Q.Can PCl5 act as an oxidising as well as a reducing agent? Justify.

Step 1: Oxidation state of P in $PCl_5$.

Phosphorus in $PCl_5$ is in the $+5$ oxidation state — the maximum (group) oxidation state possible for phosphorus (Group 15, so maximum = $+5$).

Step 2: Condition to act as an oxidising agent.

A species acts as an oxidising agent by itself being reduced, i.e. its central atom's oxidation state must be able to decrease. Since P is at $+5$ in $PCl_5$, it can indeed be reduced (e.g. to $+3$ in $PCl_3$), so $PCl_5$ readily acts as an oxidising agent:

(For example, $PCl_5$ oxidises $H_2S$: $PCl_5 + H_2S \rightarrow PCl_3 + S + 2HCl$.)

Step 3: Condition to act as a reducing agent.

A species acts as a reducing agent by being oxidised itself, i.e. its central atom's oxidation state must be able to increase. Since $+5$ is already phosphorus's highest attainable oxidation state (there is no $+7$ or higher state for P), phosphorus in $PCl_5$ cannot be oxidised any further.

Step 4: Conclusion. …

Identify the oxidation state of P in PCl5 (+5, the group maximum), then reason that since it can be reduced (going lower) but not oxidised (no higher state e …

• The direct "yes/no" answer must be stated clearly — some students describe PCl5's reactions without ever committing to the fact that it CANNOT act as a reducing agent.
• Do not confuse this with PCl₃ (P = +3), which genuinely CAN act as both an oxidising and reducing agent since +3 is an intermediate oxidation state for P. …