Exampro questions kinetics temp, Exams of Chemistry

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3.1.5.3 Effect of temp on

reaction rate

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Time: 130 min. Marks: 126 marks

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Q1. The graph shows the Maxwell−Boltzmann distribution of molecular energies in a sample of gas at a fixed temperature. (a) Label the horizontal axis on the graph. (1) (b) On the graph, sketch a distribution of molecular energies for this sample of gas at a higher temperature. (2) (c) This gas decomposes on heating. Explain why an increase in temperature increases the rate at which this gas decomposes.

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(2) (Total 5 marks) Q2. Nitryl chloride reacts with nitrogen monoxide according to the equation: ClNO 2 (g) + NO (g) ⟶ NO 2 (g) + ClNO(g)

Deduce the change that was made to the reaction conditions. Explain the effect that this change has on the rate of reaction. Change ____________________________________________________________

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Explanation _________________________________________________________

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(3) (Total 7 marks) Q3. The question below is about the Maxwell–Boltzmann distribution shown for a sample of a gas, X, at two different temperatures.

Which statement is correct for the higher temperature? A The area under the curve to the left of E a decreases. B The total area under the curve increases. C The activation energy decreases. D More molecules have the mean energy. (Total 1 mark) Q4. This question involves the use of kinetic data to deduce the order of a reaction and calculate a value for a rate constant. The data in Table 1 were obtained in a series of experiments on the rate of the reaction between compounds A and B at a constant temperature. Table 1 Experiment Initial concentration of A / mol dm− Initial concentration of B / mol dm− Initial rate / mol dm−3^ s− 1 0.12 0.26 2.10 × 10− 2 0.36 0.26 1.89 × 10− 3 0.72 0.13 3.78 × 10− (a) Show how these data can be used to deduce the rate expression for the reaction

(3) (c) Calculate a value for the initial rate in experiment 5. Initial rate = _______________ mol dm−3^ s− (1) (d) The rate equation for a reaction is rate = k [ E ] Explain qualitatively why doubling the temperature has a much greater effect on the rate of the reaction than doubling the concentration of E.

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(3) (e) A slow reaction has a rate constant k = 6.51 × 10−3^ mol−1^ dm^3 at 300 K. Use the equation ln k = ln A – E a / RT to calculate a value, in kJ mol−1, for the activation energy of this reaction. The constant A = 2.57 × 10^10 mol−1^ dm^3. The gas constant R = 8.31 J K−1^ mol−1.

Activation energy = _______________ (2) (Total 12 marks)

Q5.

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

CO(g) + 2H 2 (g) ⇌ CH 3 OH(g) Δ H = –90 kJ mol–

The reaction is typically carried out at 300 °C and 3 × 10^7 Pa, in the presence of a catalyst. (a) The graph shows the Maxwell–Boltzmann distribution for a mixture of carbon monoxide and hydrogen at 300 °C. (i) Sketch a second curve on the graph to show the distribution of molecular energies in this mixture at a higher temperature. (1) (ii) Explain with reference to both curves on the graph how a small change in temperature leads to a large change in the rate of reaction.

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(2) (b) Both the rate of production and equilibrium yield of methanol are considered when choosing the most appropriate conditions for the operation of this process on an industrial scale. (i) State and explain the effect of a higher pressure on the equilibrium yield of methanol.

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(1) (c) Which of the following, A , B or C , describes what the value of Y represents in the figure? Write the correct letter, A , B or C , in the box. A The energy needed for a successful collision B The minimum energy needed for a reaction to occur C The most probable energy (1) (d) On the figure above, draw a distribution of molecular energies in this sample of gas at a higher temperature. (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.

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(1) (Total 6 marks) Q7. (a) In an investigation of the rate of reaction between hydrochloric acid and pure magnesium, a student obtained the following curve. The reaction of magnesium with dilute hydrochloric acid is exothermic. Use your understanding of collision theory to explain why the student did not obtain a straight line.

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(3) (b) The magnesium used in a laboratory experiment was supplied as a ribbon. The ribbon was stored in an open plastic bag exposed to the air. Explain why it is important to clean the surface of this magnesium ribbon when investigating the rate of its reaction with hydrochloric acid.

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(2) (c) Magnesium ribbon reacts with hot water. Heated magnesium ribbon reacts with steam. State two differences between these reactions. Difference 1 _________________________________________________________

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Difference 2 _________________________________________________________

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(2) (d) Pure magnesium reacts completely with an excess of dilute sulfuric acid. The reaction of pure calcium with an excess of dilute sulfuric acid is very rapid initially. This reaction slows down and stops before all of the calcium has reacted. Use your knowledge of the solubilities of Group 2 sulfates to explain why these reactions of magnesium and calcium with dilute sulfuric acid are so different.

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(2) (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 298 K. You should assume that the gas is collected at a temperature of 298 K. Figure 3 (2) (d) Explain why the slope (gradient) of curve R decreases as time increases.

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(2) (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 2 O 2 + HBr HBrO + H 2 O Step 2 HBrO + H 2 O 2 H 2 O + O 2 + HBr (i) Write an equation for the overall reaction.

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(1) (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.

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(1) (Total 10 marks) Q9. The diagram shows the Maxwell-Boltzmann distribution of molecular energies in a gas at two different temperatures. (a) One of the axes is labelled. Complete the diagram by labelling the other axis. (1) (b) State the effect, if any, of a solid catalyst on the shape of either of these distributions.

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(3) (c) Strontium metal is used in the manufacture of alloys. (i) Explain why strontium has a higher melting point than barium.

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(2) (ii) Write an equation for the reaction of strontium with water.

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(1) (d) Magnesium can be used in the extraction of titanium. (i) Write an equation for the reaction of magnesium with titanium(IV) chloride.

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(1) (ii) The excess of magnesium used in this extraction can be removed by reacting it with dilute sulfuric acid to form magnesium sulfate. Use your knowledge of Group 2 sulfates to explain why the magnesium sulfate formed is easy to separate from the titanium.

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(1) (Total 9 marks) Q11. The diagram below shows the Maxwell–Boltzmann energy distribution curve for a sample of gas at a fixed temperature. Ea is the activation energy for the decomposition of this gas.

(a) On this diagram sketch the distribution curve for the same sample of gas at a higher temperature. (3) (b) (i) What is the effect of an increase in temperature on the rate of a chemical reaction? Explain your answer with reference to the Maxwell–Boltzmann distribution. Effect ________________________________________________________ Explanation ____________________________________________________

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(ii) What is the effect of the addition of a catalyst on the rate of a chemical reaction? Explain your answer with reference to the Maxwell–Boltzmann distribution. Effect ________________________________________________________ Explanation ____________________________________________________

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(6) (Total 9 marks) Q12. The gas-phase reaction between hydrogen and chlorine is very slow at room temperature. H 2 (g) + Cl 2 (g) → 2HCl(g) (a) Define the term activation energy.

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curve to represent the same sample of gas at a lower temperature. Label this curve T 2. Use these curves to explain why a small decrease in temperature can lead to a large decrease in the rate of a reaction.

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(8) (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.

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(5) (Total 15 marks) Q14. 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) Label the axes on the diagram. (2) (b) On the diagram draw a curve to show the distribution for this sample at a lower temperature. (2) (c) In order for two particles to react they must collide. Explain why most collisions do not result in a reaction.

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(1) (d) State one way in which the collision frequency between particles in a gas can be increased without changing the temperature.

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(1) (e) Suggest why a small increase in temperature can lead to a large increase in the reaction rate between colliding particles.

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