AQA A Level Chemistry
AQA A Level Chemistry 3.1.9 Rate Equations Exam Questions
Free AQA A Level Chemistry exam questions for 3.1.9 Rate Equations, with a mark scheme for every pack. Covers rate equations, orders, rate constants and units, initial rates, continuous monitoring, the Arrhenius equation and rate-determining steps.
3.1.9 Rate Equations exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About AQA A Level Chemistry 3.1.9 Rate Equations
Rate equations make kinetics quantitative. On the AQA A Level course you write rate equations of the form rate = k[A]^m[B]^n, deduce orders from initial rate data and from concentration-time or rate-concentration graphs, calculate the rate constant and its units, and use the Arrhenius equation to relate k to temperature and activation energy. You also link the rate equation to the rate-determining step of a mechanism. Required practical 7 covers measuring rate by an initial rate method and by continuous monitoring. Examiners like table-based order deductions, k unit questions, ln k against 1/T graphs and mechanism questions.
What you need to know
- Rate equation rate = k[A]^m[B]^n, where m and n are orders with respect to each reactant and the overall order is m + n. Orders can only be found by experiment, not from the balanced equation.
- Orders from data Zero order: changing concentration has no effect on rate. First order: doubling concentration doubles rate. Second order: doubling concentration quadruples rate. Compare experiments where only one concentration changes.
- Graphs On a concentration-time graph, a zero order reaction gives a straight line and a first order reaction has a constant half-life. On a rate-concentration graph, zero order is horizontal, first order is a straight line through the origin and second order is a curve.
- Rate constant Rearrange the rate equation to find k and work out units by cancelling. For overall second order, k has units of mol-1 dm3 s-1. The rate constant increases with temperature.
- Arrhenius equation k = Ae^(-Ea/RT), which gives ln k = -Ea/RT + ln A. A plot of ln k against 1/T gives a straight line with gradient -Ea/R. T must be in kelvin and R = 8.31 J K-1 mol-1.
- Rate-determining step The slowest step of a mechanism. Species that appear in the rate equation are involved in or before the rate-determining step; species that are zero order are not.
- Required practical 7 Initial rate methods, such as an iodine clock, measure the time for a fixed small change. Continuous monitoring follows a concentration, volume of gas or mass over time, and the tangent at t = 0 gives the initial rate.
Common mistakes in 3.1.9 Rate Equations
- Taking orders from the coefficients in the balanced equation. Orders must come from experimental data.
- Getting the units of k wrong. Write the rearranged expression, substitute units and cancel step by step.
- Using temperatures in Celsius in the Arrhenius equation. Always convert to kelvin.
- Forgetting the minus sign or R when finding Ea from the gradient. Ea = -gradient x R, then convert J to kJ if asked.
- Deducing an order by comparing two experiments in which two concentrations change at once. Choose a pair where only one changes, or allow for the known effect of the other.
Try three 3.1.9 Rate Equations exam-style questions
1. The rate equation for a reaction is rate = k[A][B]^2. When [A] = 0.20 mol dm-3 and [B] = 0.10 mol dm-3, the rate is 3.0 x 10^-4 mol dm-3 s-1. Calculate k and give its units. (3 marks)
Show the mark scheme answer
k = rate / [A][B]^2 = 3.0 x 10^-4 / (0.20 x 0.10^2); k = 0.15; units mol-2 dm6 s-1.
2. A plot of ln k against 1/T has a gradient of -6500 K. Calculate the activation energy in kJ mol-1. R = 8.31 J K-1 mol-1. (2 marks)
Show the mark scheme answer
Ea = -gradient x R = 6500 x 8.31 = 54 015 J mol-1; Ea = 54.0 kJ mol-1.
3. The hydrolysis of 2-bromo-2-methylpropane by hydroxide ions has the rate equation rate = k[(CH3)3CBr]. Suggest which step of the mechanism is rate-determining and explain your answer. (2 marks)
Show the mark scheme answer
The rate-determining step is the first step, in which the C-Br bond breaks to form a carbocation; hydroxide ions do not appear in the rate equation, so they react in a later, fast step.
3.1.9 Rate Equations FAQs
What does AQA A Level Chemistry 3.1.9 Rate Equations cover?
It covers rate equations and orders, rate constants and their units, deducing orders from data and graphs, the Arrhenius equation, rate-determining steps, and Required practical 7 on measuring rates.
Which paper is 3.1.9 Rate Equations examined in?
It is examined in Paper 2, alongside organic chemistry, which is why mechanism and rate-determining step questions are common. Paper 3 can also test it.
How do I work out the units of k quickly?
Rearrange the rate equation for k, replace each concentration with mol dm-3 and the rate with mol dm-3 s-1, then cancel. Learn the pattern: zero order mol dm-3 s-1, first order s-1, second order mol-1 dm3 s-1, third order mol-2 dm6 s-1.
