Class.X-Oxidation and Reduction Reactions

Oxidation and Reduction Reactions

Text book Activity 1.11

*Heat a china dish containing about 1 g copper powder (Fig. 1.10).

* What do you observe?

The surface of copper powder becomes coated with black copper(II) oxide.

Why has this black substance formed?
This is because oxygen is added to copper and copper oxide is formed.

2Cu + O₂ Heat →2CuO                                                     (1.28)
If hydrogen gas is passed over this heated material (CuO), the black coating on the surface turns brown as the reverse reaction takes place and copper is obtained.

CuO +H₂ Heat →Cu+H₂O                                                   (1.29)

If a substance gains oxygen during a reaction, it is said to be oxidized.

If a substance loses oxygen during a reaction, it is said to be reduced.

During this reaction (1.29), the copper(II) oxide is losing oxygen and is being reduced. The hydrogen is gaining oxygen and is being oxidized.

In other words, one reactant gets oxidized while the other gets reduced during a reaction. Such reactions are called oxidation-reduction reactions or redox reactions.

                        

Some other examples of redox reactions are:

ZnO + C → Zn +CO                                                             (1.31)

MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂                                         (1.32)

In reaction (1.31) carbon is oxidised to CO and ZnO is reduced to Zn.

In reaction (1.32) HCl is oxidised to Cl₂ whereas MnO₂ is reduced to MnCl₂.

From the above examples we can say that if a substance gains oxygen or loses hydrogen during a reaction, it is oxidized. If a substance loses oxygen or gains hydrogen during a reaction, it is reduced.