What is a Limiting Reactant? When you see a chemical equation in a textbook, it looks like the reactants are completely used up and all made into product. But in the real world, you usually have some of one or more reactants left over. The reagent that is used up first is known as the “Limiting Reagent” (also known as the “Limiting Reactant.”) This is especially important if you are working with a very expensive chemical that you can't afford to waste.