Classes at: - Topic: Thermodynamics

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1 PHYSICAL CHEMISTRY by: SHAILENDRA KR. Classes at: - Meq. Approach SCIENCE TUTORIALS; Opp. Khuda Baksh Library, Ashok Rajpath, Patna PIN POINT STUDY CIRCLE; House No. 5A/65, Opp. Mahual Kothi, Alpana Market, Patna Topic: Thermodynamics 1. For a process at 451 K and constant pressure, S is -326 J surr K 1. Calculate the quantity of heat (in kj) absorbed by the system. 2. Which of the following produces an INCREASE in entropy of the system? a. O (l) O (s) b. 2 O SO 2 SO 3 c. 2 CH 3 OH + 3 O 2 2 CO O (l) d. I 2 (s) I 2 (l) e. None of the above. 3. The gas-phase reaction between and can be initiated by ultraviolet light at 25 C, + 2 HCl Using the thermodynamic data provided below, calculate the standard free energy change (in kj) at 25 C for this reaction. compound H f, kj mol 1 S, J mol 1 K HCl In lecture, you observed the following reaction, 2 + O 2 2 O For this reaction, H = kj and S = JK 1 Assumeing that H = H and S = S at all temperatures, calculate the temperature (in K) at which this reaction would become non-spontaneous. 5. For a particular chemical reaction at 400 C, G = -67 kj. Calculate the time (in seconds) it will take for the reaction to reach completion. 6. For a particular chemical reaction, H is positive and S is negative. Which of the following statements about the spontaneity of the reaction under standard is conditions TRUE? a. The reaction will be spontaneous only if the magnitude of H is large enough to overcome the unfavorable entropy change Page No.: 1

2 b. The reaction will be spontaneous only if the magnitude of S is large enough to overcome the unfavorable enthalpy change. c. The reaction will be spontaneous regardless of the magnitudes of H and S. d. The reaction cannot be spontaneous. e. The reaction will be spontaneous only of G is positive. 7. Which of the following is TRUE for an operating voltaic cell? a. G > 0 ; E = 0 b. G < 0 ; E < 0 c. G = 0 ; E > 0 d. G = 0 ; E = 0 e. G < 0 ; E > 0 8. Which of the following produces a DECREASE in entropy of the system? Hint :- the system is shown in bold a. Dissolving sugar in a cup a coffee. b. Condensation of water on the surface of a glass of iced tea on a hot summer day. c. Boiling water in a pot on the stove to make spaghetti. d. Allowing the liquid propane in a gas grill to escape from the tank. e. Producing CO 2 gas from baking soda (NaHCO 3 ) when baking a cake. 9. Consider the following reaction and thermodynamic data, 2 O 2 (aq) º 2 O (l) + O 2 substance H f, kj mol 1 S, J mol 1 K 1 O 2 (aq) O (l) O Calculate the value (in kj) of G at 25 C. 10. For particular chemical reaction, both H and S are negative. Which of the following statements about the spontaneity of the reaction under standard conditions is TRUE? a. The reaction will be spontaneous only if the magnitude of H is large enough to overcome the unfavorable entropy change. b. The reaction will be spontaneous only if the magnitude of S is large enough to overcome the unfavorable entropy change. c. The reaction will be spontaneous regardless of the magnitudes of H and S. d. The reaction cannot e spontaneous. e. The reaction will be spontaneous only of G is positive. Page No.: 2

3 11. The boiling point of a substance is the temperature at which equilibrium is established between the liquid and the vapor. CH 3 C OH (l) º CH 3 C OH Calculate the boiling point (in K) of ethanol, CH 3 C OH, if H = 42.6 kj and S = 122 J/K for this process. 12. What is the significance of the Third Law of Thermodynamics? Choose the best answer. a. The absolute entropy of a substance decreases with increasing temperature. b. The change in entropy of the universe must be positive for a spontaneous process. c. The absolute value of entropy can be measured for some very pure substances. d. The change in entropy of the universe equals the sum of the change in entropy of the system plus the change in entropy of the surroundings. e. The entropy of the universe is constant. 13. The K sp for the sparingly soluble salt barium chromate (BaCrO 4 ) at 25 C is equal to 2.0 x Calculate G (in kj) for the following reaction. BaCrO 4 (s) º Ba 2+ (aq) + CrO 4 2 (aq) 14. A student of chemistry determined the value of K sp for a saturated solution of borax at several different temperatures. The value for S for the dissolution of borax in water can be determined from: a. the slope of the line resulting from a plot of ln K SP versus (1/T). b. the y-intercept of the line resulting from a plot of ln K SP versus T. c. the slope of the line resulting from a plot of K SP versus (1/T) d. the y-intercept of the line resulting from a plot of K SP versus (1/T) e. the slope of the curve at 25 C from a plot of ln K SP versus T. 15. Consider the following gas-phase reaction 2NO 2 º 2NO + O 2, G = kj If 1.00 mole of NO 2 is placed in a 1.00-L flask (no NO or O 2 initially) at 25, Which of the following statements is TRUE? a. The reaction will occur spontaneously from left to right b. The reaction will occur spontaneously from right to left c. The reaction is not spontaneous in either direction. d. The reaction is spontaneous in both directions. e. At equilibrium, the concentrations of NO and O 2 will be much larger than the concentration of NO 2. Page No.: 3

