Define the limiting reagent in a chemical reaction and give an example.

• A. The reactant consumed first that limits product formation; e.g., in A + B → AB with 2 moles A and 1 mole B, B is limiting
• B. The reactant present in the smallest amount
• C. The reactant that speeds up the reaction
• D. The product that forms last

Answer: (A)

Limiting reagent is the reactant that is consumed first, which sets the maximum amount of product that can be formed given the amounts and the ratios in the balanced equation. Once this reactant is used up, the reaction stops even if other reactants are still present in excess. For example, in the reaction A + B → AB, with 2 moles of A and 1 mole of B, you need a 1:1 ratio of A to B to make AB. Since there is only 1 mole of B, only 1 mole of AB can be produced, using 1 mole of A and leaving 1 mole of A unreacted. Thus the reactant present in the smaller proportion relative to its required amount becomes the limiting reagent. It’s not simply about which reactant starts with the smaller amount, because the limiting reagent depends on the amounts and the stoichiometric coefficients, not just the initial quantities.