Final answer:
Excess reagents in a chemical reaction are the reactants present in greater amounts than necessary to fully react with the limiting reactant, remaining partially unreacted after the reaction completes.
Step-by-step explanation:
Excess reagents are the reactants present in quantities greater than necessary to react with the quantity of the limiting reactant (LR).
In a chemical reaction, the limiting reactant is the one that is completely used up first, restricting the amount of product that can be formed. In contrast, the excess reagent is the substance that is initially present in greater amounts than needed and is not entirely consumed when the reaction reaches completion. For example, if we react hydrogen gas (H₂) with nitrogen gas (N₂) to form ammonia (NH₃), and H₂ is the limiting reactant, any remaining N₂ after the reaction would be considered the excess reagent. It is critical for evaluating the conditions of a chemical process to determine which reactant is in excess, as this knowledge influences the calculation of the theoretical yield and informs on potential waste or additional costs associated with unreacted chemicals.