Reaction Rates Review with Answers

Reaction Rates Review CHEMISTRY 12 1. Page 3 REACTION RATES REVIEW Given the following reaction, which one of the following changes would increase the rate of evolution of CO2 ? CaCO3 (s) + 2 H+(aq) ® Ca2+(aq) + H2O(l) + CO2 (g) + heat a) b) c) d) 2. Which of the following represent an endothermic reaction ? a) b) c) d) 3. Decrease the temperature of the system. Increase the concentration of Ca2+(aq). Finely powder the CaCO3 (s). Add water to the system. H2 (g) ® 2 H (g) DH = 436 kJ/mol ½ H2 (g) + ½ Br2 (l) ® HBr (g) DH = -36.23kJ/mol ½ H2 (g) + ½ Cl2 (g) ® HCl (g) + heat H2 (g) + ½ O2 (g) ® H2O (g) + 241.8 kJ In the following P.E. diagram, which interval represents the enthalpy change, DH, for the reaction depicted ? a) b) c) d) e) I II III IV None of the above Reaction Rates Review 4. Page 4 Which one of the following is the basic premise expressed in Collision Theory ? a) Chemical reactions can occur only if reacting particles collide. b) At a high temperature, more gas particles collide than at a low temperature. c) In order to react, particles must have the correct geometry when they collide. d) Catalysts affect the rate at which reacting particles collide. 5. At 25oC and considering only the nature of the reactants, which one of the following reactions most probably has the highest rate? a) b) c) d) Ca2+(aq) + CO32-(aq) ® CaCO3 (s) CH4 (g) + 2 O2 (g) ® CO2 (g) + 2 H2O (g) H2 (g) + I2 (g) ® 2 HI (g) C (s) + O2 (g) ® CO2 (g) 6. For the reaction depicted by the above potential energy diagram, which one of the following statements is true about the rate of the reaction ? a) It depends on the concentration of the reactants and upon the temperature. b) It depends upon the concentration of the reactants but is temperature independent. c) It depends upon the temperature but is independent of the concentration. d) It is independent of both the concentration of the reactants and the temperature. Reaction Rates Review 7. In the following potential energy diagram, which interval represents the activation energy for the reaction proceeding from right to left ? a) b) c) d) 8. The mass of the system. The pressure of the system. The concentration of water. The concentration of the Cl-(aq). Which of the following factors affects the rate of heterogeneous reactions only ? a) b) c) d) 10. I II III IV The following reaction occurs at constant temperature and constant volume in a closed system : + 2+ CaCO3 (s) + 2 H (aq) + 2 Cl (aq) ® CO2 (g) + H2O (l) + Ca (aq) + 2 Cl (aq) Changes in which one of the following would be useful in experimentally measuring the rate of this reaction ? a) b) c) d) 9. Page 5 Presence of a catalyst Temperature of reactants Concentration of reactants Surface area of reactants Which of the following changes would lead to a decrease in enthalpy ? a) b) c) d) NaCl (s) ® NaCl (l) H2O (l) ® H2O (g) H2O (s) ® H2O (l) H2O (g) ® H2O (l) Reaction Rates Review 11. Which one of the following reactions is MOST likely to have the highest reaction rate ? a) b) c) d) 12. Page 6 2H2 (g) + O2 (g) ® 2H2O(g) Mg2+(aq) + 2OH-(aq) ® Mg(OH)2 (s) C2H5OH(l) + 3O2 (g) ® 2CO2 (g) + 3H2O (g) 2MnO4-(aq) + 16H+(aq) + 5C2O42-(aq) ® 2Mn2+(aq) + 10CO2 (g) + 8H2O (g) Which of the following statements best describes the enthalpy change (DH) of a reaction ? a) DH is the maximum amount of useful work obtainable from a reaction at a given temperature. b) DH measures the change in randomness of a reaction taking place at a constant temperature. c) DH is equal to the energy required to initiate a chemical reaction under given conditions. d) DH is equal to the change in energy content of the system at a constant temperature and pressure. 13. on Which one of the following statements correctly describes the effect of catalysts reaction rates ? a) They have no effect on reaction rate. b) They increase the rates of both the forward and reverse reactions. c) They decrease the rate of the reverse reaction but not that of the forward reaction. d) They increase the rate of the reverse reaction but not that of the forward reaction. 