Excess reactants, also known as excess reagents, are the reactants in a chemical reaction that are present in quantities greater than required for complete reaction with the limiting reactant. While the limiting reactant determines the maximum amount of product that can be formed, the excess reactants remain unreacted after the limiting reactant is completely consumed.

Excess reactants are typically used to ensure that the limiting reactant is fully consumed, maximizing the yield of the desired product. They provide an excess supply of reactant, allowing the reaction to proceed to completion even if the stoichiometric ratios are not perfectly balanced.

The presence of excess reactants can affect the efficiency and cost-effectiveness of a chemical reaction. It is important to consider the excess reactants when calculating the actual yield of the reaction, as the excess reactant may still be present in the final mixture. However, excess reactants do not contribute to the stoichiometry of the reaction beyond their role in ensuring the complete consumption of the limiting reactant.

Managing and controlling the use of excess reactants is crucial in optimizing reaction conditions, minimizing waste, and maximizing the yield of the desired product. By carefully selecting the appropriate amounts of reactants, chemists can ensure efficient use of resources and reduce the cost and environmental impact of chemical processes.