Define rate law and order of a reaction. Explain how the order is found by the initial rate method, with an example.

1. 1 · Define the rate law. Rate = k[A]ˣ[B]ʸ: rate in terms of reactant concentrations raised to powers found by experiment.
2. 2 · Define order. Order = x + y, the sum of the powers; it may be zero or a fraction.
3. 3 · Describe the method. Measure the initial rate in runs where only one reactant's concentration changes; compare runs to find each power.
4. 4 · Work an example. Doubling [A] doubles the rate: 2ˣ = 2, x = 1. Doubling [B] makes it 4 times: 2ʸ = 4, y = 2. Rate = k[A][B]², order 3.
5. 5 · Give the unit of k. For order 3: mol⁻² L² s⁻¹.
6. 6 · Close in one line. So the order cannot be read from the balanced equation; it comes from experiment.

Scoring tip: Write the ratio of two experiments in full (rate₂/rate₁ = 2ˣ) so the examiner sees the method, not just the answer.