Redox Reactions

 The transfer of electrons from one reactant to another...When there is oxidation, there is also reduction.
The substance which loses electrons is oxidised.
The substance which gains electrons is reduced.

For example: Fe (metal) + Cu2+ -> Fe2+ + Cu (metal)

Fe donates two electrons to the Cu2+ to form Cu (metal). The Fe lost 2 electrons, so is oxidised.

The Cu2+ gained 2 electrons, so is reduced (in its valency).

Simply put:The chemical which gains electrons is reduced (reduces its valency) and is called the oxidising agent.

The chemical which loses electrons, is oxidised (increases its valency) and is called the reducing agent.

Please do not get these terms "oxidised" and "oxidising agent" confused.

Confusing isn’t it! - Lets try some more... Fe + Cu2+ -> Fe2+ + Cu Oxidised
Reducing
Agent Reduced
Oxidising
Agent
The chemical which is oxidised is the reducing agent.
The chemical which is reduced is the oxidising agent.

For Example: Zn + 2HCl -> Zn2+ + H2 +2Cl-

In this reaction Zn + 2H+ -> Zn2+ + H2 (the chlorine is not changed in its ion state, so is not oxidised or reduced).

Zn is oxidised to Zn2+ (loses 2 electrons)

H+ is reduced to H2 (gains 2 electrons)

H+ has an oxidation number (valency) of +1, and is reduced to an oxidation number (valency) of 0.

So, reduction decreases the oxidation number (valency).

Zn has an oxidation number (valency) of 0, and is oxidised to Zn2+, an oxidation number of 2+,

So oxidation increases the oxidation number (valency).