4 16. For which one of the following reactions does the entropy of the system INCREASE? a. NH 3 +HCl NH 4 Cl (s) b. Ag + (aq) + Cl (aq) AgCl (s) c. 2 + O 2 2 O d. + NH 3 + O (l) NH 4 (aq) + OH (aq) e. 2 O 2 (l) 2 O (l) + O Consider the following characteristics of chemical reactions, I. spontaneity II. maximum amount of work that can be done III. speed The value of G for a chemical reaction under a given set of conditions provides information about: a. I only b. II only c. III only d. I and II e. I, II and III 18. Consider the following standard reduction potentials, Half-reaction E,V I 3 (aq) 2e 3 I (aq) 0.53 Cr 3+ (aq) + e Cr 2+ (aq) Calculate G (in kj) for the following reaction at 25 C. Cr 2+ (aq) I 3 (aq) Cr 3+ (aq) + I (aq) 19. Consider the following reaction, 2 SO 2 + O 2 º 2 SO 3 and the following thermochemical data, Substance G f, kj/mol SO O SO Calculate the value of the equilibrium constant for this reaction at 25 C. 20. For the dissociation of acetic acid in water at 25 C, CH 3 COOH (l) + O º H 3 O + (aq) + CH 3 COO (aq) H =284.3kJ and S = J K 1. Calculate the value of G (in kj) for this reaction at equilibrium. 21. Consider the carbonylation (i.e., addition of CO) of methanol, CH 3 OH, to produce acetic acid, CH 3 COOH, in a closed system at constant temperature and pressure, and the following data : CH 3 OH CO º CH 3 COOH Page No.: 4

5 substance H f, kj mol 1 S, J mol 1 K 1 CH 3 OH CO CH 3 COOH If the initial concentration of CH 3 OH is 1.5 M and the initial concentration of CO is 2.0 M, what is the concentration (in M) of CH 3 OH at equilibrium, at 298 K? 22. For which of the following process does the entropy of the system DECREASE? a. Melting of ice b. Dissolving table salt in water. c. Decomposition of liquid hydrogen peroxide to produce liquid water and oxygen gas. d. Precipitation of silver chloride from a solution containing silver ions and chloride ions. e. The entropy of the system increases in all of the above processes. 23. For a particular chemical reaction, the values for H and S are both positive. Which of the following statements is TRUE about this reaction? a. The reaction is spontaneous at any temperature. b. The reaction is not spontaneous at any temperature. c. The reaction is spontaneous when the temperature is high enough to overcome H. d. The reaction is spontaneous when the temperature is low enough to overcome H. e. The reaction is spontaneous when the temperature is low enough to overcome S. 24. For the boiling of methanol, CH 3 OH (l) CH 3 OH H = kj, and S = J/K. Assume that methanol boils at a temperature at which the value of G is equal to zero. Using the values of H and S, calculate the temperature (in K) at which methanol boils. 25. Which of the following reactions is unfavorable at low temperatures but becomes favorable as the temperature increases? a. 2 CO + O 2 CO 2 ; H = -566 kj; S = -173 J/K b. 2 O 2 + O 2 ; H = 484 kj; S = 90.0 J/K c. 2 N 2 O 2 N 2 + O 2 ; H = -164 kj; S = 149 J/K d. Pb (s) Pb 2+ (aq) + O 2 ; H = 23.4 kj; S = J/K 26. The following reaction is spontaneous as written: Zn (s) + Cu 2+ (aq) Zn 2+ (aq) + Cu (s) Which of the following statement is TRUE : a. K eq > 1 and G = 0 b. K eq > 1 and G < 0 c. K eq < 1 and G < 0 d. K eq > 1 and G > 0 e. K eq < 1 and G = 0 Page No.: 5

6 27. A spontaneous reaction ALWAYS occurs when: a. H < 0 and S < 0 b. H > 0 and S < 0 c. H < 0 and S > 0 d. H > 0 and S = 0 e. H > 0 and S > When a reaction is at equilibrium, which of the following statements is TRUE? a. G = G b. ln K eq = 0 c. G = 0 d. Q = 0 e. G = For mercury the molar entropy of vaporization is J K 1 mol 1 and the molar enthalpy of vaporization is kj mol 1. Therefore, the normal boiling point of mercury in C is : a. 273 C b. 357 C c. 516 C d. 670 C e. unable to determine unless G of vaporization is known. 30. Which statement is TRUE for an operating electrolytic cell? a. G > 0 and E Cell < 0 b. G = 0 and E Cell > 0 c. G < 0 and E Cell = 0 d. G = 0 and E Cell = 0 e. G > 0 and E Cell > For which ONE of the following reactions does the entropy of the system DECREASE? a. NH 4 NO 3 (s) NH 4+ (aq) + NO 3 (aq) b. 2 O 2 (l) 2 O(l) + O 2 c. Ba(OH) 2.8 O(s) + 2 NH 4 SCN(s) Ba(SCN) 2 (aq) + 2 NH 3 (aq) + 10 O(l) d. NO + NO 2 N 2 O 3 e. CO 2 (s) CO For a particular chemical reaction at 25 C, H = -432 kj. Which of the following statement is TRUE? a. The reaction will be spontaneous, at any temperature, if S is positive. b. The reaction will be spontaneous, at any temperature, if S is Negative. c. The reaction will be spontaneous only if G is positive. d. The reaction can never be spontaneous, at any temperature. e. There is insufficient information provided to answer this question. 33. For a particular chemical reaction, H = kj and S = +121 J/K. At what temperature (in K) would this reaction become spontaneous? 34. Consider the gas-phase hydrogenation of ethylene, C 2, to produce ethane, C 2 H 6, C C 2 H 6 Page No.: 6

