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Redox reactions

This is short for reduction-oxidation. In order to make sense of this we must understand the principle behind oxidation and reduction. We are all surrounded by air. This contains the very reactive element, oxygen. We frequently see combustion reactions where various materials react with oxygen. It is not surprising, therefore, that an expression was developed for this group of reactions with oxygen. When a substance reacts with oxygen it is said to be oxidized. If a substance loses oxygen it is said to be reduced.

However, as the chemistry of these reactions, and the bonding involved, became understood it was clear that the definition would need to be expanded. For example, when magnesium metal reacts with oxygen to form magnesium oxide it is clear that the magnesium has been oxidized. We now know that it has formed an ionic compound by loss of electrons from the magnesium atom:

At first inspection the reaction of magnesium metal with chlorine gas to form magnesium chloride is not an oxidation. However, magnesium chloride is also an ionic compound which has been formed by the loss of electrons from the magnesium atom:

We cannot say that the magnesium has been oxidized in the first case but not in the second case, because exactly the same thing has happened to the magnesium atom. The definition of oxidation is expanded to include increase in charge on the atom / ion and loss of electrons. We also include a definition from organic chemistry which is loss of hydrogen. To recap:

Oxidation is:

Reduction is:

Oxidation and reduction always go together - if something gains something, something else must lose it and vice versa. For the loss and gain of electrons the mnemonic OILRIG is sometimes used (Oxidation Is Loss, Reduction Is Gain).

Another way of looking at redox reactions is to use the concept of oxidation number.

Test time

Consider each of the following changes, and use the rules in the table above to decide whether the material in bold has been oxidized, reduced or no change has occurred. You can check your answers by clicking on each example. If it has been oxidized it will turn red. If it has been reduced it will turn green . If there is no change, no change will occur! With ionic compounds you will have to work out the charge on the ion to see whether it has increased, decreased or stayed the same. Remember the charge on an element is zero. Be careful when using the oxygen loss or gain definition - if another element has also been lost or gained you may not get the right answer.