Question

Balance the following redox reaction if it occurs in acidic solution. What are the coefficients in front of Zn and H⁺ in the balanced reaction?

Zn²⁺ (aq) + NH₄⁺ (aq) ? Zn (s) + NO₃

Answer 1

There are several steps to balance redox reactions via the half-reaction

Split the equation into half reactions.
balance the main atoms first except O and H
balance O by adding water
balance H by adding H+
balance the charge by adding electrons, add the two equations and cancel the similar species to get the final one.

The two half reactions will be:

Zn Zn+2
NO3- NH4+


Zn Zn+2 + 2e- (Acting as reductant and itself is gettinf oxidized)....(a)

Now for the nitrogen half reaction. N atoms are already balanced but we need to balance O and H.So the equation is as follows.

NO3- + 10H+ NH4+ + 3H2O

Now still charge is not same on both sides, the charge on the left is +9. On the right, the total charge is +1 due to the amonium ion. since the charge on the left is higher than the charge on the right, we balance it by adding 8 e- to the left side and we get the following equation

8e- + NO3- + 10H+ NH4+ + 3H2O........(b)

In order to cancel the electrons, we need to multiply the equation (a) by 4 to get the following equation.

4 Zn   4 Zn+2 + 8e-.............(c)

Now add the equation (b) and (c) to get the final balanced redox equation

8e- + NO3- + 10H+ + 4Zn NH4+ + 3H2O + 4Zn+2 + 8e-

the electrons cancel and the final result is

NO3- + 10H+ + 4Zn NH4+ + 3H2O + 4Zn+2

The number of each atom is the same on both sides and that the total charge on either side is the same.