Acids, equilibrium & kinetics · HL
Rate order (HL)
Determine reaction order from two concentration–rate trials.
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Use decimal dots in datasets. Single values also accept decimal commas and scientific notation such as 1.2e-3 or 1.2 × 10⁻³.
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Formula & example
The relationship
What the variables mean
| Symbol | Meaning |
|---|---|
| Order with respect to the changing reactant. |
Worked example
- Concentration 1
- 0.100
- Rate 1
- 0.0200
- Concentration 2
- 0.200
- Rate 2
- 0.0800
order n = 2
Follow the example steps
- r₂/r₁ = 0.08/0.02 = 4
- c₂/c₁ = 0.2/0.1 = 2
- n = ln(r₂/r₁)/ln(c₂/c₁) = 2
- k = r₁/c₁ⁿ = 0.02 / (0.1)^(2) = 2
Common mistakes
Determine the order from a concentration ratio and a rate ratio.
Incorrect approach
c₂/c₁ = 2, rate₂/rate₁ = 8: order = 8/2 = 4
Correct approach
order = ln(8)/ln(2) = 3
More checks before you submit
- If c₂/c₁ = 1, the two trials cannot determine this order.
Method notes
The order obtained from experimental data need not equal a stoichiometric coefficient. For a multivariable rate law, isolate one reactant at a time with the others effectively constant.