7 and the following thermodynamic data: substance H f, kj mol 1 S, J mol 1 K 1 C C 2 H Under standard conditions, which of the following statements is TRUE? a. The reaction is spontaneous; H is favorable while S is not. b. The reaction is spontaneous; S is favorable while H is not. c. The reaction is not spontaneous; S is favorable while H is not. d. The reaction is not spontaneous; H is favorable while S is not. e. The reaction is not spontaneous; neither H nor S are favorable. 35. Consider the gas-phase reaction of hydrogen,,and oxygen, O 2, to produce water, 2 + O 2 2 O and the following thermodynamic data: substance G f, kj/mol 0 O 2 0 O Calculate the value of the equilibrium constant for this reaction under standard conditions. 36. Consider the gas-phase reaction of hydrogen,, with carbon monoxide, CO, to from formaldehyde, CO, in a closed system at constant pressure, + CO CO and the following thermodynamic data, substance H f, kj mol 1 S, J mol 1 K CO CO If the initial concentration of CO is 1.5 M and the initial concentration of is 2.3 M, calculate the concentration (in M) of CO at equilibrium, at 298 K. 37. For a certain process, S(system) >0 and S(surroundings) > 0. The process: a. is spontaneous b. is exothermic. c. decreases the entropy of the universe. d. is endothermic. e. is at equilibrium. Page No.: 7

8 38. In which case does the spontaneity of a reaction depend on the temperature? a. H = 0 and S < 0 b. H > 0 and S > 0 c. H < 0 and S = 0 d. H > 0 and S = 0 e. H < 0 and S > Consider the following reaction: O + O º 2 HOCl For this reaction, K eq = at 25 C. Calculate the value (in kj/mol) of G for this reaction. 40. For a particular process at 500 K, G = -3.0 kj and H = kj. If the process is carried out reversibly, Calculate the amount (in kj) of useful work that can be performed. 41. Which of the following statements concerning the change in G and G during a chemical reaction is most correct? a. G remains constant while G changes and becomes equal to G at equilibrium. b. Both G and G remain constant during a chemical reaction. c. Initially both G and G are equal to zero. The value of G changes to a value determined by the equilibrium constant; the value of G peaks and then decrease to zero at equilibrium. d. G remains constant if the reaction is carried out under standard conditions; G remains constant if the reaction is carried out under non-standard conditions. e. G remains constant while G changes and becomes equal to zero at equilibrium. 42. The enthalpy of vaporization of ethanol is 38.7 kj/mol at its boiling point of 78 C. Calculate the change in entropy (in J/mol) of the surroundings when 1.00 mole of ethanol is vaporized at 78 C and 1.00 atm of pressure. 43. A certain chemical reaction carried out at constant temperature and pressure has S > 0. What can you conclude about the spontaneity of this reaction? a. The reaction is not spontaneous if it is endothermic and the temperature is sufficiently high. b. The reaction is not spontaneous at any temperature. c. The reaction is not spontaneous if it is exothermic and the temperature is sufficiently low d. The reaction is only spontaneous at high temperatures. e. The reaction is spontaneous if it is exothermic and the temperature is sufficiently high. 44. Consider the following reaction: 2 C(graphite) + C 2 G = kj at 25 C, P( ) = 100 atm, P(C 2 ) = 0.10 atm Calculate the value (in kj) of G for reaction. Page No.: 8

9 45. We observed that when two white crystalline substances were mixed the beaker froze to the wooden board it was sitting on. The reaction is as follows: Ba(OH) 2.8 O(s) + 2 NH 4 NO 3 (s) Ba(NO 3 ) 2 (aq) + 2 NH O(l) For this reaction, H = kj and S = J/K. Which of the following statements must be TRUE about the process? a. Enthalpy is favorable and entropy is unfavorable. b. Neither entropy nor entropy are favorable. c. Both enthalpy and entropy are favorable. d. Enthalpy is unfavorable and entropy is favorable. e. The process is not spontaneous because it had to be stirred. 46. In the thermite reaction we observed that after glycerin was added a vigorous reaction proceeded. The reaction is as follows: Fe 2 O 3 (s) + 2 Al(s) 2 Fe + Al 2 O 3 (s) For this reaction, H = kj and S = J/K. Which of the following statements must be TRUE about the process? a. Enthalpy is favorable and entropy is unfavorable. b. Neither entropy nor entropy are favorable. c. Both enthalpy and entropy are favorable. d. Enthalpy is unfavorable and entropy is favorable. e. The process is not spontaneous because glycerin had to be added. 47. You have observed all of the following chemical reactions in lecture. For which one of these reactions does the entropy of the system INCREASE? a. NH 3 + HCl NH 4 Cl(s) b. 2 O 2 (l) 2 O(l) + O 2 c. 2 + O 2 2 O d. NH 3 + O(l) NH 4+ (aq) + OH (aq) e. Na + (aq) + CH 3 COO (aq) CH 3 COONa(s) 48. Which of the following thermodynamic properties CANNOT be experimentally determined? a. S b. H c. G d. S e. G 49. Consider the hydrogenation of acetylene, C 2, to produce ethane, C 2 H 6, C C 2 H 6 and the following data: substance H f, kj mol 1 S, J mol 1 K 1 C Page No.: 9

