A level Chemistry · Edexcel 9CH0 · Topic 16A
Rate equations and orders
Key points
- Rate equation: rate = [A]ᵐ[B]ⁿ. Orders and are found experimentally.
- Zero order: rate unaffected by concentration; concentration–time graph is a straight line falling.
- First order: rate doubles when concentration doubles; constant half-life, .
- Second order: doubling concentration multiplies rate by four; half-life increases as concentration falls.
- Rate–concentration graphs: horizontal line (0), straight line through the origin (1), curve upwards (2).
- Initial rates method: change one concentration at a time, find the rate from the tangent at or a clock reaction (rate ∝ 1/time).
- Units of depend on overall order, e.g. s⁻¹ (first order), dm³ mol⁻¹ s⁻¹ (second order).
- Continuous monitoring: colorimetry, gas collection, titration of quenched samples.
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A rate equation sums up, in one line, how the rate of a reaction depends on the concentration of each reactant. Finding the orders and the rate constant is a core data-handling skill, tested with tables, graphs and practicals.
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