kinetics - science skool!05]_kinetics.pdfwmp/jan11/chem2 do not write outside the box 4 (04) 2 the...

Kinetics

WMP/Jan11/CHEM2

Do not writeoutside the

2 The diagram below shows a Maxwell–Boltzmann distribution for a sample of gas at a fixed temperature.Ea is the activation energy for the decomposition of this gas.

2 (a) (i) On this diagram, sketch the distribution for the same sample of gas at a higher temperature. (2 marks)

2 (a) (ii) With reference to the Maxwell–Boltzmann distribution, explain why an increase intemperature increases the rate of a chemical reaction.

............................................................................................................................................

............................................................................................................................................(2 marks)

(Extra space) .....................................................................................................................

............................................................................................................................................

Number ofmoleculeswith a givenenergy

Energy

WMP/Jan11/CHEM2

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2 (b) Dinitrogen oxide (N2O) is used as a rocket fuel. The data in the table below show how the activation energy for the decomposition of dinitrogen oxide differs withdifferent catalysts.

2N2O(g) → 2N2(g) + O2(g)

2 (b) (i) Use the data in the table to deduce which is the most effective catalyst for this decomposition.

............................................................................................................................................(1 mark)

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

............................................................................................................................................

............................................................................................................................................(2 marks)

(Extra space) .....................................................................................................................

............................................................................................................................................

Without a catalyst 245

With a gold catalyst

With an iron catalyst

With a platinum catalyst

Ea / kJ mol–1

WMP/Jun12/CHEM2

Section A

Answer all questions in the spaces provided.

1 A student carried out an experiment to determine the rate of decomposition ofhydrogen peroxide into water and oxygen gas.

The student used 100 cm3 of a 1.0 mol dm–3 solution of hydrogen peroxide at 298 Kand measured the volume of oxygen collected.

Curve R, in each of Figures 1, 2 and 3, shows how the total volume of oxygencollected changed with time under these conditions.

1 (a) Draw a curve on Figure 1 to show how the total volume of oxygen collected willchange with time if the experiment is repeated at 298 K using 100 cm3 of a 2.0 mol dm–3 solution of hydrogen peroxide.

Figure 1

(2 marks)

1 (b) Draw a curve on Figure 2 to show how the total volume of oxygen collected willchange with time if the experiment is repeated at 298 K using 100 cm3 of a 0.4 mol dm–3 solution of hydrogen peroxide.

Figure 2

(2 marks)

Time / s

Volume ofoxygen / cm3 R

Time / s

WMP/Jun12/CHEM2

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1 (c) Draw a curve on Figure 3 to show how the total volume of oxygen collected willchange with time if the original experiment is repeated at a temperature higher than298 K.

You should assume that the gas is collected at a temperature of 298 K.

Figure 3

(2 marks)

1 (d) Explain why the slope (gradient) of curve R decreases as time increases.

...........................................................................................................................................

...........................................................................................................................................(2 marks)

(Extra space) ......................................................................................................................

............................................................................................................................................

Question 1 continues on the next page

Time / s

WMP/Jun12/CHEM2

1 (e) The student discovered that hydrogen peroxide decomposes at a faster rate when afew drops of aqueous hydrogen bromide are added to the solution.

The student found on the Internet that this decomposition is thought to proceed in two steps as shown by the following equations.

Step 1 H2O2 + HBr HBrO + H2O

Step 2 HBrO + H2O2 H2O + O2 + HBr

1 (e) (i) Write an equation for the overall reaction.

............................................................................................................................................(1 mark)

1 (e) (ii) Give one reason, other than the increase in rate of reaction, why the student was ableto deduce that hydrogen bromide behaves as a catalyst in this two-step reaction.

............................................................................................................................................

............................................................................................................................................(1 mark)

WMP/Jan12/CHEM2

3 The diagram shows the Maxwell–Boltzmann distribution of molecular energies in a gasat two different temperatures.

3 (a) One of the axes is labelled. Complete the diagram by labelling the other axis.(1 mark)

3 (b) State the effect, if any, of a solid catalyst on the shape of either of these distributions.

............................................................................................................................................

............................................................................................................................................(1 mark)

3 (c) In the box, write the letter, V, W, X or Y, that represents the most probable energy ofthe molecules at the lower temperature.

(1 mark)

3 (d) Explain what must happen for a reaction to occur between molecules of two differentgases.

............................................................................................................................................

............................................................................................................................................(2 marks)

X Y Energy

WMP/Jan12/CHEM2(07)

3 (e) Explain why a small increase in temperature has a large effect on the initial rate of areaction.

............................................................................................................................................

............................................................................................................................................(1 mark)

Turn over for the next question

Turn over �

WMP/Jun13/CHEM2

3 The diagram shows the Maxwell–Boltzmann distribution for a sample of gas at a fixed temperature.

Ea is the activation energy for the decomposition of this gas.

