Page 1 of 7 AP Chemistry Chapter 16 Assignment Part I Multiple Choice 1984 47. CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O(l) H = 889.1 kj H f H 2 O(l) = 285.8 kj mol 1 H f CO 2 (g) = 393.3 kj mol 1 What is the standard heat of formation of methane, H f CH 4 (g), as calculated from the data above? (A) 210.0 kj mol 1 (B) 107.5 kj mol 1 (C) 75.8 kj mol 1 (D) 75.8 kj mol 1 (E) 210.0 kj mol 1 56. A cube of ice is added to some hot water in a rigid, insulated container, which is then sealed. There is no heat exchange with the surroundings. What has happened to the total energy and the total entropy when the system reaches equilibrium? Energy Entropy (A) Remains constant Remains constant (B) Remains constant Decreases (C) Remains constant Increases (D) Decreases Increases (E) Increases Decreases 57. For the reaction A(g) B(g) + C(g), the equilibrium constant, K p, is 2 10 4 at 25 C. A mixture of the three gases at 25 C is placed in a reaction flask and the initial pressures are P A = 21 atmosphere, P B = 0.5 atmosphere, and P C = 1 atmosphere. At the instant of mixing, which of the following is true for the reaction as written? (A) G < 0 (B) G > 0 (C) S = 0 (D) G = 0 (E) G < 0 1989 28. Which of the following is probably true for a solid solute with a highly endothermic heat of solution when dissolved in water? (A) The solid has a low lattice energy. (B) As the solute dissolves, the temperature of the solution increases. (C) The resulting solution is ideal. (D) The solid is more soluble at higher temperatures. (E) The solid has a high energy of hydration. 41. Which of the following reactions has the largest positive value of S per mole of Cl 2? (A) H 2 (g) + Cl 2 (g) 2 HCl(g) (B) C1 2 (g) + ½O 2 (g) Cl 2 O(g)
Page 2 of 7 (C) Mg(s) + Cl 2 (g) MgCl 2 (s) (D) 2 NH 4 Cl(s) 4 H 2 (g) + Cl 2 (g) (E) Cl 2 (g) 2 Cl(g) 53. Which of the following must be true for a reaction that proceeds spontaneously from initial standard state conditions? (A) G > 0 and K eq > 1 (B) G > 0 and K eq < 1 (C) G < 0 and K eq > 1 (D) G < 0 and K eq < 1 (E) G = 0 and K eq = 1 70. H 2 O(s) H 2 O(l) When ice melts at its normal melting point, 273.16 K and 1 atmosphere, which of the following is true for the process shown above? (A) H < 0, S > 0, V > 0 (B) H < 0, S < 0, V > 0 (C) H > 0, S < 0, V < 0 (D) H > 0, S > 0, V >0 (E) H > 0, S > 0, V < 0 1994 25. H 2 (g) + ½O 2 (g) H 2 O(l) H = 286 kj 2 Na(s) + ½O 2 (g) Na 2 O(s) H = 414 kj Na(s) + ½O 2 (g) + ½H 2 (g) NaOH(s) H = 425 kj Based on the information above, what is the standard enthalpy change for the following reaction? (A) 1,125 kj (B) 978 kj (C) 722 kj (D) 150 kj (E) +275 kj Na 2 O(s) + H 2 O(l) 2 NaOH(s) 35. For which of the following processes would S have a negative value? I. 2 Fe 2 O 3 (s) 4 Fe(s) + 3 O 2 (g) II. Mg 2+ + 2 OH Mg(OH) 2 (s) III. H 2 (g) + C 2 H 4 (g) C 2 H 6 (g) (A) I only (B) I and II only (C) I and III only (D) II and III only (E) I, II, and III 58. N 2 (g) + 3 H 2 (g) 2 NH 3 (g) The reaction indicated above is thermodynamically spontaneous at 298 K, but becomes nonspontaneous at higher temperatures. Which of the following is true at 298 K? (A) G, H, and S are all positive. (B) G, H. and S are all negative.
Page 3 of 7 (C) G and H are negative, but S is positive. (D) G and S are negative, but H is positive. (E) G and H are positive, but S is negative. 60. I 2 (g) + 3 Cl 2 (g) 2 ICl 3 (g) According to the data in the table below, what is the value of H for the reaction represented above? 2008 (A) 860 kj (B) 382 kj (C) +180 kj (D) +450 kj (E) +1,248 kj Bond Average Bond Energy (kj mol 1 ) I I 149 Cl Cl 239 I Cl 208 59. The diagram above represents a mixture of NO 2 (g) and N 2 O 4 (g) in a 1.0 L container at a given temperature. The two gases are in equilibrium according to the equation 2 NO 2 (g) N 2 O 4 (g). Which of the following must be true about the value of the equilibrium constant for the reaction at this temperature? (A) K = 0 (B) 0 < K < 1 (C) K = 1 (D) K > 1 (E) There is not enough information to determine the relative value of K. Part II Free Response 1986 The first ionization energy of sodium is +496 kj mol 1, yet the standard heat of formation of sodium chloride from its elements in their standard state is 411 kj mol 1. (a) Name the factors that determine the magnitude of the standard heat of formation of solid sodium chloride. Indicate whether each factor makes the reaction for the formation of sodium chloride from its elements more or less exothermic. (b) Name the factors that determine whether the reaction that occurs when such a salt dissolves in water is exothermic or endothermic and discuss the effect of each factor on the solubility.
