Chapter 06 – Thermochemistry
98. The heat of combustion of propane, C3H8, is 2220 kJ/mol. The specific heat of copper is
0.385 J/gC. How many grams of propane must be burned to raise the temperature of a 10.0
kg block of copper from 25.0C to 65.0C, assuming none of the heat is lost to the
surroundings?
99. The residential rate for natural gas is about $15 per thousand cubic foot. Burning one
cubic foot of natural gas releases about 1080 kJ of heat. How much would it cost to heat the
water in a 25,000 gallon swimming pool from 52F to 78F, assuming all of the heat from
burning the natural gas went towards warming the water? (1 gal = 3.785 L; the specific heat
of water = 4.184 J/g·C)
Chapter 06 – Thermochemistry
100.
The heat of neutralization of HCl by NaOH is Hrxn = –56.2 kJ/mol. How much heat is released when 125 mL of 1.750 M
HCl is mixed with 195 mL of 0.667 M NaOH?
101. The heat released when one mole of water is formed from the elements is 1,198 kJ. An
experiment was conducted that permitted water to form in this manner, and the heat was
contained in 2.0 liters of water. The water temperature before the reaction was 34.5C, and
after the reaction it had risen to 52.0C. How many moles of water were formed? (The
specific heat of water is 4.184 J/g·C.)
Chapter 06 – Thermochemistry
102. When an automobile engine starts, the metal parts immediately begin to absorb heat
released during the combustion of gasoline. How much heat will be absorbed by a 165 kg iron
engine block as the temperature rises from 15.7C to 95.7C? (The specific heat of iron is
0.489 J/g·C.)
103.
The value of Hrxn for the following reaction is –6535 kJ/mol.
2C6H6(l) + 15O2(g) → 12CO2(g) + 6H2O(g)
How many kilojoules of heat will be evolved during the combustion of 16.0 g of C6H6(l)?
Chapter 06 – Thermochemistry
104.
What would be the standard enthalpy change for the reaction of 1.00 mole of H2(g) with 1.00 mole of Cl2(g) to produce 2.00
moles of HCl(g) at standard state conditions?
[Hf (HCl(g))= –92.3 kJ/mol)]
105.
Calculate the heat released (kJ) in the reaction of 3.50 g of acetylene (C2H2) and excess hydrogen gas to form ethane gas:
C2H2(g) + 2H2(g) → C2H6(g)
Given:
2C2H2(g) + 5O2(g) → 4CO2(g) + 2H2O(g) H = –2320 kJ/mol
2C2H6(g) + 7O2(g) → 4CO2(g) + 6H2O(g) H = –3040 kJ/mol
2H2(g) + O2(g) → 2H2O(g) H = –572 kJ/mol
Chapter 06 – Thermochemistry
106.
Calculate the heat released (kJ) in the reaction of 2.20 g of sulfur dioxide gas and excess oxygen gas to form sulfur trioxide
gas:
2SO2(g) + O2(g) → 2SO3(g)
Given:
S(s) + O2(g) → SO2(g) H = –299 kJ/mol
S(s) + 3/2O2(g) → SO3(g) H = –395 kJ/mol
107.
Find Hrxn for the reaction
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l).
[Hf (CH4(g)) = –74.8 kJ/mol; Hf (CO2(g)) = –393.5 kJ/mol; Hf (H2O(l)) = –285.5 kJ/mol]
Chapter 06 – Thermochemistry
108.
Find Hrxn for the reaction
2Ag2S(s) + 2H2O(l) → 4Ag(s) + 2H2S(g) + O2(g).
[Hf (Ag2S(s)) = –32.6 kJ/mol; Hf (H2S(g)) = –20.5 kJ/mol; Hf (H2O(l)) = –285.5 kJ/mol]
109.
Find Hrxn for the reaction
2Na(s) + 2H2O(l) → 2NaOH(aq) + 2H2(g).
[Hf (NaOH(aq)) = –426.8 kJ/mol; Hf (H2O(l)) = –285.5 kJ/mol]
Chapter 06 – Thermochemistry
110. The specific heat of silver is 0.235 J/g·C. How many joules of heat are required to heat
a 75 g silver spoon from 20C to 35C?
111. At body temperature 2,404 joules of energy are required to evaporate 1.00 g of water.
After vigorous exercise, a person feels chilly because the body is giving up heat to evaporate
the perspiration. A typical person perspires 25 mL of water after 20. minutes of exercise. How
much body heat is this person using to evaporate this water?
Chapter 06 – Thermochemistry
112. The combustion of one mole of benzene, C6H6, in oxygen liberates 3268 kJ of heat. The
products of the reaction are carbon dioxide and water. How much heat is given off when 183
g of oxygen are reacted with excess benzene?
113. A feverish student weighing 75 kilograms was immersed in 400. kg of water at 4.0C to
try to reduce the fever. The student’s body temperature dropped from 40.0C to 37.0C.
Assuming the specific heat of the student to be 3.77 J/g·C, what was the final temperature of
the water?
Chapter 06 – Thermochemistry
114. Select True or False: The specific heats of water and iron are 4.184 and 0.444 J/gC,
respectively. When equal masses of water and iron both absorb the same amount of heat, the
temperature increase of the water will be 2.42 times greater than that of the iron.
115. Select True or False: Chemical reactions in a bomb calorimeter occur at constant
pressure.
116.
Select True or False: If 2Mg(s) + O2(g) → 2MgO(s), H = –1203.6 kJ/mol, then for
Mg(s) + (1/2)O2(g) → MgO(s), the enthalpy change is H = –601.8 kJ/mol.
Chapter 06 – Thermochemistry
118. Select True or False: The work done on the surroundings by the expansion of a gas is w
119. Select True or False: The heat absorbed by a system at constant pressure is equal to DE –
PDV.