CCEA A Level Chemistry
CCEA A Level Chemistry 4.3 Rates of Reaction Exam Questions
Free CCEA A Level Chemistry revision for 4.3 Rates of Reaction: what you need to know, common mistakes and exam-style questions with answers. Covers rate equations, orders, rate constants and units, concentration-time and rate-concentration graphs, rate-determining step, SN1 and SN2 mechanisms and experimental methods.
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About CCEA A Level Chemistry 4.3 Rates of Reaction
Section 4.3 of Unit A2 1 makes kinetics quantitative. You need to use rate equations of the form rate = k[A]^x[B]^y with orders of 0, 1 or 2, define rate, order and rate constant, and deduce orders from initial rate data and from concentration-time and rate-concentration graphs. CCEA links rate equations to mechanisms through the rate-determining step, using the alkaline hydrolysis of primary (SN2) and tertiary (SN1) halogenoalkanes. You must also suggest experimental methods, including iodine titrations and colorimetry, and explain qualitatively how temperature and activation energy affect k. This builds on AS kinetics (2.9) and halogenoalkanes (2.5).
What you need to know
- Rate equation Rate = k[A]^x[B]^y, where x and y are orders found by experiment. The overall order is x + y. Orders cannot be read from the balanced equation.
- Orders from data If doubling [A] has no effect on rate, it is zero order; if rate doubles, first order; if rate quadruples, second order.
- Graphs Concentration-time: zero order is a straight line with constant gradient; first order has a constant half-life. Rate-concentration: zero order is horizontal, first order is a straight line through the origin, second order is a curve.
- Rate constant k is found by rearranging the rate equation. Units depend on overall order: first order s-1, second order dm3 mol-1 s-1, third order dm6 mol-2 s-1. k increases with temperature and is smaller for reactions with a higher activation energy.
- Mechanisms The rate-determining step is the slowest step; species in the rate equation appear in or before it. Tertiary halogenoalkanes react by SN1 (rate = k[RX]) via a carbocation. Primary halogenoalkanes react by SN2 (rate = k[RX][OH-]) in one step.
- Methods Remove samples, quench and titrate (for example iodine with thiosulfate), or follow colour change with a colorimeter.
Common mistakes in 4.3 Rates of Reaction
- Using coefficients from the equation as orders. Orders must come from experimental data.
- Getting the units of k wrong. Rearrange k = rate / concentration terms and cancel units carefully.
- Saying the rate-determining step is the first step. It is the slowest step, wherever it occurs.
- Mixing up SN1 and SN2. SN1 is first order overall and goes via a carbocation; SN2 is second order and occurs in one step.
- Drawing a first order rate-concentration graph as a curve. It is a straight line through the origin.
Try three 4.3 Rates of Reaction exam-style questions
1. For rate = k[A][B]^2, the rate is 3.6 x 10^-4 mol dm-3 s-1 when [A] = 0.10 and [B] = 0.20 mol dm-3. Calculate k with units. (3 marks)
Show the mark scheme answer
k = 3.6 x 10^-4 / (0.10 x 0.20^2); = 0.090; dm6 mol-2 s-1.
2. In the reaction above, [B] is tripled while [A] is unchanged. By what factor does the rate change? (1 mark)
Show the mark scheme answer
3^2 = 9 times.
3. The hydrolysis of 2-bromo-2-methylpropane has rate = k[C4H9Br]. Explain what this shows about the mechanism. (2 marks)
Show the mark scheme answer
Only the halogenoalkane is in the rate-determining step; the slow step is breaking of C-Br to form a carbocation (SN1), then OH- attacks in a fast step.
4.3 Rates of Reaction FAQs
What does CCEA A Level Chemistry 4.3 Rates of Reaction cover?
It covers rate equations, orders, rate constants, graphs, the rate-determining step, SN1 and SN2 mechanisms and methods for measuring rate.
Which unit is 4.3 Rates of Reaction examined in?
It is examined in Unit A2 1: Further Physical and Organic Chemistry, a 2 hour paper with a multiple-choice section and structured questions. Related practical skills can also be assessed in Unit A2 3.
How do I find orders from a table of initial rates?
Pick two experiments where only one concentration changes, compare the factor change in concentration with the factor change in rate, and repeat for each reactant.