Page 4 of 7 1990 Standard Free Energies of Formation at 298 K Substance G f at 298 K ( kj mol -1 ) C 2 H 4 Cl 2 (g) 80.3 C 2 H 5 Cl(g) 60.5 HCl(g) 95.3 Cl 2 (g) 0 Average Bond Dissociation Energies at 298 K Bond Energy, kj mol -1 C H 414 C C 347 C Cl 377 Cl Cl 243 H Cl 431 The tables above contain information for determining thermodynamic properties of the reaction below. C 2 H 5 Cl(g) + Cl 2 (g) C 2 H 4 Cl 2 (g) + HCl(g) (a) Calculate the H for the reaction above, using the table of average bond dissociation energies. (b) Calculate the S for the reaction at 298 K, using data from either table as needed. (c) Calculate the value of K eq for the reaction at 298 K. (d) What is the effect of an increase in temperature on the value of the equilibrium constant? Explain your answer. 1991 BCl 3 (g) + NH 3 (g) Cl 3 BNH 3 (s) The reaction represented above is a reversible reaction. (a) Predict the sign of the entropy change, S, as the reaction proceeds to the right. Explain your prediction. (b) If the reaction spontaneously proceeds to the right, predict the sign of the enthalpy change, H. Explain your prediction. (c) The direction in which the reaction spontaneously proceeds changes as the temperature is increased above a specific temperature. Explain. (d) What is the value of the equilibrium constant at the temperature referred to in (c); that is, the specific temperature at which the direction of the spontaneous reaction changes? Explain. 1992 Cl 2 (g) + 3 F 2 (g) 2 ClF 3 (g) ClF 3 can be prepared by the reaction represented by the equation above. For ClF 3 the standard enthalpy of formation, H f, is 163.2 kj mol -1 and the standard free energy of formation, G f, is 123.0 kj mol -1. (a) Calculate the value of the equilibrium constant for the reaction at 298 K. (b) Calculate the standard entropy change, S, for the reaction at 298 K. (c) If ClF 3 were produced as a liquid rather than as a gas, how would the sign and the magnitude of S for the reaction be affected? Explain. (d) At 298 K the absolute entropies of Cl 2 (g) and ClF 3 (g) are 222.96 J K 1 mol 1 and 281.50 J K 1 mol 1, respectively. (i) Account for the larger entropy of ClF 3 (g) relative to that of Cl 2 (g). (ii) Calculate the value of the absolute entropy of F 2 (g) at 298 K.
Page 5 of 7 1993 2 C 4 H 10 (g) + 13 O 2 (g) 8 CO 2 (g) + 10 H 2 O(l) The reaction represented above is spontaneous at 25 C. Assume that all reactants and products are in their standard state. (a) Predict the sign of S for the reaction and justify your prediction. (b) What is the sign of G for the reaction? How would the sign and magnitude of G be affected by an increase in temperature to 50 C? Explain your answer. (c) What must be the sign of H for the reaction at 25 C? How does the total bond energy of the reactants compare to that of the products? (d) When the reactants are place together in a container, no change is observed even though the reaction is known to be spontaneous. Explain this observation. 1994 2 H 2 S(g) + SO 2 (g) 3 S(s) + 2 H 2 O(g) At 298 K, the standard enthalpy change, H for the reaction represented above is 145 kj. (a) Predict the sign of the standard entropy change, S, for the reaction. Explain the basis for your prediction. (b) At 298 K, the forward reaction (i.e., toward the right) is spontaneous. What change, if any, would occur in the value of G for this reaction as the temperature is increased? Explain your reasoning using thermodynamic principles. (c) What change, if any, would occur in the value of the equilibrium constant, K eq, for the situation described in (b)? Explain your reasoning. (d) The absolute temperature at which the forward reaction becomes nonspontaneous can be predicted. Write the equation that is used to make the prediction. Why does this equation predict only an approximate value for the temperature? 1995 (1) Propane, C 3 H 8, is a hydrocarbon that is commonly used as fuel for cooking. (a) Write a balanced equation for the complete combustion of propane gas, which yields CO 2 (g) and H 2 O(l). (b) Calculate the volume of air at 30 C and 1.00 atm that is needed to burn completely 10.0 grams of propane. Assume that air is 21.0 percent O 2 by volume. (c) The heat of combustion of propane is 2,220.1 kj mol -1. Calculate the heat of formation, H f, of propane given that H f of H 2 O(l) = 285.3 kj mol -1 and H f of CO 2 (g) = 393.5 kj mol -1. (d) Assuming that all of the heat evolved in burning 30.0 grams of propane is transferred to 8.00 kilograms of water (specific heat = 4.18 J g 1 K 1 ), calculate the increase in temperature of water. 1995 (2) Lead iodide is a dense, golden yellow, slightly soluble solid. At 25 C, lead iodide dissolves in water forming a system represented by the following equation. PbI 2 (s) Pb 2+ + 2 I H = +46.5 kj (a) How does the entropy of the system PbI 2 (s) + H 2 O(l) change as PbI 2 (s) dissolves in water at 25 C? Explain. (b) If the temperature of the system were lowered from 25 C to 15 C, what would be the effect on the value of K sp? Explain. (c) If additional solid PbI 2 were added to the system at equilibrium, what would be the effect on the concentration of I in the solution? Explain. (d) At equilibrium, G = 0. What is the initial effect on the value of G of adding a small amount of Pb(NO 3 ) 2 to the system at equilibrium? Explain.