10 C 2 H At 25 C and 1 atm pressure, which of the following statements is TRUE about the spontaneity of this reaction? a. Both H and S favor spontaneity. b. H favors spontaneity but S opposes it. c. H opposes spontaneity but S favors it. d. Both H and S opposes spontaneity. e. There is insufficient information to answer this question. 50. Consider the following information: i. Ice has a more ordered structure than liquid water. ii. Liquid water freezes spontaneously at -5 C and 1 atm. Which of the following statements is most likely TRUE? a. Liquid water freezes at -5 C because the increase in entropy overcomes the endothermicity. b. Liquid water freezes at -5 C because the exothermicity overcomes the decrease in entropy. c. Liquid water freezes at -5 C because the exothermicity overcomes the increase in entropy. d. Liquid water freezes at -5 C because the endothermicity overcomes the decrease in entropy. e. None of these statements is TRUE? 51. Solid ammonium nitrate, NH 4 NO 3 (s) dissolves spontaneously in water at 25 C, NH 4 NO 3 (s) NH 4+ (aq) + NO 3 (aq) If this process is endothermic, which of the following is most likely TRUE? a. G > 0 b. G = 0 c. S < 0 d. S > 0 e. S = Consider the following reaction for the dissociation of lactic acid, HC 3 H 5 O 3 (a weak acid): HC 3 H 5 O 3 (l) + O(l) º H 3 O + (aq) + C 3 H 5 O 3 (aq) If K a for lactic acid 1.4 x 10 4, calculate the value of G (in J) for this reaction at 25 C. 53. In lecture, you observed the following reaction, Ba(OH) 2 : 8 O(s) + 2 NH 4 SCN(s) Ba(SCN) 2 (aq) + 2 NH 3 (aq) + 10 O(l) H for this reaction is : kj, and S is : 495 J/K. Calculate the TEMPERATURE (in K) at which this reaction will become non-spontaneous. 54. Early in the development of thermodynamics, it was believed that exothermic reactions would be spontaneous. This is often true, but there are some exothermic reactions that are NOT spontaneous. In the absence of any kinetic barriers, which of the following factors must also be Page No.: 10

11 considered (in addition to the enthalpy change) to determine whether or not a reaction will be spontaneous? a. activation energy b. rate constant c. Ssys d. Concentrations of reactants 55. Which of the following statements best describes the spontaneity of a reaction at 25 C for which Ssys is negative and Hsurr is positive? a. The reaction cannot be spontaneous at 25 C. b. The reaction will be spontaneous at all temperatures. c. The reaction will be spontaneous only if Ssys is greater in magnitude than Ssurr. d. The reaction will be spontaneous only if Ssurr is greater in magnitude than Ssys. e. The reaction will only be spontaneous if the change in free energy for the system is positive. 56. Which of the following processes will result in a DECREASE in the entropy of the system? All systems are closed systems. a. CH 4 (l) CH 4 b. Zn (aq) + 4 NH 3 (aq) Zn(NH 3 ) 4 (aq) c. (298 K) (450 K) d. ZnS(s) Zn 2+ (aq) + S 2 (aq) e. NH 3 (3 atm, 298 K) NH 3 (1 atm, 298 K) 57. Which of the following statements is TRUE? a. Endothermic reactions are never spontaneous, at any temperature. b. Exothermic reactions are always spontaneous, at any temperature. c. Exothermic reactions in which the entropy change for the system is negative are spontaneous at any temperature. d. Reactions in which the entropy change for the system is negative are never spontaneous, at any temperature. e. None of the above statements is TRUE. 58. At elevated temperatures, hydrogen iodide, HI, decomposes to produce hydrogen,, and iodine, I 2, as shown in the equation below: 2 HI + I 2 Using the thermodynamic data below, calculate the standard free energy change (in KJ) for this reaction. compound H f, kj mol 1 S, J mol 1 K 1 HI I Methylene chloride, C, is common organic solvent. Using the thermodynamic data below, calculate the boiling point (in C) for C. Page No.: 11

