Chemistry › Rate equations
Rate equations
Unit C09 · 3 lessons.
The orders in a rate equation cannot be read off the balanced equation; they come only from experiment, from initial rates, from half-lives and from the shapes of graphs. A rate equation gives the rate in terms of concentrations raised to those orders and a rate constant, the Arrhenius equation links the constant to temperature and activation energy, and the orders say which species are in the rate-determining step.
The method marks are in the comparison. A table of initial rates wants each order found from two runs in which only one concentration changes, with the factor stated: concentration doubled, rate quadrupled, second order. The units of the rate constant then follow from the overall order, and stating them is often a mark of its own. An Arrhenius plot is ln k against 1/T, with T in kelvin.
AQA Papers 2 and 3, OCR A Papers 1 and 3, Edexcel Papers 2 and 3 and Cambridge Paper 4 examine it, and each paper's page under past papers lists what else it covers.
Specification sections: AQA 3.1.9.1, 3.1.9.2 · OCR A 5.1.1 · Edexcel 16.1 to 16.12, 6.34 · CAIE 26.1
Notes
Illustrated notes for each topic in this unit, with worked examples and a checklist of objectives.
- Rate equations, orders of reaction and the rate constant
The rate equation rate = k[A]ᵐ[B]ⁿ, order with respect to a reactant and overall order, the rate constant and how to work out its units for overall orders zero to three, the rate–concentration and concentration–time graphs for zero, first and second order and k from their gradients, rates of loss and formation linked by the mole ratio, why orders come from experiment and not from the balanced equation, and the new rate when concentrations change by given factors.
- Finding orders from initial rates, half-lives and graphs
The initial-rates method and how to read orders from a table of runs, clock reactions and 1/t, continuous monitoring by colorimetry, titration with quenching and gas collection, rates from tangents to a concentration–time graph, the constant half-life of a first-order reaction and k = ln 2 / t½, how to choose a monitoring method from the equation, the isolation method and pseudo-first-order runs, and required practical 7 with the iodine and propanone reaction: apparatus, method, measurements and the errors that matter.
- The Arrhenius equation and the rate-determining step
The Arrhenius equation k = Ae^(−Ea/RT) and its logarithmic form, the graph of ln k against 1/T with gradient −Ea/R, finding the activation energy and the pre-exponential factor from a graph or from two rate constants, the rate-determining step, intermediates and catalysts in a mechanism, the mechanism of the iodination of propanone, the effect of a catalyst on Ea and k, deducing a rate equation from a mechanism and proposing a mechanism from a rate equation, and SN1 and SN2 hydrolysis of halogenoalkanes.
Key definitions for this unit
Practical work
The practical capabilities this unit's work belongs to, each with its techniques, its calculations and the errors that cost marks.
Worked in full, from the apparatus to the evaluation:
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