Electrochemistry

A redox reaction is a reaction in which electrons are transferred from one species to another, and it has two inseparable halves. Oxidation is the loss of electrons and reduction is the gain of electrons, and the two always occur together, because the electrons given up by one species are exactly the electrons taken by another. In older language oxidation also meant adding oxygen or removing hydrogen, and reduction meant the reverse, but the electronic definition fits every case, from a burning metal to a battery. A species that is oxidised has its oxidation number raised and acts as a reducing agent; a species that is reduced has its oxidation number lowered and acts as an oxidising agent.

The oxidation number, or oxidation state, is the apparent charge an atom carries in a compound or ion, and it may be positive, negative or zero. Assigning it follows a short set of rules. An element in its free, uncombined state is zero. A monatomic ion takes the charge it carries. The sum of the oxidation numbers of all the atoms in a neutral compound is zero, and in a polyatomic ion it equals the net charge on the ion. Oxygen is normally $-2$, but it is $-1$ in the peroxides such as hydrogen peroxide, $-\frac{1}{2}$ in the superoxides such as $KO_2$, and $+2$ in $OF_2$. Hydrogen is $+1$ in its covalent compounds but $-1$ in the metal hydrides such as sodium hydride. Group IA elements are $+1$ and group IIA elements $+2$, and fluorine is $-1$ in every compound without exception, because it is the most electronegative element.

Two consequences follow. The first is that not every atom in a redox reaction needs to change its oxidation number; at least one must, and usually that change is easy to find. The second is a quick way to name the agents. The strongest oxidising agents hold an element in a high oxidation state that can be pulled down, and the strongest reducing agents hold an element in a low state that can rise. A reaction is not redox when no oxidation number changes, which is how a neutralisation such as $CuO + H_2SO_4 \rightarrow CuSO_4 + H_2O$ is told apart from a genuine electron transfer.