Problem 71 - Chemistry The Central Science

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?

Step-by-Step Solution

Answer

(a) A limiting reactant is the reactant that is completely consumed in a chemical reaction and determines the maximum amount of product that can be formed, while an excess reactant is the reactant that is not completely consumed and is present in excess after the reaction is complete. (b) The amount of products formed is determined by the limiting reactant because it dictates the extent to which the reaction can occur, and no more product can be formed once the limiting reactant is used up. (c) Determining the limiting reactant should be based on the initial number of moles, instead of mass, because moles are directly related to the proportion of molecules or atoms in a balanced chemical equation, and stoichiometry is based on mole ratios.

Step 1: (a) Definition of Limiting and Excess Reactant

A limiting reactant is the reactant that is completely consumed in a chemical reaction and determines the maximum amount of product that can be formed. In contrast, an excess reactant is the reactant that is not completely consumed and is present in excess after the reaction is complete.

Step 2: (b) Amounts of Products Determined by Limiting Reactant

The amount of products formed in a chemical reaction is determined by the amount of the limiting reactant because it is the substance that gets completely consumed. Once the limiting reactant is used up, no more product can be formed, regardless of the amount of the other reactants present. This is because a chemical reaction will not proceed if one or more reactants are unavailable in the required stoichiometric amounts. In other words, the limiting reactant controls the amount of products formed because it dictates the extent to which the reaction can occur.

Step 3: (c) Basing the Limiting Reactant Choice on Initial Moles, not Mass

In order to determine which reactant is the limiting reactant in a chemical reaction, it is important to focus on the initial number of moles of each reactant involved rather than their initial masses. This is because moles, not mass, are directly related to the proportion of molecules or atoms in a balanced chemical equation. The stoichiometry of a chemical reaction is based on mole ratios, and thus comparing the initial number of moles of each reactant enables us to determine which reactant will be consumed first, resulting in the limiting reactant. By comparing the initial moles of reactants, it is possible to determine the reactant that will limit the amount of product formed during the reaction. This is essential in accurately calculating theoretical yields and ensuring efficient use of resources in various chemical processes.