Question

The rate of reaction, \[\ce{A + B -> Products}\], is given by the equation, r = k [A] [B]. If B is taken in large excess, the order of reaction would be ______.

Options

• 2

• 1

• 0

• unpredictable

Answer 1

The rate of reaction, \[\ce{A + B -> Products}\], is given by the equation, r = k [A] [B]. If B is taken in large excess, the order of reaction would be 1.

Explanation:

The given rate law is r = k [A] [B]

This shows the reaction is second order overall (1 in A + 1 in B).

If [B] is very large and constant, then its concentration does not change appreciably during the course of the reaction.

So, we can combine k[B] as a new constant, k'.

r = k [A] [B] ≈ k'[A]

Now, the reaction behaves like a pseudo-first order reaction with respect to A.
So, the effective order = 1.