Chemistry 621
Chapter 17 Assignment #1
Multiple Choice. Identify the choice that best completes the statement or answers the question.
____
1. 5.00 g of sodium hydroxide is dissolved in 150 mL of water. A coffee-cup calorimeter shows the temperature change of the water
as 2.2C. The specific heat capacity of water is 4.184 J/g C. What is the enthalpy change for this dissolution?
a. 46 J
b. 1.4  103 J
c. 5.8  103 J
d. 1.7  105 J
e. 58 J
____
2. Which one of the following is not considered to be a fossil fuel?
a. coal
b. natural gas
c. oil
d. uranium
e. gasoline
____
3. Which of the following has a standard enthalpy of formation of 0 kJ/mol at 25°C and 1.00 atm?
a. CO2(g)
b. N2(g)
c. H2O(l)
d. H2O(s)
e. NaCl(l)
____
4. What is the molar enthalpy of combustion of hexane, C6H14(l), if combustion of 1.00 g of hexane causes a temperature rise of
6.09°C in a bomb calorimeter that has a heat capacity of 8.31 kJ/°C?
a. 50.6 kJ/mol
b. 63.2 kJ/mol
c. 118 kJ/mol
d. 1716 kJ/mol
e. 4.36 103 kJ/mol
____
5. What is the enthalpy change for the following reaction under standard conditions?
SO2(g) +
O2(g)  SO3(g)
Enthalpies of formation: SO3(g) =
a. + 98.9 kJ
b.
98.9 kJ
c.
197.8 kJ
d.
791.4 kJ
e. + 791.4 kJ
395.7 kJ/mol; SO2(g) =
296.8 kJ/mol
____
6. Estimate the standard enthalpy of formation of methane using the following data.
Average bond energies (kJ/mol): C–C = 346; C–H = 413; H–H = 432
Enthalpy of sublimation for graphite 716 kJ/mol
a.
72 kJ/mol
b.
442 kJ/mol
c.
504 kJ/mol
d.
874 kJ/mol
e.
1047 kJ/mol
____
7. Using the data given below, calculate the enthalpy of solution for HCl (g)  HCl(aq).
H2(g) + Cl2(g)  HCl(g) + 92.3 kJ
H2(g) + Cl2(g)  HCl(aq) + 167.5
a.
b.
c.
d.
e.
Hsoln = 167.5 kJ
Hsoln = + 259.8 kJ
Hsoln = 259.8 kJ
Hsoln = 75.2 kJ
Hsoln = + 75.2 kJ
____
8. The bond energies for H2(g) and HCl(g) are 432 kJ/mol and 432 kJ/mol, respectively. The enthalpy of formation of HCl (g) is 92.3
kJ/mol. What is the bond energy of Cl–Cl in kJ/mol?
a. 46
b. 124
c. 247
d. 308
e. 616
____
9. The energy in fossil fuels originated from
a. the food chain
b. photosynthesis
c. fossilization
d. the Sun
e. heat and pressure within Earth
____
10. A certain bomb calorimeter has a heat capacity of 7.50 kJ/°C. What is the expected temperature increase in this bomb calorimeter
when 0.270 g of octane, C8H18(l), is burned in excess oxygen?
Hcomb C8H18(l) = 5720 kJ/mol
a. 0.0404°C
b. 0.555°C
c. 0.206°C
d. 1.80°C
e. 24.7°C
____
11. Using the bond energies given below, estimate the enthalpy change for the addition reaction of ethene with hydrogen to form
ethane:
C2H4(g) + H2(g)  C2H6(g)
Bond energies (kJ/mol):
C–H 413; C–C 346; C=C 610; H–H 432
a. +394 kJ
b.
394 kJ
c. +658 kJ
d.
658 kJ
e. none of the above
____
12. A toy engine that runs on methanol fuel is known to be 23% efficient. If the engine burns 12.1 g of fuel, how much work should be
expected from the engine?
CH3OH(l) +
a.
b.
c.
d.
e.
O2(g)  CO2(g) + 2H2O(l)
H=
802 kJ/mol
1.3 103 kJ
4.9 102 kJ
3.0 102 kJ
70 kJ
2.2 kJ
____
13. Calcium chloride, CaCl2(s), is used in hot packs. A student uses a coffee-cup calorimeter to measure the molar heat of solution of
calcium chloride. When 1.25 g of calcium chloride is dissolved in 60 g of water, initially at 22.4°C, the final temperature of the
solution is 26.0°C. Calculate Hsoln for calcium chloride. The specific heat capacity of water is 4.184 J/g °C.
a. +0.72 kJ/mol
b.
0.72 kJ/mol
c.
24 kJ/mol
d.
