Problem 34 - Chemistry An Atoms First Approach

Question

What does it mean to say a reactant is present "in excess" in a process? Can the limiting reactant be present in excess? Does the presence of an excess of a reactant affect the mass of products expected for a reaction?

Step-by-Step Solution

Answer

A reactant is present "in excess" if it is present in a greater amount than required to completely react with another reactant. The limiting reactant cannot be present in excess, as its availability limits the reaction's progress. The presence of an excess reactant does not affect the mass of products expected since the mass of products is determined by the stoichiometry of the balanced chemical equation and the limiting reactant.

Step 1: Understanding "Excess" Reactants

In a chemical process, when a reactant is present in a greater amount than is required to completely react with another reactant, it is said to be present "in excess." This extra amount ensures that one reactant is completely consumed, leaving no unreacted molecules.

Step 2: Limiting Reactant and Excess

The limiting reactant is the reactant that is completely consumed and determines the amount of product produced in a chemical reaction. It cannot be present in excess, as its availability limits the reaction's progress.

Step 3: Effect of Excess Reactants on the Mass of Products

The presence of an excess reactant does not affect the mass of products expected in a reaction. The mass of products is determined by the stoichiometry of the balanced chemical equation and the limiting reactant. Since the limiting reactant determines how much product will be produced, adding more of the excess reactant will not result in the formation of more products.