14. Which of the following statements about catalysts is true ? a) b) c) d) 15. Catalysts provide energy for reactions. Catalysts provide a lower energy pathway for a chemical reaction. Catalysts are always in the same phase as the reactant molecules. Catalysts can change endothermic reactions into exothermic reactions. Coal dust is found to burn more rapidly than a lump of coal. The FACTOR that increases the rate of this chemical reaction is the a) b) c) d) presence of a catalyst. surface area of the reactants. concentration of the reactants. chemical properties of the reactants. Reaction Rates Review Page 7 16. Which of the following can be inferred from the graph above ? a) The reverse reaction is endothermic. b) The reaction mechanism has two elementary processes. c) The energy of activation of the forward reaction is greater than that of the reverse reaction. d) As the temperature increases, the rate of the forward reaction increases more than the rate of the reverse reaction. 17. Which would be the best procedure to use to increase the rate of the following reaction ? PCl5 (g) ® PCl3 (g) + Cl2 (g) a) b) c) d) 18. At 10oC, a small piece of Zn reacts with 2.0 M HCl to produce 15.0 mL of H2 gas in 25 s. What is the rate of this reaction ? a) b) c) d) 19. Increase temperature. Decrease temperature. Decrease pressure. Increase surface area. 0.060 mol Zn per second 0.4 C per second 0.60 mL H2 per second 0.80 mol/L HCl per second For the reaction, Zn (s) + S (g) ® ZnS (s) DH = -460 kJ, which of the following would increase the rate of the reaction ? a) b) c) d) Change Zn (s) to a fine powder. Decrease the pressure. Decrease the temperature. Remove ZnS (s) as it is formed. Reaction Rates Review Page 8 *** Use the following diagram for the next two questions. 20. Given the above potential energy diagram for a catalyzed and uncatalyzed reaction, the DH for the forward catalyzed reaction is a) b) c) d) 21. The activation energy for the forward uncatalyzed reaction is a) b) c) d) 22. -20 kJ -15 kJ +5 kJ +15 kJ 10 kJ 15 kJ 25 kJ 30 kJ The reaction 2 A + B + C ® D takes place through the following mechanism in which I1 and I2 represent reaction intermediates. Step 1 (slow) : A + B ® I1 Step 2 (fast) : I1 + A ® I2 Step 3 (fast) : I2 + C ® D How can you produce a significant increase in the rate of the overall reaction ? a) b) c) d) Increase [B] Decrease [B] Increase [C] Decrease [A] Reaction Rates Review 23. The potential energy diagram for the formation of C6H10 from C2H6 is given below. What is the DH of this reaction ? a) b) c) d) 24. -287 kJ -172 kJ +172 kJ +287 kJ Which of the following reactions would be expected to proceed at the greatest rate at room temperature ? a) b) c) d) 25. Page 9 Mg (s) + ½O2 (g) ® MgO (s) Br2 (g) + Cl2 (g) ® 2 BrCl (g) 2Ag+(aq) + CrO42-(aq) ® Ag2CrO4 (s) 2BrO3-(aq) + 10Fe2+(aq) + 12H+(aq) ®10Fe3+(aq) + Br2(aq) + 6H2O (l) Which of the following effects does a catalyst have on a chemical reaction ? a) It decreases the rate of the reverse reaction. b) It increases the forward rate but has no effect on the reverse rate. c) It provides an alternative reaction mechanism with a lower activation energy. d) It decreases the magnitude of the equilibrium constant for the reaction. 26. When the temperature of a gaseous mixture is increased from 10oC to 20oC, it is found that the reaction rate doubles. Which of the following could be deduced from this information ? a) At the higher temperature all collisions lead to reaction. b) The average kinetic energy has been doubled by raising the temperature. c) The frequency of effective collisions has been doubled. d) The activation energy has been reduced by raising the temperature. Reaction Rates Review 27. Page 10 Phosphorus ignites readily on exposure to air as below : P4 (s) + 5 O2 (g) ® P4O10 (s) DH = -2980 kJ/mol At a given temperature, which of the following sets of factors determines the rate of the above reaction ? a) The partial pressure of the oxygen only. b) The concentration of phosphorus and the volume of oxygen. c) The concentration of phosphorus and the partial pressure of the oxygen. d) The surface area of the phosphorus and the partial pressure of the oxygen. 28. Which of the following equations represents an endothermic reaction ? a) b) c) d) 29. One way of increasing the rate of a reaction is to a) b) c) d) 30. increase the activation energy of the reaction. increase the frequency of collision of reactant molecules. decrease the overall enthalpy change of the reaction. increase the potential energy of the activated complex. The rate-determining step in a reaction mechanism is always the a) b) c) d) 31. 