Page 6 of 7 1996 C 2 H 2 (g) + 2 H 2 (g) C 2 H 6 (g) Information about the substances involved in the reaction represented above is summarized in the following tables. Substance S (J K 1 mol 1 ) H f (kj mol -1 ) C 2 H 2 (g) 200.9 226.7 H 2 (g) 130.7 0 C 2 H 6 (g) - - - - 84.7 Bond Bond Energy (kj mol -1 ) C C 347 C=C 611 C H 414 H H 436 (a) If the value of the standard entropy change, S, for the reaction is 232.7 J K 1, calculate the standard molar entropy, S, of C 2 H 6 gas. (b) Calculate the value of the standard free-energy change, G, for the reaction. What does the sign of G indicate about the reaction above? (c) Calculate the value of the equilibrium constant, K, for the reaction at 298 K. (d) Calculate the value of the C C bond energy in C 2 H 2 in kilojoules per mole. 1997 For the gaseous equilibrium represented below, it is observed that greater amounts of PCl 3 and Cl 2 are produced as the temperature is increased. PCl 5 (g) PCl 3 (g) + Cl 2 (g) (a) What is the sign of S for the reaction? Explain. (b) What change, if any, will occur in G for the reaction as the temperature is increased? Explain your reasoning in terms of thermodynamic principles. (c) If He gas is added to the original reaction mixture at constant volume and temperature, what will happen to the partial pressure of Cl 2? Explain. (d) If the volume of the reaction mixture is decreased at constant temperature to half the original volume, what will happen to the number of moles of Cl 2 in the reaction vessel? Explain. 1998 C 6 H 5 OH(s) + 7 O 2 (g) 6 CO 2 (g) + 3 H 2 O(l) When a 2.000-gram sample of pure phenol, C 6 H 5 OH(s), is completely burned according to the equation above, 64.98 kilojoules of heat is released. Use the information in the table below to answer the questions that follow. Substance Standard Heat of Formation, H f, at 25 C (kj mol 1 ) Absolute Entropy, S, at 25 C (J mol 1 K 1 ) C(graphite) 0.00 5.69 CO 2 (g) 393.5 213.6 H 2 (g) 0.00 130.6 H 2 O(l) 285.85 69.91 O 2 (g) 0.00 205.0 C 6 H 5 OH(s)? 144.0
Page 7 of 7 (a) Calculate the molar heat of combustion of phenol in kilojoules per mole at 25 C. (b) Calculate the standard heat of formation, H f, of phenol in kilojoules per mole at 25 C. (c) Calculate the value of the standard free-energy change, G, for the combustion of phenol at 25 C. (d) If the volume of the combustion container is 10.0 liters, calculate the final pressure in the container when the temperature is changed to 110. C. (Assume no oxygen remains unreacted and that all products are gaseous.) 2001 2 NO(g) + O 2 (g) 2 NO 2 (g) H = 114.1 kj, S = 146.5 J K 1 The reaction represented above is one that contributes significantly to the formation of photochemical smog. (a) Calculate the quantity of heat released when 73.1 g of NO(g) is converted to NO 2 (g). (b) For the reaction at 25 C, the value of the standard free-energy change, G, is 70.4 kj. (i) Calculate the value of the equilibrium constant, K eq, for the reaction at 25 C. (ii) Indicate whether the value of G would become more negative, less negative, or remain unchanged as the temperature is increased. Justify your answer. (c) Use the data in the table below to calculate the value of the standard molar entropy, S, for O 2 (g) at 25 C. Standard Molar Entropy, S (J K 1 mol 1 ) NO(g) 210.8 NO 2 (g) 240.1 (d) Use the data in the table below to calculate the bond energy, in kj mol 1, of the nitrogen-oxygen bond in NO 2. Assume that the bonds in the NO 2 molecule are equivalent (i.e., they have the same energy). Bond Energy (kj mol 1 ) Nitrogen-oxygen bond in NO 607 Oxygen-oxygen bond in O 2 495 Nitrogen-oxygen bond in NO 2?