Question

What is the sum of the smallest possible integer coefficients when the skeletal redox reaction below is correctly balanced? MnO4–(aq) + H2SO3(aq) → Mn2+(aq) + SO42–(aq) (acidic solution) [Note that H2O(l) and H+(aq) may have to be added where necessary to balance the equation and should be included when you calculate the sum.]

Answer 1

Mn in MnO4- has oxidation state of +7
Mn in Mn+2 has oxidation state of +2
So, Mn in MnO4- is reduced to Mn+2
This means MnO4- is oxidising agent

S in H2SO3 has oxidation state of +4
S in SO4-2 has oxidation state of +6
So, S in H2SO3 is oxidised to SO4-2
This means H2SO3 is reducing agent





Reduction half cell:
MnO4- + 5e- --> Mn+2
Oxidation half cell:
H2SO3 --> SO4-2 + 2e-
Balance number of electrons to be same in both half reactions

Reduction half cell:
2 MnO4- + 10e- --> 2 Mn+2
Oxidation half cell:
5 H2SO3 --> 5 SO4-2 + 10e-
10 electrons are being transferred in the reaction

Lets combine both the reactions.
2 MnO4- + 5 H2SO3 --> 2 Mn+2 + 5 SO4-2
Balance Oxygen by adding water

2 MnO4- + 5 H2SO3 --> 2 Mn+2 + 5 SO4-2 + 3 H2O

Balance Hydrogen by adding H+
2 MnO4- + 5 H2SO3 --> 2 Mn+2 + 5 SO4-2 + 3 H2O + 4 H+
This is balanced chemical equation in acidic medium

Sum of coefficient = 2 + 5 + 2 + 5 + 3 + 4
= 21
Answer: 21