For a first-order reaction, what happens to the rate when the concentration doubles?

• A. The rate quadruples
• B. The rate doubles
• C. The rate halves
• D. The rate is independent of concentration

Answer: (B)

In a first-order reaction, the rate depends directly on the concentration of the reactant, described by rate = k[A]. The rate constant k is fixed at a given temperature, so increasing [A] scales the rate proportionally. If the concentration doubles, the rate becomes rate = k(2[A]) = 2k[A], which is twice the original rate. This linear relationship means doubling the concentration always doubles the rate. (Zero-order would keep the rate the same regardless of [A], and second-order would make the rate ∝ [A]^2, giving a quadruple increase when [A] doubles.)