Thermochemistry Unit Section 16.2

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Thermochemistry Unit Section 16.2
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Practice Problem 15
a. H2O(g)
H2(s) 1/2O2(g) ? H2O(g) 241.8 KJ
b. CaCl2(s)
Ca(s) Cl2(g) ? CaCl2(s) 795.4 KJ
c. CH4(g)
C(s) 2H2(g) ? CH4(g) 74.6 KJ
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d. C6H6(l)
6C(s) 3H2(g) 49 KJ ? C6H6(l)
e. Show the standard molar enthalpy of parts c)
and d) using another method
C(s) 2H2(g) ? CH4(g) ?Hof -74.6 KJ
6C(s) 3H2(g) ? C6H6(l) ?Hof 49 KJ
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Practice Problem 16 Draw an enthalpy diagram to
represent the standard molar enthalpy of
formation of sodium chloride.
NaCl(s) ?Hof -411.1 KJ/mol Exothermic
Na(s) 1/2Cl2(g) ? NaCl(s) 411.1 KJ
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Practice Problem 17
a. Ethane, C2H6(g)
C2H6(g) 7/2O2(g) ? 2CO2(g) 3H20(l) 1250.9
KJ
b. Propane, C3H8(g)
C3H8(g) 5O2(g) ? 3CO2(g) 4H20(l) 2323.7 KJ
c. Butane, C4H10(g)
C4H10(g) 13/2O2(g) ? 4CO2(g) 5H20(l) 3003.0
KJ
c. Pentane, C5H12(l)
C5H12(l) 8O2(g) ? 5CO2(g) 6H20(l) 3682.3 KJ
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Practice Problem 18 Draw an enthalpy diagram to
represent the standard molar enthalpy of
combustion of heptane, C7H16(l) (use Table 16.3).
C7H16(l) 11O2(g) ? 7CO2(g) 8H20(l) 5040.9
KJ
?Exothermic reaction, so the enthalpy of
reactants is higher than the enthalpy of the
products.
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Sample Problem (page 644) Methane is the main
component of natural gas. Natural gas undergoes
combustion to provide energy for heating homes
and cooking food. a) How much heat is released
when 500.00 g of methane forms from the elements?
q n?Hof
q ?
nmethane m/M
m 500.0 g
(500.00 g) / (16.05 g/mol)
?Hof -74.6 KJ/mol
3.115 mol
?q n?Hof (3.115 mol)(-74.6 KJ/mol) -232.4
KJ
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b) How much heat is released when 50.00 g of
methane undergoes complete combustion?
q n?Hocomb
q ?
nmethane m/M
m 50.0 g
(500.00 g) / (16.05 g/mol)
?Hocomb -965.1 KJ/mol
3.115 mol
?q n?Hocomb (3.115 mol)(-965.1 KJ/mol)
-3006.29 KJ
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Practice Problem 19 a) Hydrogen gas and oxygen
gas react to form 0.534 g of liquid water. How
much heat is released to the surroundings?
q n?Hof
q ?
nwater m/M
m 0.534 g
(0.534 g) / (18.02 g/mol)
?Hof -285.8 KJ/mol
0.0296 mol
?q n?Hof (0.0296 mol)(-285.8 KJ/mol) -8.47
KJ
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b) Hydrogen gas and oxygen gas react to form
0.534 g of gaseous water. How much heat is
released to the surroundings?
q n?Hof
q ?
nwater m/M
m 0.534 g
(0.534 g) / (18.02 g/mol)
?Hof -241.8 KJ/mol
0.0296 mol
?q n?Hof (0.0296 mol)(-241.8 KJ/mol) -7.16
KJ
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• Practice Problem 21
• Determine the heat released by the combustion of
  56.78 g of
• pentane, C5H12(l)

