question_answer

Phosphorus acid can act both as oxidising agent as well as reducing agent while phosphoric acid is only an oxidising agent. Explain.

Answer:

                The behaviour is linked with the oxidation state of phosphorus atom in the two acids. In phosphorus acid \[({{H}_{3}}P{{O}_{3}})\] the oxidation state of P is + 3. It is in a position to increase its oxidation state (acts as reducing agent) as well as decrease it (acts as oxidising agent). The oxidation state of phosphorus in phosphoric acid \[({{H}_{3}}P{{O}_{3}})\] is + 5. Since it cannot increase it beyond 5, it cannot act as a reducing agent. However, it can behave as oxidising agent by decreasing its oxidation state.