Advanced Placement (AP) Chemistry Practice Exam

What defines the limiting reactant in a chemical reaction?

1. The substance that is present in excess

2. The reactant that produces the most moles of product

3. The substance that controls the quantity of product that can form

4. The reactant that is consumed first in the reaction

The correct answer is: The substance that controls the quantity of product that can form

The limiting reactant is defined as the substance that controls the quantity of product that can form in a chemical reaction. This is because it is the reactant that will be completely consumed first during the reaction, which in turn limits the extent to which products can be formed. Once the limiting reactant is used up, the reaction cannot proceed further, even if there are other reactants present in excess. Understanding the concept of the limiting reactant is crucial in stoichiometry, as it allows for the accurate calculation of how much product can be generated from given amounts of reactants. Identifying the limiting reactant involves comparing the mole ratio of the reactants used in the reaction to their coefficients in the balanced chemical equation. The reactant that produces fewer moles of product based on this ratio is the one that will limit the production of the desired product. In summary, the limiting reactant is integral to determining how far the reaction can go and how much product can result.