What is the purpose of excess reactants?

• A. To make sure all the other reactant will be used
• B. To ensure the limiting reactant is used up
• C. To increase the rate of reaction
• D. To reduce costs

Answer: (A)

Having excess of one reactant is used to ensure the other reactant (the limiting reactant) is used up. In any reaction, the amount of product formed is limited by whichever reactant runs out first. By providing extra of one reactant, you guarantee that the reaction goes as far as the limiting reactant allows, leaving any excess reagent untouched. This is why the purpose is described as making sure the other reactant is used up. Increasing rate or cutting costs aren’t the primary goals here; excess may even waste material, since the extra reactant remains unreacted after the reaction ends.