Final answer:
The limiting reactant controls the amount of product formed in a chemical reaction while the excess reactant is the reactant that is left over after the reaction is complete.
Step-by-step explanation:
The limiting reactant (or limiting reagent) is the reactant that determines the amount of product that can be formed in a chemical reaction. The reaction proceeds until the limiting reactant is completely used up. In our example above, the H₂ is the limiting reactant. The excess reactant (or excess reagent) is the reactant that is initially present in a greater amount than will eventually be reacted. In other words, there is always excess reactant left over after the reaction is complete. In the above example, the N₂ is the excess reactant.