Chem 401 Unit 1 Exam: Thermodynamics & Kinetics (Nuss: Spr 2018)

Transcription

1 Date: Exam # Chem 401 Unit 1 Exam: Thermodynamics & Kinetics (Nuss: Spr 2018) Multiple Choice Identify the choice that best completes the statement or answers the question. (3 pts each) 1. Which of the following statements regarding spontaneous changes is false? a. Spontaneity is favored when heat is released. b. Spontaneity is favored when the dispersal of matter is increased. c. Spontaneous changes occur at a given state without any outside influence. d. Ice melting at 25 C is spontaneous primarily due to the increase in molecular disorder (dispersal of matter). e. All exothermic reactions are spontaneous. 2. Which one of the following reactions has a positive entropy change? a. H 2 O(g) H 2 O(ª) b. BF 3 (g) + NH 3 (g) F 3 BNH 3 (s) c. 2SO 2 (g) + O 2 (g) 2SO 3 (g) d. N 2 (g) + 3H 2 (g) 2NH 3 (g) e. 2NH 4 NO 3 (s) 2N 2 (g) + 4H 2 O(g) + O 2 (g) 3. A process occurs spontaneously and DS system < 0. Which statement below must be true? a. DS surroundings > 0 b. DS universe > 0 c. The pressure is constant. d. Both (a) and (b) are correct. e. All three answers are correct. 4. A process cannot be spontaneous (product-favored) if. a. it is exothermic, and there is an increase in disorder b. it is endothermic, and there is an increase in disorder c. it is exothermic, and there is a decrease in disorder d. it is endothermic, and there is a decrease in disorder e. the entropy of the universe increases 5. Which of the following expressions does not represent a proper expression for the rate of this reaction? 2A + 3B F + 2G a. b. -D[ A] Dt -D[ B] 3Dt c. d. D[ F] Dt D[ G] 2Dt e. -D[ A] 2Dt 1

2 6. In the following reaction the rate of change of reactant B, D[ B] Dt overall rate of the reaction at this time? a mol/l s b mol/l s c mol/l s d. 2.7 mol/l s e mol/l s A + 3B AB 3 = mol/l s. What is the value of the 7. A hypothetical reaction X + 2Y Products is found to be first order in X and second order in Y. What are the units of k, the specific rate constant, if reaction rate is expressed in units of moles per liter per second? a. M s -1 b. M -2 s -1 c. M -3 s d. M 2 s -1 e. M -1 s 8. Which statement is incorrect? a. The reaction rate for a zero-order reaction is independent of concentrations. b. The specific rate constant for a second-order reaction is independent of temperature. c. The half-life for a first-order reaction is independent of initial concentrations. d. The rate law expression relates rate and concentration. e. The integrated rate equation relates time and concentration. 9. Which idea listed below is not a part of the collision theory of reaction rates? a. Molecules must be properly oriented when they collide to react. b. Molecules must collide to react. c. Molecules must collide with enough kinetic energy to overcome the potential energy stabilization of the bonds. d. Effective collisions result in a chemical reaction. e. All molecular collisions result in a reaction. 10. Consider the hypothetical reaction shown below. 2A + C 2 A 2 C + C Assume that the following proposed mechanism is consistent with the rate data. A + C 2 AC + C slow AC + A A 2 C fast 2A + C 2 A 2 C + C overall Which one of the following statements must be true? The reaction is. a. first order in A, first order in AC b. second order in C 2 c. first order in A and first order in C 2 d. second order in C 2, zero order in A e. second order in A and first order in C 2 2

3 11. A reaction has an activation energy of 40 kj/mol and an overall energy change of reaction of -100 kj/mol. In each of the following potential energy diagrams, the horizontal axis is the reaction coordinate and the vertical axis is potential energy in kj. Which potential energy diagram best describes this reaction? a. c. e. b. d. 12. Which of the following statements regarding the rate constant in the rate law expression is incorrect? a. Its value increases with increasing temperature. b. Its value is independent of initial concentration at a given temperature. c. Its units depend on the overall order of reaction. d. Its value is experimentally determined. e. The larger its value, the slower the reaction rate. 13. Reaction rates increase with increasing temperature because. a. the activation energy increases b. larger molecules collide more frequently c. the energy of the transition state is lowered d. the activation energy is decreased e. a greater fraction of molecules possess the activation energy when they collide 14. A student analyzed a first-order reaction and obtained the graph below. Unfortunately, the student forgot to label the axes. What are the correct labels for the X and Y axes? a. X = time, Y = ln[a] b. X = time, Y = 1/[A] c. X = time, Y = ln(1/[a]) d. X = 1/time, Y = [A] e. X = 1/time, Y = 1/[A] 3

