3.1.9
Rate Equations
Section 3.1.9 covers the rate equation, order and the rate constant, calculations with the rate equation and the units of k, the Arrhenius equation in both forms with calculations and the ln k against 1/T graph, and the determination of rate equations from concentration–time and rate–concentration graphs, initial rates and the rate-determining step, and what the orders reveal about the mechanism. Required practical 7 supplies the rate data.
Revision Notes
7 revision notes, in the order the specification teaches them.
Rate Equations, Orders and the Rate Constant
Rate equations: orders of reaction, the rate constant k, overall order and working out the units of k.
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Techniques for Measuring Rates
Measuring rates: titration, colorimetry, mass and gas-volume methods, continuous monitoring and the initial-rates method with clock reactions.
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Concentration–Time Graphs and Half-Life
Concentration–time graphs: deducing order from the shape, tangents and rates, half-life and first-order reactions.
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Rate–Concentration Graphs and the Initial-Rates Method
The initial-rates method: rate–concentration graphs, deducing orders from tables of data, writing the rate equation and calculating k.
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The Iodine–Propanone Reaction
The iodine–propanone reaction: collecting rate data, the orders with respect to iodine, propanone and H⁺, and deducing the mechanism.
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Rate-Determining Step and Reaction Mechanisms
The rate-determining step: rate equations from mechanisms, mechanisms from rate equations, intermediates, and Sₙ1/Sₙ2 evidence from halogenoalkanes hydrolysis.
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Activation Energy and the Arrhenius Equation
The Arrhenius equation: temperature and the rate constant, ln k against 1/T graphs and finding the activation energy from the gradient.
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Rates and Rate Equations Map
AQA A-level Chemistry (7405) · 3.1.9 Rates and rate equations
The whole section on one A4 page: measuring a rate, rate equations, units of k, initial rates, graph shapes and the initial rate from a tangent, with worked examples.
Mechanisms and Temperature Map
AQA A-level Chemistry (7405) · 3.1.9 Mechanisms and temperature
The whole section on one A4 page: the slow step, mechanisms from rate equations, activation energy and temperature, the Arrhenius equation and plot, and catalysts.
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3.1.9.1
- i
know that the rate of a chemical reaction is related to concentration by a rate equation of the form Rate = k[A]m[B]n
Revise - v
know that the rate constant k varies with temperature according to k = Ae-Ea/RT where A is the Arrhenius constant, Ea is the activation energy and T is the temperature in K
Revise - viii
understand that k = Ae-Ea/RT can be rearranged to ln k = -Ea/RT + ln A and use this with experimental data to plot a straight line graph with slope -Ea/R
Revise
3.1.9.2
- v
derive the rate equation for a reaction from the orders with respect to each of the reactants
Revise - vi
use the orders with respect to reactants to provide information about the rate determining step of a reaction
Revise - vii
understand that the orders with respect to reactants can provide information about the mechanism of a reaction
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