Why are excess reactants used?

• A. To make sure all the other reactant will be used
• B. To speed up the reaction
• C. To reduce temperature
• D. To purify product

Answer: (A)

Using an excess of one reactant is about ensuring the reaction goes as far as possible toward making product. In any reaction, one reactant is the limiting one—the one that runs out first. If you mix reactants in equal or exact proportions, the reaction can stop as soon as the limiting reactant is used up, leaving some of the other reactant unused. By having more of one reactant than needed, you make sure the other reactant is completely consumed, so you can produce as much product as possible. The extra amount of the excess reactant just remains unused after the reaction ends. It’s not about speeding up the reaction, lowering temperature, or purifying the product.