4 15. Given the following mechanism: A + C AC step 1 B+ AC AB + C step 2 A + B AB overall AC is a: a. catalyst c. transition state complex b. reaction intermediate d. coordination compound 16. The rate constant for the first order reaction A B + C is k = min -1 at 57 K. What is the half-life for this reaction at 57 K? a. 21 min b. 30 min c. 61 min d. 9.1 min e min 17. Given the following data for the NH NO 2 - N 2 + 2H 2 O reaction Trial [NH 4+ ] [NO 2- ] Rate M M M/s M M M/s M M M/s The rate law for the reaction is a. rate = k[nh 4+ ][NO 2- ] b. rate = k[nh 4+ ] 2 [NO 2- ] c. rate = k[nh 4+ ][NO 2- ] 2 d. rate = k[nh 4+ ] 2 [NO 2- ] 2 e. None of the above 18. Evaluate DS 0 for the reaction below at 25 C and 1 atm. 3NO 2 (g) + H 2 O(ª) 2HNO 3 (aq) + NO(g) S 0 (J/mol K) a J/K b J/K c J/K d J/K e J/K 4

5 19. For the following reaction at 25 C, DH 0 = kj and DS 0 = 11.2 J/K. Calculate DG 0 for the reaction at 25 C in kilojoules. I 2 (g) + Cl 2 (g) 2ICl(g) a kj b kj c kj d kj e kj 20. Evaluate the specific rate constant for this reaction at 800 C. The rate-law expression is rate = k[no] 2 [H 2 ]. (Choose the closest answer.) 2NO(g) + 2H 2 (g) (g) + 2H 2 O(g) Experiment Initial [NO] Initial [H 2 ] Initial Rate of Reaction (M s -1 ) M M M M M M a. 22 M -2 s -1 b. 4.6 M -2 s -1 c M -2 s -1 d M -2 s -1 e M -2 s -1 Completion (2 pts each) Complete each statement. 21. A catalyst works by lowering the for a reaction. 22. A reaction whose rate law can be written directly from the stoichiometric coefficients of the reaction must be a(n) reaction. 23. The third law of thermodynamics states that at zero Kelvin, the entropy of a pure cyrstal. 24. A(n) reaction is a spontaneous reaction with a (-)DG. 5

6 True/False (2pts each) Indicate whether the statement is true or false. 25. For a typical substance, DS of fusion (melting) is expected to be greater than DS of vaporization. 26. An elementary step involving a colission between two particles is a dimolecular step. 27. The entropy at standard state is greater for CH 3 CH 2 CH 3 than it is for CH 4. Short Answer 28. The reaction SF 4 SF 2 + F 2 has the following rate law: rate = M -1 s -1 [SF 4 ] 2. How many minutes will it take for the concentration of SF 4 to be reduced from 2.5 M to 0.25 M? (4 pts) 29. Estimate the boiling point in Celcius of tin(iv) chloride, SnCl 4, at one atmosphere pressure: (4pts) SnCl 4 (l) SnCl 4 (g) For SnCl 4 (l), DH f 0 = kj/mol, S = J/mol K, and for SnCl 4 (g), DH f 0 = kj/mol, S = 366 J/mol K at 298 K. 6

7 30. For a particular reaction at 25 C, DH o = -297 kj/mol, and DS o = J/mol K. a) At what temperature would the reaction switch between being spontaneous and nonspontaneous? (3 pts) b) Would it be spontaneous above or below that temperature? (1pts) 31. Calculate DG 0 at 298 K for the reaction below. (5 pts) Fe 2 O 3 (s) + 13CO(g) 2Fe(CO) 5 (g) + 3CO 2 (g) DH f 0 (kj/mol) S 0 (J/mol K)

8 32. Based on the data below, what are the average rates of change of [O 2 ] and [NO 2 ] over the interval 285 to 1175 seconds for the reaction 2NO(g) + O 2 (g) 2NO 2 (g)? (4pts) Time (s) [NO] [O 2 ] [NO 2 ] M M M M M M M M M Average Rate of Change of O 2 = Average Rate of Change of NO 2 = 33. The specific rate constant, k, for a reaction is 0.44 s -1 at 298 K, and the activation energy is 245.kJ/mol. Calculate k at 398 K. (5 pts) Extra Credit (5pts) 34. A student measured the rate constant of a reaction at two different temperatures. At 30 C k = M -1 s -1. At 78 C k = M -1 s -1. Sketch an Arrhenius plot for this data set and use it to estimate the activation energy of the reaction. 8