Define reduction in terms of (i) electron transfer, (ii) change in oxidation number - Leaving Cert Chemistry - Question b - 2016

Reduction can also be defined as a decrease in oxidation number. When a substance is reduced, its oxidation state decreases, indicating a gain of electrons.

In the equation $ClO^- + SO_2^{2-} + OH^- \rightarrow Cl^- + SO_4^{2-} + H^+$, the reducing agent is $SO_2^{2-}$. This species donates electrons to the $ClO^-$, leading to the reduction of chlorine.

In the same equation, the oxidizing agent is $ClO^-$. It accepts electrons from the $SO_2^{2-}$, thus causing the oxidation of sulfur.

To balance the equation $4ClO^- + SO_2^{2-} + 4OH^- \rightarrow 4Cl^- + 2SO_4^{2-} + 2H^+$, we first determine the oxidation states:

• Chlorine in $ClO^-$ is +1, and in $Cl^-$ is -1.
• Sulfur in $SO_2^{2-}$ is +4, and in $SO_4^{2-}$ is +6.

The balanced equation maintains conservation of mass and charge, ensuring that the number of each type of atom and the charges are equal on both sides.