Exam Qs

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Rates of Reaction (Kinetics)

1 Initial rates data and the rate equation

2 The Maxwell-Boltzmann distribution

You are advised to work through these questions a screen at a time, writing your answers down on paper and then clicking the Answer link to check.

1 (a) The following data were obtained in a series of experiments on the rate of the reaction between compounds X and Y at a constant temperature.

(i) Show with reasoning how the data in the table can be used to deduce that the reaction is first-order with respect to X.

(ii) Deduce with reasoning the order with respect to Y.

(iii) State the rate equation and overall order for this reaction.

(5 marks)

Experiment

Initial concentration of

X/mol dm–3

Initial concentration of Y/mol dm–3

Initial rate/mol dm–3 s–1

1 0.05 0.08 0.15 x 10–6

2 0.10 0.08 0.30 x 10–6

3 0.20 0.16 2.40 x 10–6

Question page 1

Answer 1 (a)

1 (b) The following data were obtained in a second series of experiments on the rate of the reaction between compounds M and N at a constant temperature.

The rate equation for this reaction is rate = k [M]2 [N]

Experiment

Initial concentration of M/mol dm–3

Initial concentrationof N/mol dm–3

Initial rate/mol dm–3 s–1

4 0.25 0.50 3.10 x 10–6

5 0.40 0.20 To be calculated

Question page 2

(i) Use the data from Experiment 4 to calculate a value for the rate constant, k, at this temperature. State the units of k.

Value for k ............... ...............

Units of k ............... ...............

(ii) Calculate the value of the initial rate in Experiment 5.

(4 marks)

Answer 1 (b)

Question page 3

2 (a) State what is meant by the term activation energy of a reaction.

(1 mark)

(b) Explain how a catalyst increases the rate of a chemical reaction.(2

marks)

Answers 2 (a) and (b)

Number ofmolecules

Emp Ea Energy

Question page 4

2 (c) The Maxwell–Boltzmann distribution of energies, at a constant temperature, for a molecular gas is shown below. The most probable molecular energy at this temperature is indicated by Emp and the activation energy by Ea.

Consider the following changes:(i) The number of molecules is increased at constant temperature.(ii) The temperature is decreased without changing the number of molecules.(iii) A catalyst is introduced without changing the temperature or number of molecules.

For each change state how, if at all, the following would vary:• the value of the most probable energy, Emp

• the number of molecules with the most probable energy, Emp

• the area under the molecular energy distribution curve• the number of molecules with energy greater than the activation energy, Ea

(12 marks)Answer 2 (c) i ii iii

References

An illustration of how exam questions and model answers might be presented for use in the classroom or independently.

Steve Lewis for the Royal Society of Chemistry