Chem1 Chapter 6 HW

99. Evaporating sweat cools the body because evaporation is an endothermic process: H2O(l)hH2O(g) Ho rxn = +44.01 kJ Estimate the mass of water that must evaporate from the skin to cool the body by 0.50 C. Assume a body mass of 95 kg and assume that the specific heat capacity of the body is 4.0 J/g # C .

99. 78 g

⁰101. Use standard enthalpies of formation to calculate the standard change in enthalpy for the melting of ice. (The Hf for H2O(s) is -291.8 kJ/mol.) Use this value to calculate the mass of ice required to cool 355 mL of a beverage from room temperature (25.0 C) to 0.0 C. Assume that the specific heat capacity and density of the beverage are the same as those of water.

101. H = 6.0 kJ>mol, 1.1 * 102 g

109. One tablespoon of peanut butter has a mass of 16 g. It is combusted in a calorimeter whose heat capacity is 120.0 kJ> C. The temperature of the calorimeter rises from 22.2 C to 25.4 C. Find the food caloric content of peanut butter.

109. 5.7 Cal>g

49. Suppose that 25 g of each substance is initially at 27.0 C. What is the final temperature of each substance upon absorbing 2.35 kJ of heat? a. gold b. silver c. aluminum d. water

49. a. 7.6 * 102 C b. 4.3 * 102 C c. 1.3 * 102 C d. 49 C

53. The air within a piston equipped with a cylinder absorbs 565 J of heat and expands from an initial volume of 0.10 L to a final volume of 0.85 L against an external pressure of 1.0 atm. What is the change in internal energy of the air within the piston?

53. 489 J

55. When 1 mol of a fuel burns at constant pressure, it produces 3452 kJ of heat and does 11 kJ of work. What are E and H for the combustion of the fuel?

55. E = -3463 J, H = -3452 kJ

59. Consider the thermochemical equation for the combustion of acetone (C3H6O), the main ingredient in nail polish remover. C3H6O(l) + 4O2(g) -> 3CO2(g) + 3H2O(g) Hrxn = -1790 kJ If a bottle of nail polish remover contains 177 mL of acetone, how much heat is released by its complete combustion? The density of acetone is 0.788 g/mL.

59. -4.30 * 103 kJ

65. A silver block, initially at 58.5 C, is submerged into 100.0 g of water at 24.8 C, in an insulated container. The final temperature of the mixture upon reaching thermal equilibrium is 26.2 C. What is the mass of the silver block?

65. mass of silver 77.1 grams

67. A 31.1 g wafer of pure gold, initially at 69.3 C, is submerged into 64.2 g of water at 27.8 C in an insulated container. What is the final temperature of both substances at thermal equilibrium?

67. final temperature 28.4 C

73. When 0.514 g of biphenyl (C12H10) undergoes combustion in a bomb calorimeter, the temperature rises from 25.8 C to 29.4 C. Find Erxn for the combustion of biphenyl in kJ/mol biphenyl. The heat capacity of the bomb calorimeter, determined in a separate experiment, is 5.86 kJ/C.

73. -6.3 * 103 kJ>mol

79. Calculate Hrxn for the reaction: Fe2O3(s) + 3 CO(g) -> 2 Fe(s) + 3 CO2(g) Use the following reactions and given H 's. 2 Fe(s) + 3/2O2(g) -> Fe2O3(s) H = -824.2 kJ CO(g) + 1/2O2(g) -> CO2(g) H = -282.7 kJ

79. -23.9 kJ

81. Calculate Hrxn for the reaction: 5 C(s) + 6 H2(g) -> C5H12(l) Use the following reactions and given H 's. C5H12(l) + 8 O2(g) -> 5 CO2(g) + 6 H2O(g) H = -3244.8 kJ C(s) + O2(g) -> CO2(g) H = -393.5 kJ 2 H2(g) + O2(g) ->2 H2O(g) H = -483.5 kJ

81. -173.2 kJ

83. Write an equation for the formation of each compound from its elements in their standard states, and find Hf for each from Appendix IIB . a. NH3(g) b. CO2(g) c. Fe2O3(s) d. CH4(g)

83. a. N2(g) + 3 H2(g)h 2 NH3 (g), Hf = -45.9 kJ>mol b. C(s, graphite) + O2(g)h CO2(g), Hf = -393.5 kJ>mol c. 2 Fe(s) + 3>2 O2(g) h Fe2O3(s), Hf = -824.2 kJ>mol d. C(s, graphite) + 2 H2(g) h CH4(g), Hf = -74.6 kJ>mol

85. Hydrazine (N2H4) is a fuel used by some spacecraft. It is normally oxidized by N2O4 according to the equation: N2H4(l) + N2O4(g)h2 N2O(g) + 2 H2O(g) Calculate Hrxn for this reaction using standard enthalpies of formation.

85. -382.1 kJ>mol

87. Use standard enthalpies of formation to calculate Hrxn for each reaction: a. C2H4(g) + H2(g)hC2H6(g) b. CO(g) + H2O(g)hH2(g) + CO2(g) c. 3 NO2(g) + H2O(l)h2 HNO3(aq) + NO(g) d. Cr2O3(s) + 3 CO(g)h2 Cr(s) + 3 CO2(g)

87. a. -137.1 kJ b. -41.2 kJ c. -137 kJ d. 290.7 kJ

37. Which statement is true of the internal energy of a system and its surroundings during an energy exchange with a negative Esys? a. The internal energy of the system increases and the internal energy of the surroundings decreases. b. The internal energy of both the system and the surroundings increases. c. The internal energy of both the system and the surroundings decreases. d. The internal energy of the system decreases and the internal energy of the surroundings increases.

d

41. A system releases 622 kJ of heat and does 105 kJ of work on the surroundings. What is the change in internal energy of the system?

41. -7.27 * 102 kJ

43. The gas in a piston (defined as the system) warms and absorbs 655 J of heat. The expansion performs 344 J of work on the surroundings. What is the change in internal energy for the system?

43. 311 kJ

47. How much heat is required to warm 1.50 L of water from 25.0 C to 100.0 C? (Assume a density of 1.0 g/mL for the water.)

47. 4.7 * 105 J

57. Determine whether each process is exothermic or endothermic and indicate the sign of H . a. natural gas burning on a stove b. isopropyl alcohol evaporating from skin c. water condensing from steam

57. a. exothermic, - b. endothermic, + c. exothermic, -