For a reversible reaction A ⇌ B with forward rate kf[A] and backward rate kb[B], which statement is true at equilibrium?

• A. [B]eq/[A]eq = kb/kf
• B. [A]eq = [B]eq
• C. kf = kb
• D. kf[A]eq = kb[B]eq

Answer: (D)

At equilibrium, the forward and backward steps occur at the same rate, so there’s no net change in the concentrations. For A ⇌ B, the forward rate is kf[A] and the backward rate is kb[B]. When equilibrium is reached, kf[A]eq = kb[B]eq. This rate-balance condition is the precise statement of equilibrium for a reversible reaction.

From this equality you can also see that [B]eq/[A]eq = kf/kb, which explains why the ratio is the forward-to-backward rate constant ratio (not kb/kf). Equal concentrations would only happen if kf equals kb (giving an equilibrium constant of 1).