Excess reactant is

• A. The reactant that remains after the reaction
• B. The reactant that is completely consumed
• C. The reactant present in minimal amounts
• D. The reactant that remains in excess

Answer: (D)

In a reaction, one reactant determines how far the reaction can go—the one that runs out first is the limiting reagent. The other reactants are often present in larger amounts and aren’t completely used up. The excess reactant is simply the reactant that remains unconsumed after the reaction has proceeded to the extent allowed by the limiting reagent.

For example, in a reaction where one molecule of A reacts with two molecules of B to form AB2, if you start with 1 mole of A and 3 moles of B, A is consumed completely (it’s limiting), using up 2 moles of B. That leaves 1 mole of B unreacted. That leftover B is the excess reactant.