Dr. Arrington Exam 4 (100 points), ChemActivities Thursday, April 21, 2011

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1 Chemistry 124 Honor Pledge: Dr. Arrington Exam 4 (1 points), ChemActivities Thursday, April 21, 211 Show all work on numeric problems in Section II to receive full or partial credit. Give all answers with correct units. Your exam should contain 6 pages of questions. Multiple Choice: (3 points each) Place answer in the blank next to the problem. Only answers in this blank will be graded. Use the following initial rate data to answer questions 1 and 2. For the reaction, H 2 PO 2 + OH sd HPO H 2, the following data on initial rates are obtained. [OH ] o (M) [H 2 PO 2 ] o (M) Initial rate (M s 1 ) Exp x 1 6 Exp x 1 6 Exp x 1 6 Exp ??? 1. The rate equation for this reaction is: (A) rate = k [OH ] [H 2 PO 2 ] (B) rate = k [OH ] 2 [H 2 PO 2 ] (C) rate = k [OH ] 2 [H 2 PO 2 ] 2 (D) rate = k [OH ] [H 2 PO 2 ] 2 (E) rate = k [OH ] (F) rate = k [H 2 PO 2 ] 2. The initial rate expected for Experiment 4 is (A) M s 1 (B) M s 1 (C) M s 1 (D) M s 1 (E) M s 1 (F).11 M s 1 3. The mechanism for formation of the product X is: A + B sd C+ D A + D sd X +B The intermediate(s) in the reaction is(are) (A) C only (B) D only (C) A and B (D) C and D (E) B and D (F) X only 4. For the production of one mole of nitrosyl chloride (below), the activation energy for this reaction is 92.2 kj. The activation energy for the reverse reaction is NO(g) + ½Cl 2 (g) qwe NOCl(g) (A) 9. kj (B) kj (C) 92.2 kj (D) kj (E) 53.7 kj (F) 13.7 kj 5. Hydrogen peroxide decomposes by the following reaction, 2H 2 O 2 2H 2 O + O 2. This reaction is first order in hydrogen peroxide. When hydrogen peroxide disappears at a rate of 8. x 1 6 M s 1, what is the rate of formation of molecular oxygen? (A) 4. x 1 6 M s 1 (B) 8. x 1 6 M s 1 (C) 12. x 1 6 M s 1 (D) 16. x 1 6 M s 1. (E) 8. x 1 6 s 1 (F) 16. x 1 6 s 1

2 6. For a certain reaction, the rate law is found to be, rate = k[x] 3/2 [Y] 1/2. The units of the rate constant, k, are? 2 2 mole 1 L mole (A) (B) (C) (D) (E) s 2 2 L s mole s L mole s L s 7. Which plot of the reactant A would you expect for the reaction mechanism: A sd B (slow) B+ A sd P (fast) 8. The reactions equations and the rate laws for several reactions are given below. Of these which reaction could be an elementary reaction? i.) 2A P ; rate = k[a] 2 ii.) A P ; rate = k[a][p] iii.) A + 2B P ; rate = k[a][b] (A) i only (B) ii only (C) iii only (D) i and ii (E) i, ii, and iii (F) none are elementary 9. Acetylene reacts with atomic oxygen at room temperature according to the overall reaction 2O(g) + C2H2(g) 2 CO(g) + 2 H(g). The experimentally determined rate law is (d[o]/dt) = k [O] [C2H2]. Which mechanism is consistent with this rate law? (A) C2H2 2 CH slow (B) O + C2H2 C2H2O fast CH + O CO + H fast O + C2H2O 2 CO + 2 H slow (C) O + O O2 fast (D) O + C2H2 CO + CH2 slow O2 + C2H2 2 CO + 2 H slow O + CH2 CO + 2 H fast (E) None of these mechanisms is consistent with the rate equation. 1. Which chemical reactions show a decrease in half-life, t ½, as the reaction proceeds? (A) th order (B) 1 st order (C) 2 nd order (D) th order and 2 nd order (E) t ½ never decreases 11. Which one of the following will change the value of an equilibrium constant in a reaction mechanism? (A) changing temperature (C) decreasing the catalyst concentration (E) increasing the pressure (B) removing products (D) diluting the reaction mixture (F) increasing the catalyst concentration

