If [A] doubles in a first-order reaction, what happens to the rate?

• A. Doubling [A] doubles the rate
• B. Doubling [A] quadruples the rate
• C. Doubling [A] halves the rate
• D. Doubling [A] leaves the rate unchanged

Answer: (A)

In first-order kinetics, the rate is directly proportional to the concentration of the reactant: rate = k[A]. That means if the concentration of A doubles, the rate doubles as well, since the proportionality constant k stays the same. This linear relationship is what defines first-order behavior. If the reaction were second-order, doubling [A] would quadruple the rate, and if it were zero-order, doubling [A] would leave the rate unchanged.