Question

Balance each of the following equations according to the half- reaction method:

(a) MnO4- + NO2- --> MnO2 + NO3- (in base)
(b) MnO42- --> MnO4- +MnO2 (in base)
(c) Br2 + SO2 --> Br- + SO42- (in acid)

Answer 1

Explanation :

Redox reaction or Oxidation-reduction reaction : It is defined as the reaction in which the oxidation and reduction reaction takes place simultaneously.

Oxidation reaction : It is defined as the reaction in which a substance looses its electrons. In this, oxidation state of an element increases. Or we can say that in oxidation, the loss of electrons takes place.

Reduction reaction : It is defined as the reaction in which a substance gains electrons. In this, oxidation state of an element decreases. Or we can say that in reduction, the gain of electrons takes place.

(a) The given chemical reaction is :

`MnO_4^-+NO_2^-\rightarrow MnO_2+NO_3^-`

The oxidation-reduction half reaction will be :

Oxidation :
`NO_2^-+2OH^-\rightarrow NO_3^-+H_2O+2e^-`

Reduction :
`MnO_4^-+2H_2O+3e^-\rightarrow MnO_2+4OH^-`

In order to balance the electrons, we multiply the oxidation reaction by 3 and reduction reaction by 2 then added both equation, we get the balanced redox reaction.

The balanced chemical equation in a basic solution will be,

`2MnO_4^-+2H_2O+3NO_2^-\rightarrow 3MnO_2+6OH^-+2NO_3^-`

(b) The given chemical reaction is :

`MnO_4^(2-)\rightarrow MnO_4^-+MnO_2`

The oxidation-reduction half reaction will be :

Oxidation :
`MnO_4^(2-)\rightarrow MnO_4^-+1e^-`

Reduction :
`MnO_4^(2-)+2H_2O+2e^-\rightarrow MnO_2+4OH^-`

In order to balance the electrons, we multiply the oxidation reaction by 2 and then added both equation, we get the balanced redox reaction.

The balanced chemical equation in a basic solution will be,

`2MnO_4^(2-)+MnO_4^(2-)+2H_2O\rightarrow 2MnO_4^-+MnO_2+4OH^-`

(c) The given chemical reaction is :

`Br_2+SO_2\rightarrow Br^-+SO_4^(2-)`

The oxidation-reduction half reaction will be :

Oxidation :
`SO_2+2H_2O\rightarrow SO_4^(2-)+4H^++2e^-`

Reduction :
`Br_2+2e^-\rightarrow 2Br^-`

The electrons in oxidation and reduction reaction are same. Now add both the equation, we get the balanced redox reaction.

The balanced chemical equation in a acidic solution will be,

`Br_2+SO_2+2H_2O\rightarrow 2Br^-+SO_4^(2-)+4H^+`