Chapter 18, Problem 18.1P

a.

Interpretation Introduction

To determine:

The unbalanced half reactions for the given net ionic equation.

  ${\text{MnO}}_{\text{4}}^{-}{\text{(aq) + IO}}_{\text{3}}^{-}\text{(aq) }\to {\text{MnO}}_{\text{2}}{\text{(s) + IO}}_{\text{4}}^{-}\text{(aq)}$

Explanation of Solution

As per the given reaction the oxidation number on Mn decreases from +7 in ${\text{MnO}}_{\text{4}}^{-}$ to +4 in ${\text{MnO}}_{\text{2}}$. In other words, Mn gains three electrons and gets reduced. In contrast, the oxidation number on I increases from +5 in ${\text{IO}}_3^{-}$ to +7 in ${\text{IO}}_{\text{4}}^{-}$. Here, iodine (I) loses two electrons and gets oxidised. Therefore, the two half reactions are as follows:
Reduction half-reaction: ${\text{MnO}}_{\text{4}}^{-}{\text{(aq) + 3e}}^{-}\to {\text{MnO}}_{\text{2}}\text{(s) }$

Oxidation half-reaction: ${\text{IO}}_{\text{3}}^{-}\text{(aq) }\to {\text{ IO}}_{\text{4}}^{-}{\text{(aq) + 2e}}^{-}$

b.

Interpretation Introduction

To determine:

The unbalanced half reactions for the given net ionic equation.

  ${\text{NO}}_3^{-}{\text{(aq) + SO}}_2^{}\text{(aq) }\to {\text{ SO}}_{\text{4}}^{2-}{\text{(aq) + NO}}_2^{}\text{(g)}$

Explanation of Solution

As per the given reaction the oxidation number on nitrogen (N) decreases from +5 in ${\text{NO}}_3^{-}$ to +4 in ${\text{NO}}_{\text{2}}$. In other words, N gains one electron and gets reduced. In contrast, the oxidation number on sulfur (S) increases from +4 in ${\text{SO}}_2^{}$ to +6 in ${\text{SO}}_{\text{4}}^{2-}$. Here, sulfur loses two electrons and gets oxidised. Therefore, the two half reactions are as follows:
Reduction half-reaction: ${\text{NO}}_3^{-}{\text{(aq) + e}}^{-}\to {\text{ NO}}_2^{}\text{(g)}$

Oxidation half-reaction: ${\text{ SO}}_2^{}\text{(aq) }\to {\text{ SO}}_{\text{4}}^{2-}{\text{(aq) + 2e}}^{-}$

Conclusion

Therefore, in any given redox reaction, the reduction half reaction proceeds with a gain of one or more electrons. In contrast, the oxidation half reaction proceeds with a loss of one or more electrons.