Question

(a) Define the terms limiting reactant and excess reactant. (b) Why are the amounts of products formed in a reaction determined only by the amount of the limiting reactant? (c) Why should you base your choice of which compound is the limiting reactant on its number of initial moles, not on its initial mass in grams?

Short answer

(a) Limiting reactant is the substance that gets fully consumed in a chemical reaction and determines the amount of product formed. Excess reactant is a reactant that remains after the reaction is complete, as it is present in a larger quantity than required. (b) The amount of products formed in a reaction is determined by the limiting reactant because once it is completely consumed, the reaction stops and no further products can be formed. (c) The choice of limiting reactant should be based on its number of initial moles, not mass in grams, because chemical reactions progress based on the number of particles (moles) involved, not their mass. The stoichiometric ratios in balanced chemical equations represent the molar ratios needed for the reaction.

Step-by-step solution

:(a) Definition of Limiting reactant and Excess reactant

: Limiting reactant: The limiting reactant, sometimes also referred to as the limiting reagent, is a substance that gets fully consumed in a chemical reaction and determines the amount of product that can be formed. It is a reactant that produces the least amount of product according to the balanced chemical equation. Excess reactant: The excess reactant, on the other hand, is a reactant that remains after the reaction is complete because not all of it gets consumed during the reaction. It is present in a larger quantity than required according to the balanced chemical equation.

:(b) Amount of products formed dependent on the limiting reactant

: The amount of products formed in a reaction is determined only by the amount of the limiting reactant because it is the reactant that gets fully consumed first in the reaction. As the limiting reactant affects the reaction completion, it determines the maximum amount of product that can be formed. Once it is completely consumed, the reaction stops and no further products can be formed. Excess reactant cannot contribute to forming more products because there is not enough limiting reactant left to react with.

:(c) Choosing the limiting reactant based on initial moles, not initial mass

: The choice of which compound is the limiting reactant should be based on its number of initial moles, not on its initial mass in grams because the chemical reaction progresses based on the number of particles (moles) involved in the reaction, not their mass. The stoichiometric ratios in balanced chemical equations represent the molar ratios, which explain how many moles of a reactant are required to form a certain number of moles of products in the reaction. In other words, the limiting reactant is the reactant that runs out first and controls the amount of the final product formed. Since chemical reactions consume reactants in certain molar ratios, it is essential to look at the initial moles of reactants to determine which one will limit the amount of products formed. If we base our choice on the mass in grams without considering molar ratios, we might miscalculate which reactant will get consumed first and thus not correctly identify the limiting reactant.