2.2.10 (HL)—Reaction order
- Syllabus
- First assessment 2025
- Objective
- 2.2.10
- Level
- HL
Reaction Order
HL onlyoverallorder=x+yinrate=k[A]x[B]y
The exponent gives order with respect to that reactant. Use rate factors to identify zero, first or second order, then match the order to the concentration–time or rate–concentration graph.
Use multiple representations as cross-checks: zero-order rate is independent of concentration and [A] falls linearly; first-order rate is proportional to [A] and gives exponential decay; second-order rate curves upward on a rate-versus-concentration plot. Do not infer order from one balanced equation.
Determining Overall Order
HL onlyAssessment in practice
Representative question
Question 1
[Maximum number: 1]
Compounds P and Q were mixed together at various concentrations and the initial rate of each reaction was measured.
| Experiment | [P] mol dm −3 | [Q] mol dm −3 | Initial rate of reaction (mol dm −3 s−1 ) |
|---|---|---|---|
| 1 | 0.20 | 0.15 | 0.50 |
| 2 | 0.10 | 0.15 | 0.25 |
| 3 | 0.20 | 0.30 | 1.00 |
What are the orders of reaction with respect to P and Q ?
Order with respect to P
Order with respect to Q
1
1
1
2
2
1
1
0
A
Rate and Mechanisms Summary
Retrieve the route: measure a tangent rate, explain effective collisions, map rate factors, read Ea and energy profiles, evaluate mechanisms, determine molecularity and orders, calculate k, then use Arrhenius gradient and intercept for Ea and A.
Check tangent versus average slope, energy versus orientation, barrier labels, intermediate versus transition state, one-variable trial comparisons, order-dependent units, kelvin temperature and the signs of gradient and Ea.