Edexcel A Level Chemistry
Edexcel A Level Chemistry Topic 16: Kinetics II Exam Questions
Free Edexcel A Level Chemistry exam questions for Topic 16: Kinetics II, with a mark scheme for every pack. Covers rate equations, orders of reaction, rate constants, initial rates and continuous monitoring methods, half-life, rate-determining steps and the Arrhenius equation.
Topic 16: Kinetics II 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 Edexcel A Level Chemistry Topic 16: Kinetics II
Topic 16 of Edexcel A Level Chemistry, Kinetics II, turns the collision theory of Topic 9 into quantitative kinetics. You determine orders of reaction from initial rates data and from concentration-time graphs, including constant half-life for first order reactions, write rate equations and calculate rate constants with units. You then link the rate equation to the rate-determining step of a mechanism, for example SN1 and SN2 hydrolysis of halogenoalkanes and the iodination of propanone. The Arrhenius equation is used to find activation energy from a graph of ln k against 1/T. It is assessed in Paper 2. Examiners focus on deducing orders and units of k.
What you need to know
- Rate equations Rate = k[A]^m[B]^n, where m and n are orders found by experiment. The overall order is the sum of the individual orders.
- Determining orders Use initial rates from experiments where only one concentration changes. Continuous monitoring gives a concentration-time graph: zero order is a straight line, first order has a constant half-life.
- Rate-concentration graphs Zero order gives a horizontal line, first order a straight line through the origin and second order an upward curve.
- Units of k Rearrange the rate equation for k and cancel units. First order k has units s-1, second order dm3 mol-1 s-1.
- Mechanisms The rate-determining step is the slowest step, and only species involved up to and including it appear in the rate equation. Iodination of propanone is zero order in iodine, showing I2 is not in the slow step.
- SN1 and SN2 Tertiary halogenoalkanes react by SN1 (rate = k[RX]) through a carbocation. Primary halogenoalkanes react by SN2 (rate = k[RX][OH-]) in one step.
- Arrhenius equation k = Ae^(-Ea/RT), so ln k = -Ea/RT + ln A. Plotting ln k against 1/T gives a straight line with gradient -Ea/R.
Common mistakes in Topic 16: Kinetics II
- Taking orders from the stoichiometric equation.
- Forgetting to convert Ea to kJ mol-1 or leaving the negative sign in the final answer.
- Writing units of k with the wrong powers after cancelling.
- Saying a zero order reactant does not take part in the reaction; it takes part after the rate-determining step.
- Using temperature in Celsius in Arrhenius calculations instead of kelvin.
Try three Topic 16: Kinetics II exam-style questions
1. A graph of ln k against 1/T has a gradient of -6000 K. Calculate the activation energy. R = 8.31 J K-1 mol-1. (2 marks)
Show the mark scheme answer
Ea = -gradient x R = 6000 x 8.31 = 49900 J mol-1; Ea = 49.9 kJ mol-1.
2. 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; so the slow step is the breaking of the C-Br bond to form a carbocation (SN1), with OH- reacting in a later fast step.
3. When [A] is doubled the initial rate quadruples, and when [B] is doubled the rate is unchanged. Write the rate equation and give the units of k. (2 marks)
Show the mark scheme answer
Rate = k[A]^2; units dm3 mol-1 s-1.
Topic 16: Kinetics II FAQs
What does Edexcel A Level Chemistry Topic 16 Kinetics II cover?
It covers rate equations, orders, rate constants, initial rates and continuous monitoring, half-life, rate-determining steps and the Arrhenius equation.
Which paper is Kinetics II examined in?
Topic 16 is assessed in Paper 2, Advanced Organic and Physical Chemistry, and its practical work is tested in Paper 3.
How do I find activation energy from experimental data?
Calculate ln k and 1/T for each temperature, plot ln k against 1/T, draw a best fit line and use Ea = -gradient x R.
