Dr. Steward s Spring 2014 Exam #1 Kinetics, Equilibrium Review

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1 Caveat Lector: This review cannot possibly cover all of the types of problems that could be on the exam. Use this review as a good starting point to study. Dr. Steward s Spring 2014 Exam #1 Kinetics, Equilibrium Review 1. Consider the following reaction, 2 ClO 2 (aq) + 2 OH - (aq) -> ClO 3 - (aq) + ClO 2 - (aq) + H 2 O (l) Write an expression that describes the relationship between the rates of disappearance of ClO 2 and OH - and the rates of appearance of ClO 3 -, ClO 2 - and H 2 O 2. Consider the following reaction in aqueous solution, 5Br - (aq) + BrO 3 - (aq) + 6H + (aq) --> 3Br 2 (aq) + 3H 2 O(l) If the rate of appearance of Br 2 at a particular moment during the reaction is M s -1, what is the rate of disappearance (in M s -1 ) of Br - at that moment? 3. Consider the following reaction at 25 o C, (CH 3 ) 3 COH(l) + HCl(aq) --> (CH 3 ) 3 CCl(l) + H 2 O(l) The experimentally determined rate law for this reaction indicates that the reaction is first-order in (CH 3 ) 3 COH and that the reaction is first-order overall. Which of the following would produce an increase in the rate of this reaction? a. increasing the concentration of (CH 3 ) 3 COH b. increasing the concentration of HCl c. decreasing the concentration of HCl d. decreasing the concentration of (CH 3 ) 3 CCl e. It is impossible to tell. 4. A certain first-order reaction has a rate constant, k, equal to 2.1 x 10-5 s -1 at 355 K. If the activation energy for this reaction is 135 kj/mol, calculate the value of the rate constant (in s -1 ) at 550 K. 5. Laughing gas, N 2 O, can be prepared (ha, ha!) from H 2 and NO: H 2 (g) + 2 NO(g) --> N 2 O(g) + H 2 O(g) A study of initial concentration (ha, ha!) versus initial rate at a certain temperature yields the following data for this reaction (ha, ha!): [H 2 ], M [NO], M initial rate, M s x x x x 10-6

2 Determine the rate law for the reaction. Determine the value of the rate constant, k. 6. The following data were obtained for a reaction at 25 o C, A + 3B 2C + D time, min [A], M Determine the average rate of disappearance of A from t 0 to t 36. Determine the average rate of disappearance from t 0 to t 6. What reactant / product would have the same rate of disappearance / appearance as A? Explain. 7. Which of the following are the correct units for the rate constant, k, for a zero-order reaction? a. M s -1 b. M -1 s -1 c. M -2 s -1 d. M -3 s -1 e. M 8. Radioactive phosphorus is used in the study of biochemical reaction mechanisms. The isotope phosphorus-33 decays by first-order kinetics with a half-life of 14.3 days. If a chemist initially has a 7.5 M solution of pure phosphorus-33, calculate the concentration (in M) of phosphorus-33 in the solution after 2.4 days. 9. Consider the following reaction: 2 NOBr(g) -> 2 NO(g) + Br 2 (g) The reaction is known to be second order with k = 0.80 L/mol s. If you start with a concentration of mol/l of NOBr, what will be its concentration after 22 seconds?

3 10. The reaction 2 NO 2 + O 3 -> N 2 O 5 + O 2 obeys the rate law, Rate = k observed [NO 2 ][O 3 ] Which of the following mechanisms is consistent with this experimental rate law? (a) NO 2 + NO 2 <=> N 2 O 4 (fast equilibrium) N 2 O 4 + O 3 -> N 2 O 5 + O 2 (slow) (b) NO 2 + O 3 -> NO 5 NO 5 + NO 5 -> N 2 O 5 + 5/2 O 2 (c) NO 2 + O 3 -> NO 3 + O 2 NO 3 + NO 2 -> N 2 O 5 (slow) (slow) (d) NO 2 + NO 2 -> N 2 O 2 + O 2 (slow) N 2 O 2 + O 3 -> N 2 O 5 (e) None of these mechanisms are possible. 11. The radioactive element, thallium-201, is used medicinally as a radiotracer to study damage in heart tissue. If a patient is injected with a g dose of pure thallium-201, calculate the amount of time that would be required for the amount of thallium-201 in the patient's body to reach Thallium-201 decays by a first-order process with a half-life, t 1/2 = 73.0 hours. 12. Homes in certain parts of the country contain high levels of the radioactive isotope, radon-222. Radon-222 decays by first-order kinetics with a half-life of 3.82 days. Calculate how long it would take for 95% of a sample of radon-222 to decay. 13. The reaction in which NO 2 forms a dimer, 2NO 2 (g) <==> N 2 O 4 (g) has the following experimentally determined rate law: Rate = k[no 2 ] 2 where k = 400 L mol -1 s -1 How long (in seconds) would it take for a sample of NO 2 with an initial concentration of 0.50 M to decrease to a concentration of M? USE THE FOLLOWING INFORMATION TO ANSWER THE NEXT THREE (3) QUESTIONS. Mn n+ Tl + + 2Ce > Tl Ce 3+ The experimentally determined rate law is: Rate = k[ce 4+ ][Mn 2+ ] with the following proposed mechanism involving ions of manganese:

4 Step 1: Ce 4+ + Mn 2+ --> Ce 3+ + Mn 3+ Step 2: Ce 4+ + Mn 3+ --> Ce 3+ + Mn 4+ Step 3: Tl + + Mn 4+ --> Tl 3+ + Mn Which ion of manganese is the catalyst according to the information provided above? 15. Which step in the proposed mechanism is the rate-limiting step? 16. What is the overall order of the reaction? Equilibrium (Use the equation for the next 6 questions). At 300 o C, gaseous sulfur trioxide decomposes into gaseous sulfur dioxide and oxygen as follows: 2 SO 3 2 SO 2 + O 2, K c = 1.6 x Describe, in relative terms, the amount of reactants and products there should be at equilibrium. 2. Calculate K c of the reverse reaction. 3. Calculate K p of the forward 25.0ºC. 4. If you started with each chemical species at 1M, describe how the reaction would proceed and why. 5. Describe what would happen to the reaction if more SO 3 were added to the equilibrium mixture. 6. Describe what would happen if the volume of the container were increased after the reaction reached equilibrium. 7. For the reaction A(aq)+B(aq) 2C(aq) with a K c of 5.0, determine the equilibrium concentrations of all species starting with 3.0M for both A and B. Before you set up this problem, answer the following question: Is it reasonable to assume that the change in x is small and thus can be ignored?

5 8. Use the following reaction to answer the following questions about how each change with affect the reaction at equilibrium. CaCO 3 (s) CaO(s) + CO 2 (g) ΔH = + Addition of CaCO 3 : Addition of CaO: CaO: CaCO 3 : CO 2 : CO 2 : Addition of heat: Addition of CO 2 : CaCO 3 : CaO: CaO: CaCO 3 : CO 2 :