of oxygen, gain of electrons and gain of hydrogen.