3.2.2 Kinetics. Effect of temperature. 145 minutes. 145 marks. Page 1 of 22

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1 3.. Kinetics Effect of temperature 145 minutes 145 marks Page 1 of

2 Q1. (a) State what is meant by the term activation energy of a reaction. (b) (c) State in general terms how a catalyst increases the rate of a chemical reaction. The curve below shows the Maxwell Boltzmann distribution of molecular energies, at a constant temperature, in a gas at the start of a reaction. On this diagram the most probable molecular energy at this temperature is indicated by the symbol E mp and the activation energy by the symbol E a. Consider the following changes. (i) (ii) (iii) The number of molecules is increased at constant temperature. The temperature is decreased without changing the number of molecules. A catalyst is introduced without changing the temperature or the number of molecules. For each of these changes state how, if at all, the following would vary: the value of the most probable energy, E mp the number of molecules with the most probable energy, E mp the area under the molecular energy distribution curve the number of molecules with energy greater than the activation energy, E a (1) (Total 15 marks) Page of

3 Q. Hydrogen is produced by the reaction between steam and methane when the following dynamic equilibrium is established. CH 4 (g) + H O(g) CO(g) + 3H (g) ΔH = +06 kj mol 1 (a) Use Le Chatelier s principle to predict the separate effects of an increase in temperature and of an increase in pressure on the yield of hydrogen obtained in the above reaction. In each case, explain your answer. (6) (b) State how, and explain why, the use of a catalyst might or might not change the equilibrium yield of hydrogen, and also the amount of hydrogen produced, in a given time. (4) (Total 10 marks) Q3. (a) Define the term activation energy for a reaction. (b) Give the meaning of the term catalyst. (c) Explain in general terms how a catalyst works. Page 3 of

4 (d) In an experiment, two moles of gas W reacted completely with solid Y to form one mole of gas Z as shown in the equation below. W(g) + Y(s) Z(g) The graph below shows how the concentration of Z varied with time at constant temperature. (i) (ii) (iii) On the axes above, sketch a curve to show how the concentration of W would change with time in the same experiment. Label this curve W. On the axes above, sketch a curve to show how the concentration of Z would change with time if the reaction were to be repeated under the same conditions but in the presence of a catalyst. Label this curve Z. In terms of the behaviour of particles, explain why the rate of this reaction decreases with time. (6) (Total 1 marks) Page 4 of

5 Q4. (a) A sample of a gas was sealed into a flask at temperature T and pressure P. The Maxwell Boltzmann distribution of energies for the molecules in this sample is shown below. (i) (ii) Using the axes above, sketch the curve that you would expect if this sample of gas at pressure P had been cooled. Label this curve X. Using the axes above, sketch the curve that you would expect if another sample of the same gas was sealed in the same flask at the original temperature, T, but at a higher pressure. Label this curve Y. (4) (b) Gas A decomposes slowly to form gases B and C. An equilibrium is established as shown by the following equation. A(g) B(g) + C(g) ΔH is positive (i) In terms of the behaviour of molecules, state what must happen before molecules of A can react to form B and C. (ii) Explain why the decomposition of A is faster at higher temperatures. (4) Page 5 of

6 (c) The graphs below show how, starting from A alone, the concentration of A varies with time at temperatures of 300 K and 30 K for the reversible reaction given in part (b). (i) Suggest why, as shown on the graphs, the concentration of A remains constant after a time. (ii) Explain why, at 30 K, the concentration of A falls to a lower value compared with the reaction at 300 K. (3) (Total 11 marks) Q5. The diagram below represents a Maxwell Boltzmann distribution curve for the particles in a sample of a gas at a given temperature. The questions below refer to this sample of particles. (a) (b) Label the axes on the diagram. On the diagram draw a curve to show the distribution for this sample at a lower temperature. Page 6 of

7 (c) In order for two particles to react they must collide. Explain why most collisions do not result in a reaction. (d) State one way in which the collision frequency between particles in a gas can be increased without changing the temperature. (e) Suggest why a small increase in temperature can lead to a large increase in the reaction rate between colliding particles. (f) Explain in general terms how a catalyst works. (Total 10 marks) Q6. Gas G decomposes as shown in the equation below. G(g) X(g) + Y(g) (a) Draw, on the axes below, a Maxwell Boltzmann distribution curve for a sample of G in which only a small proportion of molecules has energy greater than the activation energy, E a. (3) Page 7 of

