For a zero-order reaction, which plot yields a straight line?

• A. ln[A] vs time
• B. 1/[A] vs time
• C. [A]^2 vs time
• D. [A] vs time

Answer: (D)

Zero-order kinetics means the reaction rate is constant and does not depend on how much reactant is present. Because d[A]/dt = -k, integrating gives [A] = [A]0 - kt. This is linear in time, so a plot of concentration [A] versus time is a straight line with slope -k and intercept [A]0. The other plots don’t stretch into a straight line for zero order: ln[A] versus time would be ln([A]0 - kt), which isn’t linear; 1/[A] versus time would be 1/([A]0 - kt), also not linear; and [A]^2 versus time would be ([A]0 - kt)^2, a quadratic relationship. So the straight-line plot for zero-order kinetics is [A] versus time.