What does 'excess' mean in a chemical reaction?

• A. More than what you need
• B. Less than what you need
• C. Exactly the amount you need
• D. The catalyst is used up.

Answer: (A)

Excess means having more of one reactant than is required by the reaction to go to completion. In a reaction, one reactant is often used up first—the limiting reactant—while any extra amount of the other reactant remains unspent. For example, if A and B react in a 1:1 ratio and you start with 5 units of A and 3 units of B, all of B is used, and 2 units of A are left over. That leftover A is in excess. The idea is about what’s left over after the reaction runs as far as it can. The other ideas don’t fit: less than needed would prevent the reaction from going to completion, exactly the right amount would leave nothing unused, and a catalyst being used up describes a catalyst, not the concept of excess.