Chemistry 30: Reaction Kinetics. Practice Problems

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1 Name: Period: Chemistry 30: Reaction Kinetics Practice Problems Date: Measuring Reaction Rates 1. A chemist wishes to determine the rate of reaction of zinc with hydrochloric acid. The equation for the reaction is: Zn (s) + 2 HCl (aq) H 2 (g) + ZnCl 2 (aq) A piece of zinc is dropped into 1.00 L of M HCl and the following data were obtained: Time (s) Mass of Zinc (g) a. Calculate the rate of reaction in grams of zinc per second. b. Calculate the rate of reaction in moles of zinc per second. c. What will happen to [H + ] as the reaction proceeds? (Recall that [ ] means concentration!) d. What will happen to [Cl - ] as the reaction proceeds? 2. On a set of axes, sketch the shape of the curve you would expect if you plotted [HCl] versus time, starting immediately after the two reactants were mixed. Explain why you chose that shape. The equation for the reaction is: Mg (s) + 2 HCl (aq) H 2 (g) + MgCl 2 (aq) 3. When pentane (C 5H 12) is burned in air (O 2), carbon dioxide and water are formed. a. Write the balanced formula for the reaction. b. If pentane is consumed at an average rate of 2.16 g/s, determine the consumption of pentane in mol/s. c. If pentane is consumed at an average rate of mol/s, determine the rate of consumption of oxygen in mol/s. 4. The following table relates the time and mass of zinc during the reaction between zinc and 0.5M nitric acid: Zn (s) + 2HNO 3 (aq) H 2 (g) + Zn(NO 3) 2 (aq) Time (s) Mass of Zinc (g) a. Calculate the reaction rate in g/s from 0 to 60 seconds. b. Calculate the reaction rate in g/s from 120 to 180 seconds. c. Why would these two values be different? 5. Consider the following reaction: 1

N 2(g) + 3 H 2(g) 2 NH 3(g) If the rate of decomposition of N 2(g) is 0.03 mol/l s, what would you expect to be: a. The approximate rate of decomposition of H 2? b. The approximate rate of formation of NH 3?

2 N 2(g) + 3 H 2(g) 2 NH 3(g) If the rate of decomposition of N 2(g) is 0.03 mol/l s, what would you expect to be: a. The approximate rate of decomposition of H 2? b. The approximate rate of formation of NH 3? 6. Consider the following graph, which shows the reaction rate of a species in a system. a. What is the reaction rate of the compound over the first ten seconds? b. What is the reaction rate of the compound over the first 50 seconds? c. What is the instantaneous rate of reaction at 6 s? d. What is the instantaneous rate of reaction at 40 s? Collision Theory and Energy Diagrams 7. Explain the three requirements for a successful reaction. 8. Answer the following questions based on the potential energy diagram shown here: a. Does the graph represent an endothermic or exothermic reaction? b. Label the position of the reactants (R), products (P), and activated complex (AC). c. Determine ΔH rxn for this reaction. d. Determine the activation energy, E a for this reaction. 2

3 9. Sketch a potential energy curve that is represented by the following values of ΔH rxn and E a. The energy of the reactants is 40 kj. ΔH rxn = -20 kj E a = 60 kj Label the reactants (R), the products (P), the activated complex (AC), ΔH and E a. Is this an endothermic or exothermic reaction? 10. Sketch a potential energy curve that is represented by the following values of ΔH rxn and E a. The energy of the products is 30 kj. ΔH rxn = 20 kj E a = 50 kj Label the reactants (R), the products (P), the activated complex (AC), ΔH and E a. Is this an endothermic or exothermic reaction? 11. Sketch a potential energy curve that is represented by the following values. The energy of the reactants is 40 kj and the energy of the products is 80 kj. E a for the reaction is 60 kj. Label the reactants (R), the products (P), the activated complex (AC), ΔH and E a. Is this an endothermic or exothermic reaction? 12. Draw the collision theory diagram for H 2 + Cl 2 2 HCl, including the reactants, activated complex and products. 13. Draw the collision theory diagram for 2 Mg + O 2 2 MgO, including the reactants, activated complex and products. Factors Affecting Reaction Rate 14. Consider the following reaction that occurs between hydrochloric acid, HCl, and zinc metal: HCl (aq) + Zn (s) H 2 (g) + ZnCl 2 (aq) a. Will this reaction occur fastest using a 6 M solution of HCl or a 0.5 M solution of HCl? Explain. b. Which reaction will take longer? Why? 3

