📘 IB Understanding
The order of a reaction with respect to a reactant is the power to which its concentration is raised in the rate equation. The overall order is the sum of the individual orders.
What Is Reaction Order?
For the rate law \(\text{Rate} = k[A]^x[B]^y\):
- The order with respect to A = \(x\)
- The order with respect to B = \(y\)
- The overall order = \(x + y\)
Summary of Orders
| Order | Rate Law | Effect of Doubling [A] | Units of k |
|---|---|---|---|
| Zero (0) | Rate = k | No effect on rate | mol dm⁻³ s⁻¹ |
| First (1) | Rate = k[A] | Rate doubles (×2) | s⁻¹ |
| Second (2) | Rate = k[A]² | Rate quadruples (×4) | mol⁻¹ dm³ s⁻¹ |
Worked Example
Reaction: NO₂(g) + CO(g) → NO(g) + CO₂(g)
Experimental rate law: Rate = k[NO₂]²[CO]⁰
This simplifies to Rate = k[NO₂]²
- Second order with respect to NO₂
- Zero order with respect to CO
- Overall order = 2 + 0 = 2
📋 Exam Tip
If a reactant is zero order, it is not involved in the rate-determining step. Its concentration has no effect on the rate. Only integer values (0, 1, 2) are assessed in the IB exams.