Redox reactions

The word ’redox’ is derived from two words –reduction and oxidation. Oxidation is a process of addition of oxygen or removal of hydrogen. Reduction is a process of addition of hydrogen or removal of oxygen. Let us consider following examples.

1. Take about two grams of copper powder in a china dish. Heat the china dish for about ten minutes. Cool the dish. The surface of copper powder becomes coated with black copper (II) oxide. Copper combines with oxygen to form copper oxide. Following figure shows formation of copper oxide.

2 Cu + O2 → 2 CuO ; CuO + H2 → Cu + H2O If hydrogen gas is passed over this heated material (CuO ) the black coating of the surface turns brown as the reverse reaction takes place and copper is obtained.

 

 

2.    When gaseous hydrogen and chlorine are heated together, hydrogen chlo- ride gas is formed. H2 + Cl2 → 2 HCl In this reaction, addition of hydrogen takes place to chlorine. So the process is called reduction. Chlorine is said to be reduced. When hydrogen peroxide is treated with hydrochloric acid, chlorine gas is evolved. H2 O2 + 2 HCl → Cl2 + 2 H2O In this reaction, hydrogen is removed from chlorine. So the process is called oxidation. Hydrochloric acid is said to be oxidized.

A chemical reaction in which addition of oxygen or removal of hydrogen takes place is called oxidation and the reaction in which addition of hydrogen or removal of oxygen takes place is called reduction.

It should be noted that oxidation and reduction reactions take place simultaneously. When one substance is oxidized, another substance is reduced. In the first example given above, copper is oxidized while oxygen is reduced, similarly, copper oxide is reduced while hydrogen is oxidized. In the second example given above, hydrogen is oxidized while chlorine is reduced, similarly, hydrochloric acid is oxidized while hydrogen peroxide is reduced.

According to modern theory, oxidation and reduction are defined in terms of loss and gain of electrons respectively.

Redox reactions play an important role in metallurgy i.e. an art of extracting metals. Most of the metals occur in the earth crust in the form of oxide or a form convertible to oxide. These oxides can be reduced to metals by using suitable substances.

Some more examples of redox reactions are given below.

1. 2 Al + 3 O2 → Al2 O3

2. Fe2 O3 + 3 CO → 2 Fe + 3 CO2

3. SnCl2 + Cl2 → SnCl4

4. MnO2 + 4 HCl → MnCl2 + 2H2O + Cl2