AP Chemistry · Exam guide
AP Chemistry Kinetics Question: Rate Laws from Initial Rates
A kinetics question usually gives a table of initial rates and asks for a rate law. The method is to compare two experiments in which only one concentration changes. Then you find the rate constant and its units. This guide shows the method on one table.
The data
| Experiment | [A] (M) | [B] (M) | Initial rate (M/s) |
|---|---|---|---|
| 1 | 0.10 | 0.10 | 2.0 x 10^-3 |
| 2 | 0.20 | 0.10 | 8.0 x 10^-3 |
| 3 | 0.10 | 0.20 | 4.0 x 10^-3 |
Find each order
- Experiments 1 and 2: [A] doubles, [B] stays the same, and the rate goes up by a factor of 4. So the rate is proportional to [A] squared, and the reaction is second order in A.
- Experiments 1 and 3: [B] doubles, [A] stays the same, and the rate doubles. So the reaction is first order in B.
The rate law is \(\text{rate} = k[A]^2[B]\), and the overall order is 3.
Find k and its units
Use experiment 1: \(k = \dfrac{2.0\times 10^{-3}\ \mathrm{M/s}}{(0.10\ \mathrm{M})^2(0.10\ \mathrm{M})} = 2.0\ \mathrm{M^{-2}\,s^{-1}}\). The units come from solving rate = k times three concentrations for k.
It gives no units. The units tell the grader whether you know the overall order.
It substitutes the data into the rate law and gives units.
Connecting to a mechanism
- The rate law is found from experiment, not from the balanced equation.
- A proposed mechanism must add up to the overall equation, and its slowest step must match the rate law.
- A catalyst changes the mechanism and lowers the activation energy, but it is not used up.
Common mistakes
- Using the coefficients in the balanced equation as the orders.
- Comparing two experiments in which more than one concentration changes.
- Giving k without units, or with the wrong units.
- Confusing the rate of reaction with the rate constant.
Frequently asked questions
Do the orders have to be whole numbers?
On this exam they are usually 0, 1, or 2.
What is a zero-order reactant?
One whose concentration does not change the rate, so it does not appear in the rate law.
How do I know the units of k?
Solve the rate law for k and cancel. Second order overall gives M^-1 s^-1, and third order gives M^-2 s^-1.
Where can I find the official scoring rules?
Exam formats change a little from year to year, so check the current course and exam description on the College Board site before test day.
Keep practicing
Every lesson in the AP Chemistry course ends with practice problems and an Exam focus box that shows how the topic is tested, and every unit has a review with worked answers. These free lessons also build the skills the free-response section tests:
See the AP Chemistry course Free lessons and lesson list
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