2 BrCl (g) - 29.3 kJ ® Br2 (g) + Cl2 (g) CaO (s) + CO2 (g) ® CaCO3 (s) + 178 kJ H2 (g) + I2 (g) ® 2 HI (g) DH = +13.4 kJ H2 (g) + ½O2 (g) ® H2O (g) DH = -241 kJ final step. first step. fastest step. slowest step. H + I ®J J + K ®L + H L ®M + N In the above reaction mechanism, which species is acting as a catalyst ? a) H b) I c) J d) L Reaction Rates Review 32. 35. A possible mechanism for a reaction between selenium and oxygen is Step 1 : Se + O2 ----> SeO2 (slow) Step 2 : SeO2 + O2 ----> 2 SeO (fast) Which graph illustrates the potential energy changes for this reaction mechanism? a) b) c) d) A 10oC increase in temperature frequently doubles the rate of reaction because a) b) c) d) 36. the increase represents a doubling of the temperature the average PE of the colliding molecules is doubled the average combined KE of the colliding molecules is doubled the fraction of molecules with sufficient KE to react is doubled Which one of the following statements is TRUE about the activated complex in a reaction ? a) b) c) d) 37. Page 11 It acts as a catalyst. It is a stable compound. It always forms products. It possesses more energy than the reactants or the products. Which one of the following describes one effect of a catalyst ? a) It increases the total energy of the products. b) It decreases the energy released in a reaction. c) It provides a new mechanism for the reaction involving a lower activation energy. d) It speeds up the rate of reaction but is used up in the overall process. Reaction Rates Review 38. Which one of the following terms refers to the slowest step in a reaction mechanism ? a) b) c) d) 39. The forward rate only will increase. The reverse rate only will increase. The forward and reverse reaction rates will both increase. The forward and reverse reaction rates will both remain unchanged. Which of the following factors affects the rate of heterogeneous reactions only ? a) b) c) d) 41. Uncatalyzed. Rate-determining. Activated complex. Activation energy. Which one of the following statements is TRUE about what occurs after a catalyst has been added to a system ? a) b) c) d) 40. Page 12 Presence of a catalyst. Temperature of the reactants. Surface area of the reactants. Concentration of the reactants. Which of the following statements best explains the increase in the rate of a chemical reaction when the temperature is increased? a) b) c) d) Activation energy is raised. A new reaction path is provided. The percentage of effective collisions is increased. There is more favorable collision geometry. Reaction Rates Review 42. A sample of magnesium having a mass of 0.360 g is dropped into dilute hydrochloric acid. At the end of 4.00 minutes, the magnesium is removed and it is found to have a mass of 0.240 g. The average rate at which the reaction took place was a) b) c) d) 43. 0.003 mol/min 0.120 mol/min 2.06 x 10-5 mol/sec 8.33 x 10-5 mol/sec Consider the following reaction : CaCO3 (s) + 2 HCl (aq) ® CO2 (g) + CaCl2 (aq) + H2O (l) + heat In which of the following will BOTH of the described procedures cause an increase in the rate at which products are formed ? a) b) c) d) 44. Page 13 Increase [HCl] and decrease pressure Increase [HCl] and increase temperature Increase [HCl] and decrease temperature Grind up the CaCO3 and decrease temperature The following equation represents the reaction between copper metal and aqueous silver nitrate solution : Cu (s) + 2 AgNO3 (aq) ® Cu(NO3)2 (aq) + 2 Ag (s) brown solid colourless solution blue solution grey solid Which of the following properties would BEST monitor the rate of this reaction ? a) b) c) d) 45. Which of the following is an endothermic reaction ? a) b) c) d) 46. concentration of the NO3-(aq) gas pressure colour of the solution mass of the system S + O2 ® SO2 DH = -297 kJ NO2 - 33.8 kJ ® ½N2 + O2 N2 + O2 + 90.4 kJ ® 2 NO N2H4 + O2 ® N2 + 2 H2O + 627.6 kJ Consider the reaction : Mg (s) + 2HNO3 (aq) ® Mg(NO3)2 (aq) + H2 (g) + heat Which of the following procedures will increase the rate of the reaction ? a) b) c) d) Precipitate out the Mg(NO3)2 Allow the H2 (g) to escape Decrease the temperature Increase the [HNO3] Reaction