WA 7 Dearing

4. A sheet of gold weighing 10.0 g and at a temperature of 18.0 °C is placed flat on a sheet of iron weighing 20.0 g and at a temperature of 55.6 °C. What is the final temperature of the combined metals? Assume that no heat is lost to the surroundings. (5 points) Specific heat Au = 0.129 J/g o C, Specific heat Fe = 0.444 J/g o C 10g x 0.129J x (t-18C) = 20g x 0.444J x (t-55.6) 10g x 0.129 = 1.29 x -18 = -23.2C, 20 x 0.444 = 8.88 x 55.6 = 494 1.29 t -23.2 = -8.88 t + 494, 1.29 t -23.2 + (-494) = 517 -8.88 + -1.29 = 10.17 10.17 t = 517, t = 517/10.17 = Final Temp 50.8C 5. A 0.1375 g sample of solid magnesium is burned in a constant-volume bomb calorimeter that has a heat capacity of 3024 J/°C. The temperature increases by 1.126°C. Calculate the heat given off by the burning Mg, in kJ/g and in kJ/mol. (5 points) (Reference: Chang 6.37) 3024 J/°C x 1.126°C = 3405kJ / 0.1375g = - 24.76kJ/g 24.76kJ/g / Mg 24.3 amu = - 602kJ/mol 6. Calculate the heat of decomposition for this process at constant pressure and 25°C: (10 points) CaCO 3 ( s ) → CaO( s ) + CO 2 ( g ) (1 mol CaO -635.6) + (1 mol CO 2 -393.5) – (1 mol CaCO 3 -1206.9) = 177.8 kJ Substance ∆H° f , kJ/mol CaCO 3 ( s ) –1206.9 CaO( s ) –635.6 CO 2 ( g ) –393.5 (Reference: Chang 6.51) 7. Methanol, ethanol, and n -propanol are three common alcohols. When 1.00 g of each of these alcohols is burned in air, heat is liberated as shown by the following data: (a) methanol (CH 3 OH), –22.6 kJ/g; (b) ethanol (C 2 H 5 OH), –29.7 kJ/g; (c) n - propanol (C 3 H 7 OH), –33.4 kJ/g. Calculate the heats of combustion of these alcohols in kJ/mol. (12 points) (Reference: Chang 6.55) (a) methanol (CH 3 OH), –22.6 kJ/g CH 3 OHamu 32, 1g / 32 = 0.031mol, -22.6 kJ/g / 0.031mol = -729 kJ/mol (b) ethanol (C 2 H 5 OH), –29.7 kJ/g C 2 H 5 OHamu 46, 1g / 46 = 0.022mol, -29.7 kJ/g / 0.022mol = -1350 kJ/mol (c) n -propanol (C 3 H 7 OH), –33.4 kJ/g C 3 H 7 OHamu 60, 1g / 60 = 0.017mol, -33.4 kJ/g /0.017mol = -1965kJ/mol 2