4 15. Again consider the reaction between hydrochloric acid and zinc. How will increasing the temperature of the acid affect the rate of the reaction? Explain. 16. Based on the following kinetic energy curves: a. Which reaction will have a faster rate - A or B? Explain. b. Which reaction, A or B, would benefit most in terms of increased rate if the temperature of the system were increased? A B 17. On the grid below, sketch how the Maxwell-Boltzmann curve will change for: a. A temperature increase b. A temperature decrease c. The addition of a catalyst 18. On the grid to the right: a. Sketch a potential energy curve that has reactants at 50 kj, products at 20 kj and an activation energy of 50 kj. b. Label the reactants (R), the products (P), the activated complex (AC), ΔH and E a. c. Draw a second curve on the same graph that shows the same process if a catalyst is added that lowers the activation energy to 20 kj. d. What is the benefit of using a catalyst? 19. An inhibitor slows down a chemical reaction in the same way that a catalyst speeds up a reaction. a. What other ways can a reaction be slowed down? b. Why would you want a reaction to occur more slowly? Give an example of a situation where this would be preferable. 20. In a room filled with hydrogen and oxygen gas, there are about collisions per second. a. Give a reason why this reaction at room temperature is so slow as to be unnoticed. b. Suggest two ways that the reaction rate could be increased. 4

5 Reaction Rate Laws 21. Write the experimental rate law for each of the sets of data. Reaction 1: aa + bb products Trial Initial [A] (mol/l) Initial [B] (mol/l) Initial Rate (mol/l s) Reaction 2: A products Trial Initial [A] (mol/l) Initial Rate (mol/l s) Reaction 3: aa + bb products Trial Initial [A] (mol/l) Initial [B] (mol/l) Initial Rate (mol/l s) Reaction 4: aa + bb products Trial Initial [A] (mol/l) Initial [B] (mol/l) Initial Rate (mol/l s) Reaction 5: aa + bb + cc products Trial Initial [A] (mol/l) Initial [B] (mol/l) Initial [C] (mol/l) Initial Rate (mol/l s) For each of the reactions above, indicate the reaction order. Reaction Mechanisms 23. Given the following reaction mechanism, determine the equation for the overall reaction. Step 1: A 2 (g) 2 A(g) Step 2: Step 3: Step 4: 2 A(g) + 2 B (g) 2 AB(g) 2 AB(g) + C 2(g) 2 ABC(g) 2 ABC(g) 2 AC(g) + 2 B(g) Overall: List the reaction intermediates for this reaction. 5

6 24. Hydrogen peroxide reacts with hydrogen ions and iodide ions according to the following reaction mechanism: Step 1. H + + H 2O 2 H 3O 2 + fast Step 2. H 3O I - H 2O + HOI slow a. Write the overall reaction described by this mechanism Step 1. H + + H 2O 2 H 3O 2 + Step 2. H 3O I - H 2O + HOI Overall: b. If you wanted to increase the rate of the overall reaction, would it be better to increase the concentration of H + or I -? Explain why. 23. Phosgene, COCl 2, one of the poison gases used during World War I, is formed from chlorine and carbon monoxide. The mechanism is thought to proceed by: Step 1: Cl + CO COCl Step 2: COCl + Cl 2 COCl 2 + Cl a. Write the overall reaction equation. b. Identify any reaction intermediates. c. Identify any catalysts. 24. Consider the energy diagram for a two-step reaction: a. What is ΔH for the overall reaction? b. What is ΔH for the first step of the reaction mechanism? c. What is ΔH for the second step of the reaction mechanism? e. What is E a for the first step? f. What is E a for the second step? g. Which is the rate-determining step - step 1 or step 2? How do you know? i. Is the overall reaction endothermic or exothermic? 6

7 25. Draw a potential energy diagram that shows a series of two reactions: Reaction 1 has reactants at 40 kj and activation energy of 60 kj. Reaction 2 has an activation energy of 20 kj and ΔH = -20 kj. The overall ΔH is -40 kj. a. Label E a1, E a2, ΔH 1, ΔH 2 and ΔH rxn. b. Which reaction step is the ratedetermining step? c. What is ΔH for the first reaction step? 26. Draw a potential energy diagram that shows a series of two reactions: Reaction 1 has reactants at 40 kj and activation energy of 60 kj. Reaction 2 has reactants at 60 kj and an activation energy of 20 kj. The final products are at 30 kj. a. Label E a1, E a2, ΔH 1, ΔH 2 and ΔH rxn. b. Which reaction step is the ratedetermining step? c. What is ΔH for the first reaction step? d. What is ΔH for the second reaction step? e. What is ΔH rxn? Is this an endothermic or exothermic process? 27. Given the reaction: HCOOH CO + H 2O a. This reaction, without a catalyst, is very slow at room temperature. Suggest why. b. This reaction is thought to take place by means of the following mechanism: i. HCOOH + H + HCOOH 2 + (fast) ii. HCOOH 2 + H 2O + HCO + (slow) iii. HCO + CO + H + (fast) Identify the reaction intermediates. c. Which intermediate is a catalyst? d. Another catalyst can be added that will increase the rate of step 3. Would it be worth it to add this catalyst? Why or why not? e. Which step has the greatest activation energy? 7

Chemistry 12 Review Sheet on Unit 1 -Reaction Kinetics

Chemistry 12 Review Sheet on Unit 1 -Reaction Kinetics 1. Looking at the expressions for reaction rate on page 1 SW, write similar expressions with which you could express rates for the following reactions.