# Written assignment 7: energy and thermochemistry

Written Assignment 7: 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)
a) Methanol
b) Ethanol
c) n-propanol (C3H7OH), –33.4 kJ/g
(Reference: Chang 6.55)
8. From the standard enthalpies of formation, calculate ΔH°rxn for the reaction
C6H12(l) + 9O2(g) → 6CO2(g)  + 6H2O(l)
For C6H12(l), ΔH°f = –151.9 kJ/mol (5 points)
(Reference: Chang 6.57)
9. Determine the amount of heat (in kJ) given off when 1.26 × 104 g of ammonia are produced according to the equation
N2(g) + 3H2(g) → 2NH3(g)  ΔH°= –92.6 kJ/mol
Assume that the reaction takes place under standard conditions at 25oC. (5 points)
(Reference: Chang 6.59)
10. From the following heats of combustion,
CH3OH(l) + 3/2O2(g) → CO2(g) + 2H2O(l) ΔHorxn = –726.4 kJ/mol
C(graphite) + O2(g)  → CO2(g)   ΔHorxn = –393.5 kJ/mol
H2(g) + ½O2(g) → H2O(l)   ΔHorxn = –285.8 kJ/mol
Calculate the enthalpy of formation of methanol (CH3OH) from its elements.
C(graphite) + 2H2(g) + ½O2(g) → CH3OH(l) (10 points)
(Reference: Chang 6.63)
11. Methanol (CH3OH) is an organic solvent and is also used as a fuel in some automobile engines. From the following data, calculate the standard enthalpy of formation of methanol:   (5 points)
(Reference: Chang 6.87)
12. A 44.0 g sample of unknown metal at 99.0 °C was placed in a constant-pressure calorimeter, with a heat capacity of 12.4 J/°C, containing 80.0 g of water at 24.0°C. The final temperature of the system was found to be 28.4 °C. Calculate the specific heat of the metal. (10 points)
(Reference: Chang 6.85)
13. A person ate 0.50 pound of cheese (an energy intake of 4000 kJ). Suppose that none of the energy was stored in his body. What mass (in grams) of water would he need to perspire in order to maintain his original temperature? (It takes 44.0 kJ to vaporize 1 mole of water.)   (5 points)
14. The total volume of the Pacific Ocean is estimated to be 7.2 × 108 km3. A medium-sized atomic bomb produces 1.0 × 1015 J of energy upon explosion. Calculate the number of atomic bombs needed to release enough energy to raise the temperature of the water in the Pacific Ocean by 1°C. (10 points)

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