Rates Review Page 14 **** Use the following graph to answer the following 3 questions. **** 47. What is DH for the catalyzed reaction ? a) b) c) d) 48. What is the activation energy for the forward uncatalyzed reaction? a) b) c) d) 49. -20 kJ +20 kJ +30 kJ +55 kJ What is the activation energy for the reverse catalyzed reaction ? a) b) c) d) 50. -23 kJ -10 kJ +10 kJ +48 kJ +19 kJ +23 kJ +30 kJ +48 kJ Which of the following reactions is most likely to have the greatest reaction rate at room temperature ? a) b) c) d) Pb2+(aq) + 2Cl-(aq) ® PbCl2 (s) H2 (g) + Cl2 (g) ® 2 HCl (g) C2H4 (g) + 3 O2 (g) ® 2CO2 (g) + 2H2O (g) Zn (s) + S (s) ® ZnS (s) Reaction Rates Review 51. Page 15 Which interval in the potential energy diagram below represents the DH value for the reaction ? a) b) c) d) 1 2 3 4 52. the The energy diagram below represents a four step reaction mechanism. Which is rate-determining step? 53. a) 1 b) 2 c) 3 d) 4 Consider the following SLOW reaction : Reaction Rates Review Page 16 CO (g) + NO2 (g) ® CO2 (g) + NO (g) + heat Which of the following potential energy diagrams BEST represents the above reaction ? a) b) c) 54. Consider the reaction : P2O (g) + O2 (g) ® P2O5 (g) + 114 kJ This reaction may be described as a) b) c) d) 55. d) exothermic and DH = -114 kJ endothermic and DH = 114 kJ exothermic and DH = 114 kJ endothermic and DH = -114 kJ Equal volumes of I-(aq) and H2O2 (aq) solutions were individually reacted with 0.10 M Cr2O72-(aq) and the following data were obtained : Reactant IH2O2 Concentration 0.10 M 0.20 M Temp. 20oC 30oC Time for complete reaction 1.5 s 12 s The factor that accounts for the shorter reaction time for I- is a) b) c) d) 56. temperature surface area concentration nature of reactants Consider the following reaction occurring in a closed Reaction Rates Review Page 17 container : Br2 (g) + Cl2 (g) ® 2 BrCl (g) DH = -61 kJ Which change will increase the frequency of effective collisions between Br2 and Cl2 molecules ? a) b) c) d) 57. Which interval in the diagram below represents the activation energy for the overall reaction ? a) b) c) d) 58. Remove some BrCl (g) Decrease the pressure Increase the temperature Increase the container volume 1 2 3 4 A reaction mechanism is a) b) c) d) the order in which the chemicals are mixed together the experimental apparatus used for a chemical reaction the combination of reactants used to carry out a reaction the series of collisions resulting in the overall reaction Reaction Rates Review 59. When reacting zinc with sulphuric acid, which experimental conditions would be expected to result in the greatest rate ? a) b) c) d) 60. X + Y + 20 kJ ® 2 Z X + Y ® 2 Z + 20 kJ X + Y ® 2 Z DH = 10 kJ X + Y ® 2 Z DH = -10 kJ An activated complex may be described as a molecular species which is a) b) c) d) 62. 0.50 M H2SO4 , powdered zinc, 50oC 1.0 M H2SO4 , powdered zinc, 50oC 1.0 M H2SO4 , block of zinc, 50oC 1.0 M H2SO4 , powdered zinc, 25oC The reaction depicted below is correctly represented as a) b) c) d) 61. Page 18 long-lived and has low PE short-lived and has low PE short-lived and has high PE long-lived and has high PE For the overall reaction : OCl- + I- ® OI- + Cl- the mechanism is as follows : Step 1 : OCl- + H2O ® HOCl + OHStep 2 : I- + HOCl ® HOI + ClStep 3 : HOI + ??? ® H2O + OIThe missing particle in step 3 is a) b) c) d) ClOHOClHOCl Reaction Rates Review Page 19 For the remainder of the questions, marks will be awarded as shown. Your steps and assumptions leading to a solution must be shown. In questions involving calculations, full marks will not be given for providing only an answer. Students will be expected to communicate the knowledge and understanding of chemical principles in a clear and logical manner. 