In a reaction, the reactant that limits the extent of the reaction is known as the

• A. Excess reactant
• B. Catalyst
• C. Equilibrium
• D. Limiting reactant

Answer: (D)

The extent of a reaction is governed by the reactant that is used up first—the limiting reactant. When you have more than one reactant, you can only form as much product as the limiting reagent allows based on the stoichiometric ratios. Whichever reactant would be exhausted first sets the maximum amount of product; once that reagent is gone, the reaction cannot proceed further even if other reactants remain in excess. For a generic reaction aA + bB → products, starting with certain amounts, the maximum product corresponds to the smaller value of n_A/a and n_B/b, identifying the limiting reactant. The terms excess reactant, catalyst, and equilibrium describe different ideas: excess means there is more of a reactant than needed, a catalyst speeds the reaction without being consumed, and equilibrium is the balance point between forward and reverse reactions.