Chapter 6: Thermochemistry

Transcription

1 1. Radiant energy is A) the energy stored within the structural units of chemical substances. B) the energy associated with the random motion of atoms and molecules. C) solar energy, i.e. energy that comes from the sun. D) energy available by virtue of an object's position. Ans: C Category: Easy Section: An endothermic reaction causes the surroundings to A) warm up. D) decrease in temperature. B) become acidic. E) release CO 2. C) condense. Ans: D Category: Easy Section: An exothermic reaction causes the surroundings to A) increase in temperature D) decrease in temperature. B) become acidic. E) release CO 2. C) expand. Ans: A Category: Easy Section: Calculate the amount of heat necessary to raise the temperature of g of water from 50.4 F to 85.0 F. The specific heat of water = J/g C. A) 1.1 kj B) 10.9 kj C) 16.6 kj D) 19.5 kj E) 48.0 kj Ans: B Category: Medium Section: Three separate 3.5g blocks of Al, Cu, and Fe at 25 C each absorb kj of heat. Which block reaches the highest temperature? The specific heats of Al, Cu, and Fe are J/g C, 0.385J/g C, and J/g C, respectively. A) Al B) Cu C) Fe D) Al and Cu E) Fe and Cu Ans: B Category: Easy Section: Suppose a 50.0 g block of silver (specific heat = J/g C) at 100 C is placed in contact with a 50.0 g block of iron (specific heat = J/g C) at 0 C, and the two blocks are insulated from the rest of the universe. The final temperature of the two blocks A) will be higher than 50 C. B) will be lower than 50 C. C) will be exactly 50 C. D) is unrelated to the composition of the blocks. E) cannot be predicted. Ans: B Category: Medium Section: 6.5 Page 119

2 21. Naphthalene combustion can be used to calibrate the heat capacity of a bomb calorimeter. The heat of combustion of naphthalene is 40.1 kj/g. When g of naphthalene was burned in a calorimeter containing 1,000. g of water, a temperature rise of 4.21 C was observed. What is the heat capacity of the bomb calorimeter excluding the water? A) 1.76 kj/ C B) 3.64 kj/ C C) 7.8 kj/ C D) 15.3 kj/ C E) 32.9 kj/ Ans: B Category: Medium Section: Which of the following processes is exothermic, given the following: N 2 (g) + 2 O 2 (g) N 2 O 4 (l) H = 9.67 kj/mol N 2 (g) + 2 O 2 (g) 2 NO 2 (g) H = kj/mol A) 2 N 2 (g) + 4 O 2 (g) 2 N 2 O 4 (l) B) ½ N 2 (g) + O 2 (g) ½ N 2 O 4 (l) C) N 2 O 4 (l) N 2 (g) + 2 O 2 (g) D) 2 N 2 (g) + 4 O 2 (g) 2 NO 2 (g) + N 2 O 4 (l) E) 2 N 2 (g) + 4 O 2 (g) 4 NO 2 (g) Ans: C Category: Easy Section: A 100. ml sample of M aqueous hydrochloric acid is added to 100. ml of M aqueous ammonia in a calorimeter whose heat capacity (excluding any water) is 480. J/K. The following reaction occurs when the two solutions are mixed. HCl(aq) + NH 3 (aq) NH 4 Cl(aq) The temperature increase is 2.34 C. Calculate H per mole of HCl and NH 3 reacted. A) 154 kj/mol D) 154 kj/mol B) 1.96 kj/mol E) 485 kj/mol C) 1.96 kj/mol Ans: A Category: Difficult Section: Which of the following has a H f = 0 kj/mol? A) CO 2 (g) B) O 3 (g) C) Cl - (aq) D) NH 3 (aq) E) I 2 (s) Ans: E Category: Easy Section: Which of the following has a H f = 0 kj/mol? A) NO(g) B) CS 2 (l) C) Fe 2+ (aq) D) H 2 O(l) E) N 2 (g) Ans: E Category: Easy Section: When g of Na(s) reacts with excess F 2 (g) to form NaF(s), 13.8 kj of heat is evolved at standard-state conditions. What is the standard enthalpy of formation ( H f ) of NaF(s)? A) -570 kj/mol D) 24.8 kj/mol B) 24.8 kj/mol E) 570 kj/mol C) 7.8 kj/mol Ans: A Category: Medium Section: 6.6 Page 120