12 compound H f, kj mol 1 S, J mol 1 K 1 C (l) C Consider the gas-phase reaction of chlorine,, with fluorine, F 2, to produce chlorine trifluoride, ClF 3, in a closed system at constant temperature and pressure, + 3 F 2 2 ClF 3 and the following data : compound H f, kj mol 1 S, J mol 1 K F ClF If the initial concentration of is 2.4 M and the initial concentration of F 2 is 5.7 M, what is the molar concentration of F 2 at equilibrium, at 298 K? 61. For each of the following processes, predict whether the change in entropy for the system will be positive or negative. Briefly explain your reasoning. a. 2 C 8 H O 2 16 CO O b. AgCl (s) + 2 NH 3 (aq) Ag(NH 3 ) + (aq) + Cl (aq) c. O O (l) 62. Exothermicity greatly favors the spontaneity of a chemical process or reaction. Briefly explain, in thermodynamic terms, why this is true. 63. For a certain process S(system) > 0 and S(surroundings) >0. The process a. is spontaneous b. is exothermic c. decreases the entropy of the universe d. is endothermic e. is at equilibrium 64. Arrange the following compounds in order of increasing standard molar entropy at 25 C. a. ZnS (s) < O (l) < C 3 H 8 < C 2 H 4 b. C 2 H 4 < O (l) < C 3 H 8 < NaCl (aq) c. ZnS (s) < C 3 H 8 < C 2 H 4 < O d. C 3 H 8 < C 2 H 4 < O (l) < ZnS (s) e. ZnS (s) < O (l) < C 2 H 4 < C 3 H For HI the molar entropy of vaporization is 89.0 J/K mol and the molar enthalpy of vaporization is kj/mol. Therefore, the normal boiling point of HI is: a C b C c K d. 42 C e. Unable to determine unless G of vaporization is known. Page No.: 12

13 66. Hydrogen peroxide decomposes according to the following reaction: O 2 (l) O (l) + 1/2O 2 For this reaction H = kj and S = 70.1 J/K. Which is the approximate value of K p for this reaction at 25 C? a. -1 x b. -3 x 10 2 c. 5 x 10 2 d. 7 x e. 9 x The significance of the Second Law of Thermodynamics is that for spontaneous processes the: a. entropy of the universe is increasing b. entropy of the surrounding must decrease c. entropy is the driving force of all chemical reactions d. entropy of an ideal solid does not change e. absolute value for entropy can be calculated. 68. For a particular process at 300 K, G = kj and H = -3.0 kj. If the process is carried out reversibly, the amount of useful work that can be performed is: a kj b kj c kj d kj d kj 69. The reaction rates for many spontaneous reactions are actually very slow. Which of the following is the best explanation for this observation? a. K p for the reaction = 0 b. The activation energy is high c. The standard free energy change for the reaction is > 0 d. These reactions are endothermic e. The standard entropy change is < The normal freezing point of NH 3 is -78 C at one atmosphere. Predict the signs of H, S, and G for NH 3 when NH 3 supercools and then freezes at -80 C and one atmosphere. a. H < 0, S < 0, G = 0 b. H < 0, S > 0, G < 0 c. H > 0, S < 0, G > 0 d. H > 0, S > 0, G = 0 e. H < 0, S < 0, G < We observed a demonstration of the reaction : O 2 O + 1/2O 2 to which I was added. A large foaming mass of bubbles was formed. The following mechanism has been proposed for this reaction: O 2 + I O + IO Slow O + IO O + 1/2O 2 + I fast Page No.: 13

14 The function of the I in this reaction is to: a. Raise the H of the products b. Lower the G between reactants and products c. Increase the concentration of products d. Lower the activation energy e. Raise the S of the reactants. 72. In the thermite reaction, we observed that after glycerin was added a vigorous reaction proceeded. The reaction is as follows: Fe 2 O 3 (s) + 2 Al (s) 2 Fe (l) + Al 2 O 3 (s) For this reaction, the standard enthalpy change is = kj and the standard entropy change is = J/K. Which of the following statements must be TRUE about the process? a. Enthalpy is favorable and entropy is unfavorable b. Neither entropy or entropy are favorable c. Both enthalpy and entropy are favorable d. Enthalpy is unfavorable and entropy is favorable e. The process is not spontaneous because glycerin had to be added 73. In the thermite reaction we observed that glycerin was necessary to start the reaction. Which of the following is the most reasonable explanation of that observation? a. The reaction is exothermic b. The activation energy is high c. The process is not spontaneous d. Glycerin is a powerful explosive 74. Which of the following statements is TRUE? a. An exothermic process will always be spontaneous b. A process in which the entropy of the system increases will always be spontaneous c. An endothermic process can never be spontaneous d. A process in which the entropy of the surroundings increases will always be spontaneous e. An exothermic process that is accompanied by an increase in the entropy of the system will always be spontaneous 75. Which of the following processes will result in a INCREASE in the entropy of the system? All systems are closed systems. a. O O (l) b. Zn (aq)+4 NH 3 (aq) Zn(NH 3 ) 4 (aq) c. O 2 (373 K) O 2 (298 K) d. Zn 2+ (aq) + S 2- (aq) ZnS (s) e. NH 3 (3 atm, 298 K) NH 3 (1 atm,298 K) 76. Methylene chloride, C, is a common organic solvent. Consider the following process at 25 C, in a closed system: C (l) C This process is endothermic ( H = kj). Which of the following statements is most likely TRUE regarding the spontaneity of this process at 25 C? Page No.: 14