8 (b) Define the term activation energy. (c) At any time, most of the molecules of G have energy less than the activation energy. Suggest why, at a constant temperature, most of G eventually decomposes. (d) State the effect, if any, of adding a catalyst on the time required for G to decompose, compared with a similar sample without a catalyst. Explain in general terms how the catalyst has this effect. Time for decomposition... Explanation... (3) (Total 10 marks) Q7. The curve below shows how the volume of oxygen evolved varies with time when 50 cm 3 of a.0 mol dm 3 solution of hydrogen peroxide, H O, decomposes at 98 K. (a) State how you could use the curve to find the rate of reaction at point A. Page 8 of

9 (b) Sketch curves, on the above axes, to illustrate how the volume of oxygen evolved would change with time if the experiment was repeated at 98 K using the following. (i) 100 cm 3 of a 1.0 mol dm 3 solution of H O. Label this curve X. (ii) 5 cm 3 of a.0 mol dm 3 solution of H O in the presence of a catalyst. Label this curve Y. (4) (c) Hydrogen peroxide decomposes more rapidly in the presence of aqueous hydrogen bromide. The decomposition proceeds as shown by the following equations. H O + HBr HBrO + H O HBrO + H O H O + O + HBr (i) Write an equation for the overall reaction. (ii) Define the term catalyst. (iii) Give two reasons, other than an increase in the reaction rate, why these equations suggest that hydrogen bromide is behaving as a catalyst. Reason 1... Reason... (5) (Total 10 marks) Q8. (a) Define the term activation energy for a chemical reaction. (b) Draw, with labelled axes, a curve to represent the Maxwell Boltzmann distribution of molecular energies in a gas. Label this curve T 1. On the same axes, draw a second curve to represent the same sample of gas at a lower temperature. Label this curve T. Use these curves to explain why a small decrease in temperature can lead to a large decrease in the rate of a reaction. (8) Page 9 of

10 (c) Give one reason why most collisions between gas-phase reactants do not lead to a reaction. State and explain two ways of speeding up a gas-phase reaction other than by changing the temperature. (5) (Total 15 marks) Q9. The gas-phase reaction between hydrogen and chlorine is very slow at room temperature. H (g) + Cl (g) HCl(g) (a) Define the term activation energy. (b) Give one reason why the reaction between hydrogen and chlorine is very slow at room temperature. (c) Explain why an increase in pressure, at constant temperature, increases the rate of reaction between hydrogen and chlorine. (d) Explain why a small increase in temperature can lead to a large increase in the rate of reaction between hydrogen and chlorine. (e) Give the meaning of the term catalyst. Page 10 of

11 (f) Suggest one reason why a solid catalyst for a gas-phase reaction is often in the form of a powder. (Total 9 marks) Q10. The rate of a chemical reaction is influenced by the size of the activation energy. Catalysts are used to increase the rates of chemical reactions but are not used up in the reactions. (a) Give the meaning of the term activation energy (b) Explain how a catalyst increases the rate of a reaction Page 11 of

12 (c) The diagram below shows the Maxwell Boltzmann distribution of molecular energies, at a constant temperature, in a gas at the start of a reaction. On this diagram the most probable molecular energy at this temperature is shown by the symbol E mp The activation energy is shown by the symbol E a To answer the questions (c)(i) to (c)(iv), you should use the words increases, decreases or stays the same. You may use each of these answers once, more than once or not at all. (i) State how, if at all, the value of the most probable energy (E mp ) changes as the total number of molecules is increased at constant temperature. (ii) State how, if at all, the number of molecules with the most probable energy (E mp ) changes as the temperature is decreased without changing the total number of molecules. (iii) State how, if at all, the number of molecules with energy greater than the activation energy (E a ) changes as the temperature is increased without c hanging the total number of molecules. (iv) State how, if at all, the area under the molecular energy distribution curve changes as a catalyst is introduced without changing the temperature or the total number of molecules. Page 1 of

13 (d) For each of the following reactions, identify a catalyst and name the organic product of the reaction. (i) The fermentation of an aqueous solution of glucose. Catalyst... Name of organic product... (ii) The hydration of but--ene. Catalyst... Name of organic product... (Total 1 marks) Page 13 of

