In the reaction P4(s) + 10 Cl2(g) → 4PCl5(s), which is the reducing agent?

In the given reaction, phosphorus (P4) undergoes oxidation while chlorine (Cl2) is reduced. To identify the reducing agent, we need to consider the definitions of oxidation and reduction in terms of electron transfer.

A reducing agent is a substance that donates electrons to another species, causing that species to be reduced. In this specific reaction, phosphorus starts in its elemental form (P4), which is assigned an oxidation state of 0. In forming PCl5, the oxidation state of phosphorus increases to +5, indicating that phosphorus loses electrons. Therefore, phosphorus functions as the reducing agent because it donates electrons to chlorine, allowing it to undergo reduction.

Chlorine, which is in its elemental form as Cl2, undergoes reduction from an oxidation state of 0 to -1 in PCl5. Since phosphorus is what loses electrons, it is inherently the reducing agent in this context.

The other choices either represent products of the reaction or other oxidation states that are not relevant to the role of the reducing agent in this particular reaction.