If the rate law is rate = k [A]^2, and [A] is tripled, by what factor does the rate change?

• A. 3
• B. 6
• C. 9
• D. 27

Answer: (C)

The rate responds to how the concentration appears in the rate law. Since the rate law is rate = k [A]^2, the rate grows with the square of [A]. If [A] is tripled, the rate becomes rate' = k (3[A])^2 = 9 k [A]^2, which is nine times the original rate. The rate constant k stays the same, so the only change is the [A]^2 factor. Therefore, the rate changes by a factor of 9.