q ?
q n?Hocomb
m 56.78 g
?Hocomb -3682.3 KJ/mol
Mpentane 72.17 g/mol
?q n?Hocomb (0.787 mol)(-3682.3 KJ/mol)
-2897.06 KJ
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b) Determine the heat released by the combustion
of 56.78 g of pentane, C5H12(l)
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c) Determine the heat released by the combustion
of 2.344 X 104 g of hexane, C6H14(l)
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Practice Problem 23 What mass of methanol,
CH3OH(l), is formed from its elements if 2.34 X
104 kJ of energy is released during the process?
Practice Problem 23 What mass of methanol,
CH3OH(l), is formed from its elements if 2.34 X
104 kJ of energy is released during the process?
m ?
q n?Hof
q -2.34 X 104 kJ
?n q / ?Hof (-2.34 X 104 kJ)(-238.6
KJ/mol) 98.07 mol
?Hof -238.6 KJ/mol
Mmethanol 32.05 g/mol
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Practice Problem 24 An ice cube with a mass of
8.2 g is placed in some lemonade. The ice cube
melts completely. How much heat does the ice
cube absorb from the lemonade as it melts?
q ?
q n?Hofus
mice cube 8.2 g
?Homelt ?Hofus 6.02 KJ/mol
Mice cube 18.02 g/mol
?q n?Hofus (0.455 mol)(6.02 KJ/mol) 2.74
KJ
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Practice Problem 25 A teacup contains 0.100 kg
of water at its freezing point. The water
freezes solid. a) How much heat is released to
its surroundings?
q ?
q n?Hofus
mwater 0.100 kg 100 g
?Hofreezing ?Hofus -6.02 KJ/mol
Mwater 18.02 g/mol
?q n?Hofus (5.55 mol)(-6.02 KJ/mol) -33.41
KJ
b) ?qmelting 33.41 KJ
?qfreezing -33.41 KJ
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Practice Problem 26 A sample of mercury
vaporizes. The mercury is at its boiling point
and has a mass of 0.325 g. How much heat is
absorbed or released to the surroundings?
q ?
q n?Hovap
mmercury 0.325 g
?Hovap 59 KJ/mol
Mmercury 200.59 g/mol
?q n?Hovap (0.00162 mol)(59 KJ/mol) 0.0956
KJ This is then an endothermic reaction since
heat energy is absorbed.
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• Practice Problem 27
• The molar enthalpy of solution for sodium
  chloride, NaCl, is 3.9 kJ/mol.
• Write a thermochemical reaction to represent the
  dissolution of
• sodium chloride?

Dissolution Solid state ? Liquid state
NaCl(s) 3.9 kJ ? NaCl(aq)
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b) Suppose you dissolve 25.3 g of sodium chloride
in a glass of water at room temperature. How
much heat is absorbed or released by the process?
q ?
q n?Hosol
mNaCl 25.3 g
?Hosol 3.9 KJ/mol
MNaCl 58.44 g/mol
?q n?Hosol (0.433 mol)(3.9 KJ/mol) 1.69 KJ
This is then an endothermic reaction since heat
energy is absorbed.
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c) Do you expect the glass containing the salt
solution to feel warm or cool? Explain your
answer.
Answer Since heat is absorbed to dissolve the
salt, heat is removed from the glass and it will
feel cold.
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Practice Problem 28 What mass of diethyl ether,
C4H10O, can be vaporized by adding 80.7 kJ of
heat?
q 80.7 kJ
q n?Hovap
mdiethyl ether ?
?Hovap 29 KJ/mol
Mdiethyl ether 74.14 g/mol
?m nM (2.78 mol)(74.14 g/mol) 206.08 g
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Practice Problem 29 3.97 X 104 J of heat is
required to vaporize 100 g of benzene, C6H6.
What is the molar enthalpy of vaporisation of
benzene?
q 3.97 X 104 J
q n?Hovap
mbenzene 100 g
?Hovap ?
Mbenzene 78.12 g/mol
?n m/M (100 g) / (78.12 g/mol) 1.28 mol