15 a. The process will be spontaneous because S sys is negative b. The process will be spontaneous because S surr is positive c. The process will be spontaneous if the boiling point of C is less than 25 C d. The process will be spontaneous if the freezing point of C is greater than 25 C e. The process will be spontaneous if the boiling point of C is greater than 25 C 77. Acetic acid, CH 3 COOH, has an enthalpy of vaporization equal to kj an entropy of vaporization equal to 122 J/K at its boiling point. Calculate the boiling point of acetic acid. a K b. 2.3 K c. 100 K d. 428 K d. 563 K 78. Suppose we place 1.0 mole of the brown gas, NO 2 in a large glass tube at 25 C and 1 atm pressure. We then seal the tube, and wait for equilibrium to be established between a NO 2 and the colorless species, N 2 O 4, 2 NO 2 º N 2 O 4 After equilibrium has been established at 25 C, we observe that the inside of the tube is light brown in color. Next, we cool the tube in liquid nitrogen (-196 C) and again wait until equilibrium has been established. Which of the following statements most likely describes what we would observe at -196 C (for this reaction, H = kj and S = J/K)? a. The value of the equilibrium constant at -196 C is much larger than at 25 C so the brown color should disappear b. The value of the equilibrium constant at -196 C is much smaller than at 25 C so the brown color should become darker c. The equilibrium constant does not depend on temperature so we would not observe any change d. The value of the equilibrium constant at 25 C much larger than at -196 C so the brown color should become darker e. It is impossible to predict what will happen from the data provided here 79. For each of the following processes, determine whether the change in entropy will be positive or negative. Explain your reasoning. a. O O (l) b. NH 3 1/2N 2 + 3/2 c. NaCl (s) Na + (aq) + Cl (aq) d. Cu (aq) + 4 NH 3 (aq) Cu(NH 3 ) 4 (aq) e. Precipitation of the Group I ions in the qualitative analysis scheme as AgCl, Hg 2 and Pb. 80. The melting point of tungsten (used in some light bulbs) is the second highest among the elements (only that of carbon is higher). If the enthalpy of fusion is kj/mol and the entropy of fusion is 9.65 J/mol K, calculate the melting point (in K) of tungsten. Page No.: 15

16 81. Which of the following reactions IS spontaneous under standard conditions? e. The reaction is not spontaneous; neither H nor S is favorable a. 2 Au(s) + 3 Fe 2+ (aq) º 2 Au 3+ (aq) + 3 Fe(s) b. Sn (s) + Zn 2+ (aq) º Sn 2+ (aq) + Zn (s) c. Br 2 (l) + Co(s) º 2 Br (aq) + Co 2+ (aq) d. 2 Ag + (aq) + Pt (s) º 2 Ag (s) + Pt 2+ (aq) e. 2 Cu 2+ (aq) + 4 OH (aq) º 2 Cu (s) + O O (l) 82. Consider the gas-phase reaction of hydrogen ( ) with carbon monoxide (CO) to from formaldehyde ( CO), + CO º CO and the following thermodynamic data, substance H f, kj mol 1 S, J mol 1 K CO CO Which of the following best explains why this reaction is NOT spontaneous under standard conditions? a. Both H and S oppose spontaneity b. Both H and S favor spontaneity c. H favors spontaneity but S opposes spontaneity d. H opposes spontaneity but S favors spontaneity e. There is insufficient information provided to answer the question. 83. The change in Gibbs free energy, G, for a chemical reaction provides information about: a. the balance between enthalpy and entropy changes for the reaction b. the maximum amount of work that can be harnessed from the reaction c. the amount of available energy that can potentially be harnessed to do useful work d. Both (b) and (c) are true e. All of the above are true 84. Solid ammonium nitrate, NH 4 NO 3, dissolves in water at 25 C according to the following equation, + NH 4 NO 3 (s) NH 4 (aq) + NO 3 (aq) For this process, H = kj and S = +108 J/K. Which of the following statements is TRUE? a. The reaction is spontaneous ; H is favorable while S is not b. The reaction is spontaneous; S is favorable while H is not c. The reaction is spontaneous; both H and S are favorable d. The reaction is not spontaneous; S is favorable while H is not Page No.: 16

17 85. The K f for the complex ion, [Zn(NH 3 ) 4 ] 2+, at 25 C is equal to 2.9 x Calculate G (in kj) for the following reaction. Zn 2+ (aq) + 4 NH 3 (aq) º [Zn(NH 3 ) 4 ] 2+ (aq) 86. For which one of the following processes does the entropy of the system DECREASE? a. Melting of ice b. Boiling of liquid water c. Dissolving NaCl in water d. Precipitating silver chloride e. None of these 87. Consider the gas-phase reaction of hydrogen,, and chloride,, to produce hydrogen chloride, and the following themodynamic data? + 2 HCl substance G f, kj/mol 0 0 HCl Calculate the value of H (in kj) for this reaction at 25 C if the standard state entropy change is equal to 20.0 J/K. 88. Consider the gas-phase reaction of hydrogen,, and iodine, I 2, to produce hydrogen iodide, +I 2 2 HI and the following thermodynamic data substance G f, kj/mol 0 I 2 0 HI 1.37 Calculate the value of the equilibrium constant for this reaction under standard conditions. 89. In lecture, you observed the Thermite Reaction: Fe 2 O 3 (s) + 2 Al (s) 2 Fe (s) + Al 2 O 3 (s) Using the following thermodynamic data, calculate the temperature (in K) at which the Thermite Reaction becomes non-spontaneous. substance H f, kj mol 1 S, J mol 1 K 1 Fe 2 O 3 (s) Al (s) Fe (s) Al 2 O 3 (s) Page No.: 17

