Oxidation and Reduction

In the Past

Oxidation: was the addition of oxygen to a substance.
Reduction: was the removal of oxygen from a substance or, in some cases, the addition of hydrogen.

Nowadays

Nowadays oxidation and reduction are explained in terms of electron transfer.

Oxidation: is the loss of electrons from a substance.
Reduction: is the gain of electrons by a substance.

Oxidation Numbers

An oxidation number: is the charge that an atom has or appears to have when the
electrons are distributed according to certain rules.

Rules:
1. The oxidation number of an uncombined element or a diatomic molecule is zero.
2. The oxidation number of an ion is the same as the charge on the ion.
3. The sum of the oxidation numbers in a compound must add to zero.
4. The sum of the oxidation numbers un a complex ion must add up to the charge on
the ion.
5. Oxygen always has an oxidation number of ?2, except in peroxides and in OF2>
6. Hydrogen is always +1 except in metal hydrides.
7. Oxidation number of flourine is always ?1 as it is the most electronegative element.
8. Halogens (group 7) are always ?1 unless bonded to amore electronegative atom.
9. Carbon can have an oxidation number of zero in some compounds.
Oxidation: is an increase in oxidation number.
Reduction: is a decrease in oxidation number.

Redox reaction: 

is a reaction where oxidation and reduction occur simultaneously. (One
cannot happen without the other).

Oxidising agent: is a substance that oxidises something else while it itself is reduced.

Reducing agent: is a substance that reduces something else while it itself is oxidised.

Applications of oxidising agents:

1. Swimming pools { add chlorine to kill bacteria.
CL2 is the oxidisng agent. When it oxidises the germ it kills it.

2. Bleaches as oxidising agents. Household bleach contains sodium hypochlorite Na-
CLO. H2O2 is used to turn coloured hair pigments colourless.