Emp is the most probable value for the energy of the molecules.

3 (a) On the appropriate axis of this diagram, mark the value of Emp for this distribution.

On this diagram, sketch a new distribution for the same sample of gas at a lower

temperature.(3 marks)

3 (b) With reference to the Maxwell–Boltzmann distribution, explain why a decrease in temperature decreases the rate of decomposition of this gas.

............................................................................................................................................

............................................................................................................................................(2 marks)

Turn over �

Energy Ea

Number ofmolecules

WMP/Jan13/CHEM2(02)

Section A

Answer all questions in the spaces provided.

1 (a) Figure 1 shows the volume of hydrogen gas collected when a sample of magnesiumreacted with an excess of dilute hydrochloric acid.

The rate of this reaction can be studied by measuring the time it takes for a givenvolume of hydrogen to be collected.

Figure 1

1 (a) (i) State the meaning of the term rate of reaction.

............................................................................................................................................

............................................................................................................................................(1 mark)

1 (a) (ii) State and explain what has happened to the rate of this reaction at point W in Figure 1.

............................................................................................................................................

............................................................................................................................................(2 marks)

Time / s

Volume ofhydrogen / cm3

WMP/Jan13/CHEM2

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1 (a) (iii) In terms of collision theory explain why, at a fixed temperature, the rate of this reactiondoubles when the concentration of the hydrochloric acid doubles.

............................................................................................................................................

............................................................................................................................................(2 marks)

1 (b) In a study of the reaction in part (a), a student referred to activation energy.

1 (b) (i) State the meaning of the term activation energy.

............................................................................................................................................

............................................................................................................................................(1 mark)

1 (b) (ii) Complete Figure 2 by drawing the shape of the reaction profile from reactants toproducts for an exothermic reaction.Show the position of the products. Show and label the activation energy.

Figure 2

(2 marks)

Energy

Reactants

WMP/Jun14/CHEM2

2 Figure 2 shows the Maxwell–Boltzmann distribution of molecular energies in a sample of gas at temperature T.

Figure 2

2 (a) One of the axes is labelled. Label the other axis.

[1 mark]

2 (b) State why the curve starts at the origin.[1 mark]

............................................................................................................................................

2 (c) Which of the following, A, B or C, describes what the value of Y represents in Figure 2?

Write the correct letter, A, B or C, in the box.[1 mark]

A The energy needed for a successful collision B The minimum energy needed for a reaction to occur C The most probable energy

EnergyY0

WMP/Jun14/CHEM2

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2 (d) On Figure 2, draw a distribution of molecular energies in this sample of gas at a higher temperature.

[2 marks]

2 (e) The pressure of the original sample of gas is doubled at temperature T.

State the effect, if any, of this change on the value of Y.[1 mark]

............................................................................................................................................

Turn over for the next question

7 (c) A student determined a value for the enthalpy change when a sample of propanone wasburned. The heat produced was used to warm some water in a copper calorimeter.The student found that the temperature of 150 g of water increased by 8.0 ºC when 4.50 × 10–3 mol of pure propanone was burned in air.

Use the student’s results to calculate a value, in kJ mol–1, for the enthalpy change whenone mole of propanone is burned.(The specific heat capacity of water is 4.18 J K–1 g–1)

[3 marks]

.............................................................................................................................................

(17)WMP/Jun15/CHEM2

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5 Methanol, for use as a fuel, can be produced by the reaction of carbon monoxide withhydrogen.

CO(g) + 2H2(g) CH3OH(g) ΔH = –90 kJ mol–1

The reaction is typically carried out at 300 ºC and 3 × 107 Pa, in the presence of acatalyst.

5 (a) Figure 2 shows the Maxwell–Boltzmann distribution for a mixture of carbon monoxideand hydrogen at 300 ºC.

Figure 2

5 (a) (i) Sketch a second curve on Figure 2 to show the distribution of molecular energies in thismixture at a higher temperature.

[1 mark]

5 (a) (ii) Explain with reference to both curves in Figure 2 how a small change in temperatureleads to a large change in the rate of reaction.

[2 marks]

______________________________________________________________________________

(12)WMP/Jun16/CHEM2

Eaenergy

Number ofmolecules

8 (c) The rates of hydrolysis of two chloroalkanes can be investigated by adding aqueoussilver nitrate to the chloroalkanes. During the hydrolysis reactions, chloride ions areliberated slowly. Precipitates of silver chloride are formed.

Outline a method to compare the rate of hydrolysis of 1-chlorobutane with that of 2-chlorobutane. State how the method would ensure a fair test.

[4 marks]

______________________________________________________________________________

(22)WMP/Jun16/CHEM2

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