3 12. A reaction is observed to be 3 rd order in A and 1 st order in B. If the concentration of A is doubled and B is halved (A) the rate doubles. (B) the rate triples. (C) the rate quadruples. (D) the rate is unchanged. 13. Which of the following statements is false I.) a catalyst increases the concentration of the products at equilibrium. II.) a catalyst concentration can be part of the experimental rate law. III.) a catalyst lowers the activation energy of the forward reaction but not the reverse reaction. IV.) a catalyst concentration is part of the equilibrium constant if the catalyst is in solution. (A) I only (B) II only (C) I and III (D) II and III (E) I and IV (F) I, III and IV 14. Dr. McCane is on an archeological dig in Palestine with a group of Wofford students during the summer. They find a sheep skin scroll sealed in a clay cask. The team is interested in the approximate date of the scroll. Analysis of the scroll shows that its beta decay rate from 14 C is 14 counts per hour while the same quantity of sheep skin from a recently killed sheep shows a decay rate of 16 counts per hour. Given a 14 C half-life of 573 years, your best estimate of the date of the scroll is (A) 14, BC (B) 74 BC (C) 2 BC (D) 11 AD (E) 9 AD (F) 182 AD 15. In which of the following nuclear reactions would the particle X be expected to be a positron? (A) (C) 3 (B) (D) 16. The energy released by our sun principally involves what nuclear reaction? (A) The conversion of hydrogen to helium (B) The conversion of uranium to xenon and strontium (C) The conversion of carbon 14 to nitrogen -14 (D) The conversion of helium to hydrogen (E) The conversion of uranium to iron 17. From the reaction coordinate diagram to the right, one would expect that at short reaction times (A) product W is formed as the kinetically preferred product (B) product W is formed as the thermodynamically preferred product (C) product Q is formed as the kinetically preferred product (D) product Q is formed as the thermodynamically preferred product 18. In the detection of radiation a Gieger counter I.) cannot distinguish background radiation from sample radiation. II.) creates a magnetic field that attracts fast moving radioactive particles. III.) over counts the available radiation because one particle releases a cascade of electrons. IV.) under counts the available radiation because most particles miss the detector. (A) I only (B) II only (C) I and III (D) I and IV (E) II and III (F) I, II, and III

4 Section II 19. (4 points) Circle the appropriate response. (a) Graph (a) shows the loss of reactants for a reaction at two different temperatures. Which trace represents the reaction at a lower temperature conditions (solid or dashed)? (b) Plot (b) represents the reactant vs. time data for a second order and a first order reaction. The second order reaction is the (solid or dashed) trace? (a) (b) [A] [A] time time 2. (6 points) Explain two distinct reasons that changing temperature changes the rate of a chemical reaction. 21. (9 points) During our study of kinetics we saw that for 1 st order reactions half-life was not dependent on initial concentration ( =. ). (a) In terms of the initial concentration, R, what is the concentration of reactants after the reaction has proceeded for one half-life? R = (b) Half of a half-life would be the length of time required to react one quarter of the initial reactant concentration. What is the amount of reactant remaining after one quarter of the initial reactants, R, have been consumed? R = (c) Create a formula similar to the half life formula above that determines the quarter-life (the length of time it takes to react one fourth of the initial concentration) given the first order rate constant k. t 1/4 =

5 22. (15 points) The decomposition of hydrogen peroxide is shown by the elementary reaction below, with an activation energy of 82.8 kj/mol. Measuring [H 2 O 2 ] as a function of time, the decomposition of hydrogen peroxide at 6 C has been monitored. The experimental data is plotted below. H 2 O 2 (g) H 2 O(g) + 1/2O 2 (g) Time (sec) (a) From the experimental data estimate the half-life of H 2 O 2 (g) at 6 C. (b) How long will it take for 95% of a hydrogen peroxide sample to decompose at 6 C? t 1/2 = time = (c) In order for a water treatment plant to use H 2 O 2 as a cleaner if a filtration system time they must have the H 2 O 2 decompose at a half-life of 3 seconds. At what temperature will H 2 O 2 decompose with a half-life of 3 seconds. T = K

6 24. (12 points) An important reaction in understanding urban pollution is: N 2 O 5 (g) sd 2NO 2 (g) + ½O 2 (g) Dr. Savage s Environmental Science class studied this reaction by preparing a reaction mixture that is initially (N 2 O 5 ) =.2 atm. The students have monitored the N 2 O 5 pressure for 1 hour. The students prepared the plots below, because their textbook indicated that this would help determine the reaction order. (a) Use the student data above to determine the rate law for the above reaction. Be sure to include the value of the rate constant in your rate law. (b) What is the total pressure in the reaction vessel after 45 minutes of reaction? rate = P Total = atm

7 -Useful Conversions, Chemical Data & Equations- h = 6.63 x 1 34 J s c = 3. x 1 8 m/s 4.18 joule = 1 calorie R =.821 L atm R = K mol J K mol g = 1 amu N A = 6.22 x atm = 76 mmhg 1 cm 3 = 1 ml 1 pm = m t 1 1 = ln = k t k A [ ] 1 2 = k t t1 2 = = k t k [ A 1 ( E A R T ) ] k( T ) = A e [ 1 2 = t A] 2k = e ( k t ) ln( E ) ln( k1) = R A k2 1 T2 1 T1 n ( 1 2) [ ] = A ln( k) = ln( A) EA ( R T) Compound H AC (kj/mol) NO Cl NOCl

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