18 90. For the Thermite Reaction described in the preceding question, which of the following statements is TRUE under STANDARD CONDITIONS? a. The reaction is non-spontaneous; neither H nor S are favorable b. The reaction is non-spontaneous; H is favorable while S is not c. The reaction is spontaneous; S is favorable while H is not d. The reaction is spontaneous; H is favorable while S is not e. The reaction is spontaneous; both S and H are favorable 91. A mixture of hydrogen and chlorine gas remains unreacted UNTIL exposed to ultraviolet light. Then the following reaction occurs very rapidly: + 2 HCl G = kj H = kj S = J/K Select the statements below which BEST explains this behavior. a. The reactants are thermodynamically more stable than the products b. The reaction has a small equilibrium constant c. The ultraviolet light raises the temperature of the system and makes the reaction more favorable d. The negative value for S slows down the reaction. e. The reaction is thermodynamically spontaneous, but the reactants are kinetically stable. 92. A change of state occurs within a system and produces 64.0 kj of heat. This heat is transferred to the surroundings at a constant pressure and a constant temperature of 300 K. For this process, S of the surroundings is : a J/K b J/K c J/K d. 213 J/K e J/K 93. The following reaction has a standard free energy change value of 42.6 kj/mol at 25 C : HB (aq) + O (l) º H 3 O + (aq) + B (aq) The K a of the acid HB is : a b c x 10 8 d x 10 4 e x In the lecture you saw a copper wire placed in a solution of silver nitrate (AgNO 3 ). Over a period of time, you observed a spontaneous reaction in which the solution turned blue and silvery needles formed on the copper wire. Which of the following statements must be TRUE about this process? a. G > 0 and E cell < 0 b. G = 0 and E cell > 0 c. G < 0 and E cell = 0 d. G = 0 and E cell = 0 e. G < 0 and E cell > 0 Page No.: 18

19 95. Which of the following is most likely TRUE for a dead battery? a. G < 0 and E cell = 0 b. G > 0 and E cell < 0 c. G > 0 and E cell = 0 d. G = 0 and E cell < 0 e. G = 0 and E cell = Which of the following does NOT influence the spontaneity of a chemical reaction? a. The temperature b. H sys c. S sys d. S surr e. The activation energy. 97. Calculate the boiling point (in K) of titanium tetrachloride,ticl 4, if the standard enthalpy of vaporization is: 41.0 kj mol 1 and the standard entropy of vaporization is: J mol 1 K Which of the following statements best describes the spontaneity of a reaction at 25 C for which S sys is negative and S surr is positive? a. The reaction will only be spontaneous if the change in free energy for the system is positive b. The reaction will be spontaneous at all temperatures c. The reaction will be spontaneous only if S sys is greater in magnitude than S surr. d. The reaction will be spontaneous only if S surr is greater in magnitude than S sys. e. The reaction cannot be spontaneous at 25 C. 99. The equilibrium constant for a chemical reaction will be equal to one (1) under which one of the following conditions? a. H < 0 and S = 0 b. H = 0 and S = 0 c. H < 0 and S > 0 d. H > 0 and S < 0 e. H > 0 and S = Benzene, C 6 H 6, is an extremely toxic organic solvent. Consider the following process at 25 C in a closed system: C 6 H 6 (l) C 6 H 6 (s) This process is exothermic (standard enthalpy change = kj). Which of the following statements is most likely TRUE regarding the spontaneity of this process at 25 C? a. The process will be spontaneous because S sys is positive b. The process will be spontaneous because S surr is positive c. The process will be spontaneous if the boiling point C 6 H 6 is less than 25 C d. The process will be spontaneous if the freezing point C 6 H 6 is greater than 25 C e. The process will be spontaneous if the freezing point C 6 H 6 is less than 25 C 101. Calculate the value of K a for hydrofluoric acid (HF) at 25 C if the standard free energy change for the following reaction is equal to kj/mol: Page No.: 19

20 HF + O (l) º H 3 O + (aq) + F (aq) a. 4.0 x b. 7.3 x 10 4 c. 9.2 x 10 1 d. 9.9 x 10 1 e. 1.4 x If a reaction is spontaneous at 25 C and has a standard enthalpy change equal to +100 kj/mol, which of the following must be TRUE about the standard entropy of reaction? a. Standard entropy change < 0 J/mol K b. Standard entropy change = 40 J/mol K c. Standard entropy change = 100 J/mol K d. Standard entropy change = 250 J/mol K e. Standard entropy change > 335 J/mol K 103. For HI the molar enthalpy of vaporization is kj/mol and the molar entropy of vaporization is 89.0 J/K mol. Therefore, the normal boiling point of HI is : a C b. -35 C c. 4 C d. 24 C e. The normal boiling point cannot be determined unless the free energy of vaporization is known The normal freezing point of water is 0 C. predict the signs of H, S and G when O is supercooled and freezes at -10 C and one atmosphere. a. H < 0, S < 0, G < 0 b. H < 0, S > 0, G = 0 c. H > 0, S > 0, G > 0 d. H < 0, S > 0, G < 0 e. H > 0, S > 0, G = Calculate the standard free energy change for the following reaction if the complex ion formation equilibrium constant, K f, is equal to 2.1 x Cu 2+ (aq) + 4 NH 3 (aq) º Cu(NH 3 ) 4 2+ (aq) a kj b. -76 kj c kj d. 19 kj e. 43 kj 106. Calculate the standard enthalpy change for a cell reaction which has a standard cell potential of 1.50 V. The cell reaction involves the transfer of 2 moles of electrons and the standard entropy change is 110J/K. a kj b kj c kj d. 123 kj e. 344 kj 107. Consider the following gas-phase reaction and average bond enthalpies, 2 NOCl 2 NO + Page No.: 20