80 k J/mol
e. +80 kJ/mol
____
14. Ozone, O3(g), in the stratosphere protects us from the Sun’s harmful ultraviolet rays. Calculate the enthalpy of formation of ozone
using the following thermochemical equations:
O3(g) + 2B(s)
B2O3(s)
H = 1415 kJ/mol
O2(g) + 2B(s)
a.
b.
c.
d.
e.
2687 kJ/mol
+2687 kJ/mol
143 kJ/mol
+143 kJ/mol
286 kJ/mol
B2O3(s)
H=
1272 kJ/mol
Short Answer
1. The formation and breaking of chemical bonds involves an energy change.
a) Compare the energy change that occurs when chemical bonds are formed with the energy change that occurs when chemical
bonds are broken.
b) Relate these energy changes to endothermic reactions and exothermic reactions.
2. Given Equations (1) and (2), determine
for Equation (3).
(1) C(s)  O2(g)  CO2(g)
H  393.5 kJ
(2) CO(g) 
(3) C(s) 
O2(g)  CO2(g
H  283.0 kJ
O2(g)  CO(g)
3. Consider the following reaction:
C(g) + 2H2(g)  CH4(g)
C-H bond energy = 413 kJ/mol; H-H bond energy = 432 kJ/mol
a) Calculate the heat of the reaction.
b) Is this reaction endothermic or exothermic?
4. Calculate the heat of combustion of hydrogen sulfide gas:
H2S(g) + 3/2O2(g)  H2O(g) + SO2(g) Hcomb = ?
Heats of formation (in kJ/mol): H2S(g) = –20.6; H2O(g) = –241.8; SO2(g) = –296.8
5. a) Use average bond energies to calculate the heat of the following reaction:
3C(g) + 4H2(g)  C3H8(g)
C–H bond energy = 413 kJ/mol; H–H bond energy = 432 kJ/mol; C–C bond energy = 346 kJ/mol
b) Is this reaction endothermic or exothermic?
c) Tables give the enthalpy of formation of propane as
calculated in Part a).
103.8 kJ/mol. Explain the difference between this value and the value you
6. a) Use average bond energies to estimate the enthalpy of formation of pentane gas.
5C(s) + 6H2(g)  C3H12(g)
C-C bond energy = 346 kJ/mol; C-H bond energy = 413 kJ/mol; H-H bond energy = 432 kJ/mol
b) Is this reaction endothermic or exothermic?
c) The enthalpy of formation of pentane is 173.5 kJ/mol. Explain the difference between this value and the value you calculated
in Part a).
7. Given Equations (1), (2), and (3), calculate the standard enthalpy of formation of acetylene, C 2H2, as shown in Equation (4).
(1) C(graphite)  O2(g)  CO2(g)
Hrxn  393.5 kJ
O2(g)  H2O(l)
Hrxn  285.8 kJ
(3) 2C2H2(g)  5O2(g)  4CO2(g)  2H2O(l)
(4) 2C(graphite)  H2(g)  C2H2(g)
Hrxn  2598.8 kJ
(2) H2(g) 
8. a) Given Equations (1) and (2), calculate the enthalpy change for Equation (3).
(1) Pb(s)  PbO2(s)  2SO3(g)  2PbSO4(s)
H  775 kJ
(2) SO3(g)  H2O(l)  H2SO4(aq)
H  133 kJ
(3) Pb(s)  PbO2(s)  2H2SO4(aq)  2PbSO4(s)  2H2O(l)
b) Draw an enthalpy diagram to represent Equation (3).
9. A 26.6-g sample of mercury is heated to 110.0C and then placed in 125 g of water in a coffee-cup calorimeter. The initial
temperature of the water is 23.00C. The specific heat capacity of water is 4.184 J/g C, and the specific heat capacity of mercury
is 0.139 J/g C. What is the final temperature of the water and the mercury?
10. 0.1375 g of magnesium is burned in a bomb calorimeter. There is exactly 300.0 g of water surrounding the magnesium. The change
in the temperature of the water is 1.126C. The heat capacity of the bomb calorimeter is 1770.0 J/C.
a) What is the heat change for the burning of magnesium?
b) What is the enthalpy of combustion of magnesium?
c) Write the thermochemical equation for this reaction.
11. Use standard enthalpies of formation to calculate the heat of reaction for the following equation.
2H2S(g)  3O2(g)  2H2O(l)  2SO2(g)
H°f (kJ/mol): H2S(g) 20.6; H2O(l) = 285.8; SO2(g) = 296.8
12. The main constituent of the gas used in barbecue tanks is propane, C 3H8. Use average bond energies to estimate the enthalpy of
combustion per mole of propane when complete combustion takes place.
Average bond energies (kJ/mol): C–C = 346; C-H = 413; H-H = 432; O-H = 467; C=O = 799; O=O = 498