CBSE Class 11-science Answered

Step 1:

The skeleton equation is:

As₂S₃ + NO^-₃ + H⁺ → AsO^3-₄ + S + NO₂ + H₂O

Step 2:

Oxidation number of various atoms involved in the reaction is as follows:

3+   2-    +5   2-                +5   2-       0     +4 -4

As₂  S₃ +  N   O^-₃ + H⁺ → As  O^3-₄ +  S  +  N   O₂ + H₂ O

Step 3:

We can balance S by just observation. For Nitrogen oxidation number changes from +5 to +4 so it is reduced. For Arsenic (As) oxidation number changes from +3 to +5 so it is oxidised.

Step 4:

Determine the net increase in oxidation number for the element that is oxidized and the net decrease in oxidation number for the element that is reduced.

For As  +3 to +5                 Net change = +2

For N   +5 to +4                  Net change = -1

Step 5:

Determine a ratio of oxidized to reduced atoms that would yield a net increase in oxidation number equal to the net decrease in oxidation number.

As atoms would yield a net increase in oxidation number of +2 (Two electrons would be lost by one As atom.) and 2 N atom would yield a net decrease of -2. (Two N atoms would gain two electrons.)

Thus the ratio of As atoms to N atoms is 1:2.

Step 6:

To get the ratio identified in Step 5, add coefficients to the formulas which contain the elements whose oxidation number is changing.

As₂S₃ + 2NO^-₃ + H⁺ → AsO^3-₄ + S + 2NO₂ + H₂O

Step 7:

But still the equation is not balanced. Balance the rest of the equation by inspection.

First balance the oxygen. So the balanced equation is,

As₂S₃ + 10NO^-₃ + 4H⁺ → 2AsO^3-₄ + 3S + 10NO₂ + 2H₂O