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Redox reaction 

Redox reactions are reactions where both oxidation and reduction are taking place.

Oxidation is the loss of electrons by a reactant.

When a metal element is reacting to form a compound then it is being oxidised.

For example:

                          Mg(s)+O₂(g)→MgO(s)

The metal atoms are losing electrons to form an ion. They are being oxidised.

                       Mg(s)→Mg²⁺⁽aq)+2e−

This is known as an ion-electron equation.

Reduction is the opposite of oxidation. It is the gain of electrons.

Compounds reacting that result in metal elements being formed are examples of reduction reactions.

For example:

                     Cu²⁺⁽aq)+2e−→Cu(s)

The metal ions are gaining electrons to form atoms of the element. They are being reduced.

 Redox reactions takes place in the same reaction and at the same time and they result into displacement of one species by another

For example, if magnesium was added to copper sulphate solution, the magnesium metal would be oxidised, while the copper ions were being reduced.

                   Cu(s)→Cu2+(aq)+2e− oxidation reaction

                   2Ag+(aq)+2e−→2Ag(s) reduction reaction

                 Cu(s)+2Ag+(aq)→Cu2+(aq)+2Ag(s) redox reaction

The superscript along with the sign is, called ‘oxidation state’ of the atom.

NOTE :

Reduction

A substance can undergo reduction can occur via:

1. The addition of hydrogen.
2. The removal of oxygen.
3. The acceptance of electrons.
4. A reduction in the overall oxidation state.

Oxidation

The following points describe a substance that has undergone oxidation.

1. The addition of oxygen.
2. Removal of hydrogen which was attached to the species.
3. The donation/loss of electrons.
4. An increase in the oxidation state exhibited by the substance.