07_Petrucci10e_SSM - CHAPTER 7 THERMOCHEMISTRY PRACTICE...

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154 CHAPTER 7 THERMOCHEMISTRY PRACTICE EXAMPLES 1A The heat absorbed is the product of the mass of water, its specific heat 4.18 11 J g C  c h , and the temperature change that occurs. heat energy g J gC CC kJ J kJ of heat energy = 237 4.18 37.0 4.0 1 1000 = 32.7    ch 1B The heat absorbed is the product of the amount of mercury, its molar heat capacity, and the temperature change that occurs. heat energy kg g kg mol Hg g Hg J mol C C kJ J kJ of heat energy =2 . 5 0 1000 1 1 200.59 28.0 6.0 20.0 1 1000 =4.89 F H G I K J b g 2A First calculate the quantity of heat lost by the lead. This heat energy must be absorbed by the surroundings (water). We assume 100% efficiency in the energy transfer. qq lead water kg g kg J J =1.00 1000 1 0.13 35.2 100.0 = 8.4 10 = 3  c h  3 3 2 water water water -1 4.18 J 8.4 10 J 8.4 10 J = 35.2 C 28.5 C = 28 = = 3.0 10 g 28 J g mm m 2B We use the same equation, equating the heat lost by the copper to the heat absorbed by the water, except now we solve for final temperature. qx x q Cu water g J g J =100.0 0.385 100.0 = 50.0 4.18 26.5 = c h 38.5 3850 = 209 +5539 J 38.5 + 209 = 5539 +3850 247.5 = 9389 xx x x x -1 9389 J = = 37.9 C 247.5 J C x 3A The molar mass of CHO 883 is 152.15 g/mol. The calorimeter has a heat capacity of 4.90 / kJ C . q calor kJ C C C g g mol kJ / mol = 4.90 30.09 24.89 1.013 152.15 1 =3.83 10 1 3  Hq comb calor kJ / mol = = 3.83 10 3
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Chapter 7: Thermochemistry 155 3B The heat that is liberated by the benzoic acid’s combustion serves to raise the temperature of the assembly. We designate the calorimeter’s heat capacity by C . qq rxn calorim g kJ g kJ =1.176 26.42 1 = 31.07 =  qC t C C calorim kJ C kJ C kJ C = = 31.07 = 4.96 = 31.07 4.96 =6.26 /  4A The heat that is liberated by the reaction raises the temperature of the reaction mixture. We assume that this reaction mixture has the same density and specific heat as pure water.  3 calorim 1.00 g 4.18 J = 200.0 mL 30.2 22.4 C = 6.5 10 J= 1 mL gC rxn     Owing to the 1:1 stoichiometry of the reaction, the number of moles of AgCl(s) formed is equal to the number of moles of AgNO 3 (aq) in the original sample. 3 3 1.00 M AgNO 1 L 1 mol AgCl moles AgCl = 100.0 mL = 0.100 mol AgCl 1000 mL 1 L 1 mol AgNO 3 rxn 6.5 10 J 1 kJ = = 65. kJ/mol 0.100 mol 1000 J q   Because q rxn is a negative quantity, the precipitation reaction is exothermic. 4B The assumptions include no heat loss to the surroundings or to the calorimeter, a solution density of 1.00 g/mL, a specific heat of 4.18 11 J g C , and that the initial and final solution volumes are the same. The equation for the reaction that occurs is       2 NaOH aq + HCl aq NaCl aq + H O l . Since the two reactants combine in a one to one mole ratio, the limiting reactant is the one present in smaller amount (i.e. the one with a smaller molar quantity).
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This note was uploaded on 11/07/2011 for the course CHEMISTRY 1500 taught by Professor Hammedmirza during the Fall '11 term at York University.

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07_Petrucci10e_SSM - CHAPTER 7 THERMOCHEMISTRY PRACTICE...

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