3 34. When 18.5 g of HgO(s) is decomposed to form Hg(l) and O 2 (g), 7.75 kj of heat is absorbed at standard-state conditions. What is the standard enthalpy of formation ( H f ) of HgO(s)? A) 90.7 kj/mol D) 27.9 kj/mol B) 7.75 kj/mol E) 143 kj/mol C) kj/mol Ans: A Category: Medium Section: Glycine, C 2 H 5 O 2 N, is important for biological energy. The combustion reaction of glycine is given by the equation 4C 2 H 5 O 2 N(s) + 9O 2 (g) 8CO 2 (g) + 10H 2 O(l) + 2N 2 (g) H rxn = 3857 kj/mol Given that H f [CO 2 (g)] = kj/mol and H f [H 2 O(l)] = kj/mol, calculate the enthalpy of formation of glycine. A) 3,178 kj/mol D) kj/mol B) 964 kj/mol E) 2,149 kj/mol C) kj/mol Ans: C Category: Medium Section: During volcanic eruptions, hydrogen sulfide gas is given off and oxidized by air according to the following chemical equation: 2H 2 S(g) + 3O 2 (g) 2SO 2 (g) + 2H 2 O(g) Calculate the standard enthalpy change for the above reaction given: 3S(s) + 2H 2 O(g) 2H 2 S(g) + SO 2 (g) H = kj/mol S(s) + O 2 (g) SO 2 (g) H = kj/mol A) kj/mol D) kj/mol B) kj/mol E) kj/mol C) kj/mol Ans: A Category: Medium Section: How much heat (kj) is evolved when 4.50 g of Fe 2 O 3 is reacted with excess carbon monoxide using the equation below? Fe 2 O 3 (s) + 3CO(g) 2 Fe(s) + 3 CO 2 (g) H rxn = 24.8 kj/mol, A) kj B) 2.10 kj C) 17.9 kj D) 24.8 kj E) 112 kj Ans: A Category: Medium Section: Given H 2 (g) + (1/2)O 2 (g) H 2 O(l), H = 286 kj/mol, determine the standard enthalpy change for the reaction 2H 2 O(l) 2H 2 (g) + O 2 (g). A) H = 286 kj/mol D) H = +572 kj/mol B) H = +286 kj/mol E) H = 143 kj/mol C) H = 572 kj/mol Ans: D Category: Easy Section: 6.6 Page 121

4 52. Pentaborane B 5 H 9 (s) burns vigorously in O 2 to give B 2 O 3 (s) and H 2 O(l). Calculate H rxn for the combustion of 5.00 mol of B 5 H 9. H f [B 2 O 3 (s)] = 1,273.5 kj/mol H f [B 5 H 9 (s)] = 73.2 kj/mol H f [H 2 O(l)] = kj/mol A) 45,400 kj D) 9,090 kj B) 45,400 kj E) 8,790 kj C) 22,700 kj Ans: C Category: Difficult Section: The combustion of pentane produces heat according to the equation C 5 H 12 (l) + 8O 2 (g) 5CO 2 (g) + 6H 2 O(l) H rxn = 3,510 kj/mol How many grams of CO 2 are produced per kj of heat released? A) g B) 3.56 g C) 31.3 g D) 157 g E) 309 g Ans: D Category: Medium Section: Given the thermochemical equation 2SO 2 (g) + O 2 (g) 2SO 3 (g), H rxn = 198 kj/mol, how much heat is evolved when 600. g of SO 2 is burned? A) kj D) kj B) 928 kj E) 59,400 kj C) kj Ans: B Category: Medium Section: Determine the heat given off to the surroundings when 9.0 g of aluminum reacts according to the equation 2Al + Fe 2 O 3 Al 2 O 3 + 2Fe, H rxn = 849 kj/mol. A) kj D) kj B) kj E) kj C) kj Ans: A Category: Medium Section: Find the heat absorbed from the surroundings when 15 g of O 2 reacts according to the equation O + O 2 O 3, H rxn = 103 kj/mol. A) kj B) 32 kj C) 48 kj D) 96 kj E) 110 kj Ans: C Category: Medium Section: Ethanol (C 2 H 5 OH) burns according to the equation C 2 H 5 OH(l) + 3O 2 (g) 2CO 2 (g) + 3H 2 O(l), H rxn = 1367 kj/mol. How much heat is released when 35.0 g of ethanol is burned? A) kj B) 1,040 kj C) 1,367 kj D) 1,797 kj E) kj Ans: B Category: Medium Section: 6.4 Page 122

