Energy And Thermochemistry

Answer all assigned questions and problems, and show all work.
1. Consider this reaction:
2CH3OH(l) + 3O2(g) → 4H2O(l) + 2CO2(g) ∆H = –1452.8 kJ/mol
a. Is this reaction endothermic or exothermic? (2 points)
b. What is the value of ∆H if the equation is multiplied throughout by 2? (2 points)
c. What is the value of ∆H if the direction of the reaction is reversed so that the products become the reactants and vice versa? (2 points)
d. What is the value of ∆H if water vapor instead of liquid water is formed as the product? (2 points)
(Reference: Chang 6.24)
2. The first step in the industrial recovery of zinc from the zinc sulfide ore is roasting,
that is, the conversion of ZnS to ZnO by heating:
2ZnS(s) + 3O2(g) → 3ZnO(s) + 2SO2(g)  ∆H = –879 kJ/mol
Calculate the heat evolved (in kJ) per gram of ZnS roasted. (5 points)
(Reference: Chang 6.25)
3. A 6.22 kg piece of copper metal is heated from 20.5 °C to 324.3 °C. Calculate the heat absorbed (in kilojoules) by the metal. Specific heat Cu = 0.385 J/g°C. (5 points)
(Reference: Chang 6.33)
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) (Reference: Chang 6.35)
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)
6. Calculate the heat of decomposition for this process at constant pressure and 25°C: (10 points)
CaCO3(s)  → CaO(s) + CO2(g)
(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 (CH3OH), –22.6 kJ/g; (b) ethanol (C2H5OH), –29.7 kJ/g; (c) n-propanol (C3H7OH), –33.4 kJ/g. Calculate the heats of combustion of these alcohols in kJ/mol.(12 points)

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