CBSE Class 11-science Answered

In most situations of balancing an equation, you are not told whether the reaction is redox or not.

In these circumstances, you can use a procedure called the oxidation number method.

The skeleton equation is:

P₄ + HNO₃  → H₃PO₄ + H₂O + NO₂

Step 2

The oxidation number of various atoms involved in the reaction.

 _{0       +1 +5 -2           +1 +5 -2        +1  -2       +4  -2}            

P₄     + HNO₃     →    H₃PO₄   +   H₂O   +   NO₂

Step 3

For N oxidation number changes from +5 to +4 so it is reduced. For P oxidation number changes from 0 to +5 so it is oxidized. No change in oxidation number of O.

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 P  0 to +5                                  Net change = +5     ......... Oxidation

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

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.

P atoms would yield a net increase in oxidation number of +5. (five electrons would be lost by four P atoms.). 1 N atom would yield a net decrease of -1. (One N atom would gain one electron.)

Thus the ratio of N atoms to C atoms is 5:1. But as we have P₄ as a reactant hence the ratio will be changed (5 ×4)  to 20:1

Step 6

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

and we will get the balanced equation,

P₄ +20 HNO₃  → 4H₃PO₄ + 20H₂O + 4NO₂