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

To determine the reducing agent in the given reaction, it's essential to analyze the oxidation states of the elements involved. In the reaction, phosphorus (P) starts as elemental phosphorus with an oxidation state of 0 in P4 and ultimately ends up in phosphorus pentachloride (PCl5) with an oxidation state of +5.

During the reaction, phosphorus is undergoing oxidation because it is losing electrons and increasing its oxidation state from 0 to +5. Since the substance that is oxidized is considered the reducing agent, phosphorus qualifies as the reducing agent in this scenario.

Chlorine, on the other hand, is being reduced because it is gaining electrons and changing its oxidation state from 0 in Cl2 to -1 in PCl5. This means that while phosphorus is facilitating the reduction of chlorine, it is also losing electrons, thereby acting as the reducing agent itself.

Thus, phosphorus is correctly identified as the reducing agent for this reaction.