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,

$Br_2+SO_2+2H_2O\rightarrow 2Br^-+SO_4^{2-}+4H^+$