1. Define activated complex. (2 marks) 2. Explain how a catalyst increases the rate of a chemical reaction. (2 marks) 3. Describe TWO ways of increasing the rate of the following reaction, other than by using a catalyst, and explain in terms of molecular behavior why each method would be successful. (4 marks) 2 NOCl (g) ® 2 NO (g) + Cl2 (g) DH = -56 kJ 4. For the reaction that follows, state two properties that might be monitored in order to determine the rate of the forward reaction. (2 marks) CaCO3 (s) + 2 HCl (aq) ® CaCl2 (aq) + CO2 (g) + H2O (l) 5. Explain why the mechanism for the reaction below would probably involve more than the single step implied by the equation. (2 marks) 2 NO (g) + Br2 (g) ® 2 NOBr (g) 6. The atmospheric decomposition of ozone is a slow, two-step reaction with the overall equation : 2 O3 ® 3 O2 The first step in this decomposition is O3 ® O2 + O (fast) a) Write the equation for the second step. (1 mark) b) Which is the rate determining step ? (1 mark) 7. A chemist wishes to determine the rate of reaction of magnesium with hydrochloric acid. The equation for the reaction is : Mg(s) + 2 H+(aq) ® Mg2+(aq) + H2 (g) A piece of magnesium was dropped into 1.00 L of 0.100 M HCl and the following data were collected : Time Mass of Mg 0s 0.012 g 2s 0.010 g 4s 0.008 g 6s 0.006 g 8s 0.004 g a) Calculate the rate of reaction in terms of moles of Mg per second. (3 marks) b) As this magnesium reacts with hydrochloric acid, does the pH increase or decrease ? Support your answer with an explanation. Reaction Rates Review Page 20 8. A reaction has the following mechanism : Step 1 : HCOOH + H+ ® HCOOH2+ fast Step 2 : HCOOH2+ ® HCO+ + H2O slow Step 3 : HCO+ ® H+ + CO fast a) Which is the rate determining step ? (1 mark) b) What is the NET equation for the overall reaction ? (1 mark) 9. Describe what happens to the kinetic energy, the potential energy, and the total energy of reactant molecules as they approach one another. (2 marks) 10. Explain why a lower activation energy leads to a greater reaction rate at a given temperature. (2 marks) Reaction Rates Review Page 21 ANSWERS TO REACTION KINETICS REVIEW 1. 2. 3. 4. 5. C A E A A 16. 17. 18. 19. 20. A A C A B 31. 32. A A D 46. 47. 48. 49. 50. D B B B A 35. 6. 7. 8. 9. 10. A D B D D 21. 22. 23. 24. 25. A A B C C 36. 37. 38. 39. 40. D C B C C 51. 52. 53. 54. 55. C D A A D 11. 12. 13. 14. 15. B D B B B 26. 27. 28. 29. 30. C D C B D 41. 42. 43. 44. 45. C C B C C 56. 57. 58. 59. 60. C B D B B 61. 62. C B Written responses : 1. An activated complex is a high-energy, unstable, short lived configuration of reactant atoms or molecules. 2. A catalyst provides an alternate lower energy pathway for the reaction to proceed by either combining with the reactant(s) to form an intermediate compound or by providing a site for the reaction to occur. 3. a) Increasing the temperature would cause the rate of the reaction to increase because the molecules that are colliding would collide at a greater rate (more often). This would happen because of the extra speed at which the molecules would be travelling due to the increase in energy inputted into the system. b) Increasing the pressure of the system would allow the molecules that are reacting to collide more frequently therefore the reaction rate would increase. NOTE : By increasing both of the above, the position of the equilibrium would also change and the result may be actually favouring the reactants but this shift in position was not asked for in the question. 4. a) pH : As the reaction progresses, H+ ions are used up, therefore the [H+] decreases and pH increases. b) Gas pressure : If the system is closed, the release of CO2 (g) as the reaction proceeds would cause the pressure in the system to increase. 5. The possibility of 2 molecules of NO and 1 of Br2 colliding at the same time with the proper energy and geometry (as the equation suggests) is infinitesimally small therefore it is more likely that the reaction mechanism is to be a two (or more) step process to allow the reactants to come together one step at a time. 6. a) O3 + O ® 2 O2 b) The second step is the rate determining step. Reaction Rates Review Page 22 7. a) 4 x 10-5 mol/s b) pH increases because some H+ ions are used up in the reaction therefore the [H+] decreases increasing the pH. 8. a) Step 2 is the rate determining step. b) HCOOH ® H2O + CO 9. Kinetic energy of molecules decreases as the molecules approach each other. Potential energy of molecules increases as the molecules approach each other. The total amount of energy of reactant molecules remains constant. 10. A lower activation energy means that reactant molecules need less kinetic energy in order to react, therefore, at a given temperature, a greater number of reactant molecules will possess the necessary kinetic energy, thus a greater reaction rate results.