Question

5. (four marks) Consider the reaction A + B  2C

A student performs three experiments studying initial concentrations and initial rates. The data is summarized in the table below.

[A] [B] Rate (mol/Ls)
1.60 1.60 2.8 x 10-1
3.20 1.60 5.6 x 10-1
1.60 3.20 2.8 x 10-1

a) What is the order of this reaction with respect to A? Explain how you know.

b) What is the order of this reaction with respect to B? Explain how you know.

Answer 1

Let rate = k[A]^m[B]^n.

a) Order with respect to A: first order (m = 1).
- Compare exp.2 to exp.1: [A] doubles (3.20/1.60 = 2) while [B] is constant, and rate doubles (5.6×10^-1 / 2.8×10^-1 = 2). So 2 = 2^m ⇒ m = 1.

b) Order with respect to B: zero order (n = 0).
- Compare exp.3 to exp.1: [B] doubles (3.20/1.60 = 2) while [A] is constant, but the rate stays the same (2.8×10^-1 / 2.8×10^-1 = 1). So 1 = 2^n ⇒ n = 0.

Thus rate = k[A].