What is the sign of the slope m for a zero-order plot of concentration versus time?

• A. m > 0
• B. m < 0
• C. m = 0
• D. m = any sign depending on k

Answer: (B)

Concentration versus time for a zero-order process changes linearly because the rate is constant and does not depend on how much reactant remains. The integrated rate law is [A] = [A]0 − k t, so the slope of the plot is d[A]/dt = −k. Since k is a positive constant for a decay process, the slope is negative. That’s why the sign of the slope is negative. If the plot showed concentration increasing with time, or staying constant, those would not describe a typical zero-order decay.