Chemistry 360
Spring 2017
Dr. Jean M. Standard
February 17, 2017
Problem Set 5
Good review problems in Engel & Reid (3rd ed.): 4.2, 4.3, 4.4, 4.8, 4.9, 4.11, 4.14, 4.15, 4.16, 4.17, 4.18, 4.19,
4.20, 4.23, 4.26, 4.27, 4.28
1.
Determine the amount of pressure-volume work performed by 1 mole of water freezing to ice at 0°C and 1 atm
pressure. The density of liquid water at 0°C is 0.99984 g/mL and the density of ice is 0.9168 g/mL.
2.
One of the primary components of nuclear fuel rods is uranium(IV) oxide, UO2. The melting point of UO2 is
3140 K and the enthalpy of fusion is 70.0 kJ/mol. Determine the enthalpy change per mole for the process
UO 2 (s, 3100 K )
→
UO 2 ( ℓ, 3200 K ) .
The molar heat capacities are C p,m (s) = 211.0 J mol-1K -1 and C p,m ( ℓ) = 134.2 J mol-1K -1 . Assume 1 bar
pressure and that the molar heat capacities of solid and liquid UO2 are independent of temperature.
3.
Determine the enthalpy change for one mole of water undergoing the following transformation at 1 bar
pressure,
H 2O s, – 30! C
(
)
→
H 2O g, 300! C .
(
)
The pressure is constant at 1 bar and assume that the molar heat capacities are independent of temperature.
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4.
Metabolic activity in the human body releases about 1×104 kJ of heat per day. Since the body is made up of
mostly water, approximate a body as consisting of 50 kg of water.
(a) How fast would the body temperature rise (in degrees K/day) if the body is considered to be an isolated
−1 −1
system at constant pressure (1 atm)? The molar heat capacity of water is C p,m = 75.291J mol K .
(b) How much water in kg per day must the body eliminate as perspiration to maintain normal body
temperature of 98.6°F? Assume that the energy required to vaporize water is 2.41 kJ/g.
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5.
Consider the following compounds: NaHCO3 (s) , Na 2 CO3 (s) , CO 2 (g) , and H 2 O ( ℓ) .
(a) Write out the formation reaction for each of the compounds listed above.
(b) Show that the formation reactions for the compounds listed above may be combined to produce an overall
reaction given by
2 NaHCO3 (s)
→
Na 2CO3 (s) + CO 2 (g) + H 2O ( ℓ) .
(c) Use values of standard enthalpies of formation from the CRC or NIST to calculate ΔH Ro in kJ/mol at 25°C
for the reaction given in part (b).
2
6.
The following may be considered as reactions used to power rockets,
( a)
( b)
( c)
H 2 ( g) +
1O
2 2
CH 3OH ( ℓ) +
( g)
→ H 2O ( g)
3O
2 2
( g)
→ CO 2 ( g) + 2 H 2O ( g)
H 2 ( g) + F2 ( g) → 2 HF ( g) .
(a) Using values from the tables in the Appendix of your textbook, calculate the standard molar enthalpies of
reaction at 25°C for each of these reactions. Convert your results to units of kJ per total mass of reactants
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(in kg).
(b) The thrust of a rocket is greater when the molar mass of the exhaust gas is lower. To compare the fuels
above with respect to thrust, divide the absolute enthalpy per kilogram by the average molar mass of the
products. Arrange the reactions in order of effectiveness on the basis of thrust. What is the most effective
reaction in terms of rocket thrust production?
7.
Consider the reactions
( a) C ( graphite) + O 2 ( g) → CO 2 ( g)
( b) H 2 ( g) + 12 O 2 ( g) → H 2O ( ℓ)
( c)
ΔH r! = −393.5 kJ/mol
ΔH r! = −285.5 kJ/mol
ΔH r! = −3119.6 kJ/mol.
2 C 2 H 6 ( g) + 7 O 2 ( g) → 4 CO 2 ( g) + 6 H 2O ( ℓ)
Use these reactions in conjunction with Hess' Law to determine the enthalpy of formation of ethane.
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8.
The combustion of 0.4862 g naphthalene in a constant volume bomb calorimeter caused a temperature rise in
the water bath of 1.707°C. The final temperature was 298.00 K. The constant volume heat capacity of the
calorimeter is 10290 J/K.
(a) Determine the molar internal energy of combustion of naphthalene and its molar enthalpy of combustion.
(b) Using the results from part (a), calculate the enthalpy of formation of naphthalene.
9.
A sample of solid KNO3 (1.668 g) is dissolved in 100 mL of water initially at 24.85°C in a constant pressure
solution calorimeter. The final temperature was 23.76°C. Assume the constant pressure heat capacity of the
calorimeter is 101.3 cal/K. Determine the molar enthalpy of solution of KNO3.
10. Using the values in the table below, reported at 25°C, determine ΔH Ro at 500ºC for the reaction
CH 3OH ( ℓ) → CH 4 ( g) +
1
O
2 2
( g)
.
Assume that C p,m is independent of temperature for all species in the reaction.
€
€
CH 3OH ( ℓ)
CH4 (g)
O2(g)
€
€
ΔH !f
C p,m
(kJ/mol)
–238.66
(Jmol–1K–1)
74.81
0
81.6
€
35.31
29.36
3
11. Calculate the standard molar enthalpy change at 600 K for the reaction
N 2 ( g) + 3H 2 ( g) → 2 NH 3 ( g) .
The standard molar enthalpies of formation and molar heat capacities at 25°C are listed below. Assume that the
molar heat capacities are independent of temperature.
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N2 (g)
H2 (g)
NH3 (g)
€
ΔH !f
C p,m
(kJ/mol)
0
0
–45.9
(Jmol–1K–1)
€
29.1
28.8
35.1