14 Q11. Methanol (CH 3 OH) is an important fuel that can be synthesised from carbon dioxide. (a) The table shows some standard enthalpies of formation. CO (g) H (g) CH 3 OH(g) H O(g) H f Ɵ /kj mol (i) Use these standard enthalpies of formation to calculate a value for the standard enthalpy change of this synthesis. CO (g) + 3H (g) CH 3 OH(g) + H O(g) (Extra space)... (3) (ii) State why the standard enthalpy of formation for hydrogen gas is zero. Page 14 of

15 (b) State and explain what happens to the yield of methanol when the total pressure is increased in this synthesis. CO (g) + 3H (g) CH 3 OH(g) + H O(g) Effect on yield... Explanation... (Extra space)... (3) (c) The hydrogen required for this synthesis is formed from methane and steam in a reversible reaction. The equation for this reaction is shown below. CH 4 (g) + H O(g) C0(g) + 3H (g) H = +06 kj mol 1 State and explain what happens to the yield of hydrogen in this reaction when the temperature is increased. Effect on yield... Explanation... (Extra space)... (3) Page 15 of

16 (d) The methanol produced by this synthesis has been described as a carbon-neutral fuel. (i) State the meaning of the term carbon-neutral. (Extra space)... (ii) Write an equation for the complete combustion of methanol. (iii) The equation for the synthesis of methanol is shown below. CO (g) + 3H (g) CH 3 OH(g) + H O(g) Use this equation and your answer to part (d)(ii) to deduce an equation to represent the overall chemical change that occurs when methanol behaves as a carbonneutral fuel. Equation... Page 16 of

17 (e) A student carried out an experiment to determine the enthalpy change when a sample of methanol was burned. The student found that the temperature of 140 g of water increased by 7.5 C when mol of methanol was burned in air and the heat produced was used to warm the water. Use the student s results to calculate a value, in kj mol 1, for the enthalpy change when one mole of methanol was burned. (The specific heat capacity of water is 4.18 J K 1 g 1 ). (Extra space)... (3) (Total 16 marks) Page 17 of

18 Q1. A student carried out an experiment to determine the rate of decomposition of hydrogen peroxide into water and oxygen gas. The student used 100 cm 3 of a 1.0 mol dm 3 solution of hydrogen peroxide at 98K and measured the volume of oxygen collected. Curve R, in each of Figures 1, and 3, shows how the total volume of oxygen collected changed with time under these conditions. (a) Draw a curve on Figure 1 to show how the total volume of oxygen collected will change with time if the experiment is repeated at 98 K using 100 cm 3 of a.0 mol dm 3 solution of hydrogen peroxide. Figure 1 (b) Draw a curve on Figure to show how the total volume of oxygen collected will change with time if the experiment is repeated at 98 K using 100 cm 3 of a 0.4 mol dm 3 solution of hydrogen peroxide. Figure Page 18 of

19 (c) Draw a curve on Figure 3 to show how the total volume of oxygen collected will change with time if the original experiment is repeated at a temperature higher than 98 K. You should assume that the gas is collected at a temperature of 98 K. Figure 3 (d) Explain why the slope (gradient) of curve R decreases as time increases. (Extra space)... (e) The student discovered that hydrogen peroxide decomposes at a faster rate when a few 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 H O + HBr HBrO + H O Step HBrO + H O H O + O + HBr (i) Write an equation for the overall reaction. Page 19 of

20 (ii) Give one reason, other than the increase in rate of reaction, why the student was able to deduce that hydrogen bromide behaves as a catalyst in this two-step reaction. (Total 10 marks) Q13. The diagram shows the Maxwell Boltzmann distribution for a sample of gas at a fixed temperature. E a is the activation energy for the decomposition of this gas. E mp is the most probable value for the energy of the molecules. Energy Ea (a) On the appropriate axis of this diagram, mark the value of E mp for this distribution. On this diagram, sketch a new distribution for the same sample of gas at a lower temperature. (3) Page 0 of

21 (b) With reference to the Maxwell Boltzmann distribution, explain why a decrease in temperature decreases the rate of decomposition of this gas. (Total 5 marks) Page 1 of

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