21 bond average bond enthalpy, kj mol 1 N 01 N = O 716 Cl 43 Which of the following statements about the spontaneity of this reaction is TRUE at 25 C? a. The reaction will be spontaneous only if the magnitude of H is large enough to overcome the unfavorable entropy change b. The reaction will be spontaneous only if the magnitude of S is large enough to overcome the unfavorable enthalpy change c. The reaction will be spontaneous regardless of the magnitudes of H and S. d. The reaction cannot be spontaneous e. The reaction will be spontaneous only of G is positive All of the gas-phase reactions listed below are endothermic. Which one of these reactions is most likely to be non-spontaneous at ALL temperatures? a. CO CO + b. 2 SO 3 2 SO 2 + O 2 c. N 2 O 4 2 NO 2 d. CH 3 OH CO + 2 e. 4 NO + 6 O 4NH 3 + 5O A change in which of the following will also change the value of G for a chemical reaction? a. partial pressure of a gaseous reactant b. temperature c. concentration of a product in aqueous solution d. Changes in (a), (b) or (c) will change the value of G. e. A change in neither (a), (b) nor (c) will change the value of G In order for a solution to form spontaneously at a given temperature, the change in free energy for the formation of the solution ( G soln ) must be negative. Which of the following thermodynamic properties is primarily responsible for determining whether or not a solution will form? a. S soln b. H soln c. W soln d. E soln e. B soln 111. Which of the following statements is FLASE? a. Entropy is a measure of the driving force behind chemical reactions b. Entropy is a measure of disorder or randomness c. Entropy can be created and destroyed d. Entropy is a measure of the number of ways energy can be distributed among the motions of particles e. Entropy is a measure of the tendency of energy to become less concentrated. Page No.: 21

22 112. Chemical thermodynamics predicts (correctly) that materials made of wood, metal, paper, plastic, leather and rubber react with oxygen in the atmosphere and decompose at 25 C and 1 atm pressure. Which of the following is responsible for the fact these reactions are very slow under these conditions? a. entropy b. enthalpy c. internal energy d. activation energy e. potential energy 113. A sample of He is placed in a L flask at 298 K. The sample of He is then heated to 450 K at constant value. Microscopic shapshots. For the He are shown below. He, 298 K He, 450 K using positional entropy arguments, S sys for this process is: a. positive b. negative c. equal to zero d. It is impossible to tell 114. The normal freezing point of water is 0 C. Predict the signs of S sys, S surr and S uni v when O is supercooled and freezes at -10 C and 1 atm. a. S sys < 0; S surr < 0; S uni v < 0 b. S sys > 0; S surr < 0; S uni v = 0 c. S sys = 0; S surr < 0; S uni v > 0 d. S sys < 0; S surr > 0; S uni v > 0 e. S sys > 0; S surr = 0; S uni v < At 25 C, formic acid (HCO 2 H, K a = 1.8 x 10 4 ) dissolves in water and reacts according to the following equation, HCO 2 H (aq) + O (l) º H 3 O + (aq) + HCO 2 (aq) S for this reaction is JK 1. Under standard conditions at 25 C, the reaction of formic acid with water is: a. exothermic b. endothermic c. neither exothermic nor endothermic d. both exothermic and endothermic Page No.: 22

23 A N S W E R S x A 2. D x A x x There is insufficient information provided to answer the question 6. D x E 37. A 8. B 38. B x A x E 12. C E 14. D A 45. D 16. E 46. A 17. D 47. B x C x B x B x D 22. D x C C 25. B 55. D 26. B 56. B 27. C 57. E 28. E B A x D Page No.: 23

24 61. (a) Positive. All substances are gases but 27 moles of reactants produces 34 moles of products. (b) Positive. Losing a solid, and forming aqueous solutions of ions. (c) Negative. Gases have much more entropy than liquids. 62. The Second Law of Thermodynamics states that the entropy of the universe must increase for a spontaneous process. The "universe" is composed of the "system" and the "surroundings". Exothermicity favors spontaneity because exothermic processes always have positive entropy changes in the surroundings. 63. A 64. E 65. B 66. D 67. A 68. A 69. B 70. E 71. D 72. A 73. B 74. E 75. E 76. C 77. D 78. A 79. (a) The entropy of a liquid is much lower than the entropy of a gas. Therefore, negative. (b) All components are gases, but 1 mole of reactants produces 2 moles of products. Therefore, positive. (c) The entropy of aquated ions is much higher than the entropy in a crystal. Therefore, positive. (d) All components are aqueous species; however, 5 moles of reactants produces 1 mole of product. Therefore, negative. (e) Group I ions existed as aquated ions in solution. We are precipitating these as SOLIDS and we are combining two aqueous ions to produce a single product. Therefore, negative. Page No.: 24

25 B 81. C 112. D 82. C 113. C 83. E 114. D 84. B 115. B x D x x D 91. E 92. D 93. C 94. E 95. E 96. E D 99. B 100. D 101. B 102. E 103. B 104. A 105. B 106. B 107. B 108. E 109. D 110. B Page No.: 25