5 63. Methanol (CH 3 OH) burns according to the equation 2CH 3 OH(l) + 3O 2 (g) 2CO 2 (g) + 4H 2 O(l), H rxn = 1454 kj/mol. How much heat, in kilojoules, is given off when 75.0 g of methanol is burned? A) 727 kj D) kj B) kj E) kj C) kj Ans: D Category: Medium Section: According to the first law of thermodynamics: A) Energy is neither lost nor gained in any energy transformations. B) Perpetual motion is possible. C) Energy is conserved in quality but not in quantity. D) Energy is being created as time passes. We have more energy in the universe now than when time began. Ans: A Category: Easy Section: A gas is allowed to expand, at constant temperature, from a volume of 1.0 L to 10.1 L against an external pressure of 0.50 atm. If the gas absorbs 250 J of heat from the surroundings, what are the values of q, w, and E? A) A B) B C) C D) D E) E Ans: A Category: Medium Section: Ozone (O 3 ) in the atmosphere can be converted to oxygen gas by reaction with nitric oxide (NO). Nitrogen dioxide is also produced in the reaction. What is the enthalpy change when 8.50L of ozone at a pressure of 1.00 atm and 25 C reacts with L of nitric oxide at the same initial pressure and temperature? [ H f (NO) = 90.4 kj/mol; H f (NO 2 ) = kj/mol; H f (O 3 ) = kj/mol] A) 1690 kj B) 167 kj C) 97.6 kj D) 69.2 kj E) 19.7 kj Ans: D Category: Difficult Section: The heat of solution of calcium chloride CaCl 2 is 82.8 kj/mol, and the combined heats of hydration of 1.00 mole of gaseous calcium ions and 2.00 mole of gaseous chloride ions is 2327 kj. What is the lattice energy of calcium chloride? Ans: 2,244 kj/mol Category: Medium Section: 6.7 Page 123

6 98. The heat of combustion of propane, C 3 H 8, 2220 kj/mol. The specific heat of copper is J/g C. How many grams of propane must be burned to raise the temperature of a 10.0 kg block of copper from 25.0 C to 65.0 C, assuming none of the heat is lost to the surroundings Ans: 3.06 g Category: Medium Section: Calculate the heat released (kj) in the reaction of 3.50 g of acetylene (C 2 H 2 ) and excess hydrogen gas to form ethane gas: C 2 H 2 (g) + 2H 2 (g) C 2 H 6 (g) Given: 2C 2 H 2 (g) + 5O 2 (g) 4CO 2 (g) + 2H 2 O(g) H = 2320 kj/mol 2C 2 H 6 (g) + 7O 2 (g) 4CO 2 (g) + 6H 2 O(g) H = 3040 kj/mol 2H 2 (g) + O 2 (g) 2H 2 O(g) H = 572 kj/mol Ans: 28.5 kj Category: difficult Section: 6.6 Page 124