# What is an example of a rate law practice problem?

Jan 30, 2015

Here's an example that uses the method of initial rates.

Determine the rate law for the reaction A + 2B →C + 2D.

Expt.·····[A]₀/mol·L⁻¹·····[B]₀/mol·L⁻¹·····Init. Rate /mol·L⁻¹min⁻¹
···1··············0.10···················0.10·······················3.0 x 10⁻⁴
···2·············0.30··················0.30·····················9.0 x 10⁻⁴
···3·············0.10···················0.30······················3.0 x 10⁻⁴

Solution

The rate law has the form "rate" = k["A"]^m["B"]^n.

In experiments 1 and 3, we have doubled [B]₀ and kept [A]₀ constant.

Doubling [B]₀ did not change the rate, so the order is zero with respect to B ($n = 0$).

In experiments 3 and 2, we have tripled [A]₀ and kept [B]₀ constant.

Tripling [A]₀ tripled the rate, so the reaction is first order with respect to A ($m = 1$).

The rate law is "rate" = k["A"]^1["B"]^0 = k["A"].

The video below gives another example of using the method of initial rates to determine a rate law.