CHAPTER 6
Thermochemistry
1.
A gas absorbs 0.0 J of heat and then performs 15.2 J of work. The change in internal energy of the gas is
a)
30.4 J
b)
15.2 J
c)
–30.4 J
d)
–15.2 J
e)
none of these
2.
What is the kinetic energy of a 1.50-kg object moving at 86.0 km/hr?
a)
4.28  102 kJ
b)
5.55  103 kJ
c)
4.28  10–4 kJ
d)
8.56  102 kJ
e)
1.79  101 kJ
3.
Which of the following statements correctly describes the signs of q and w for the following exothermic
process at P = 1 atm and T = 370 K?
H2O(g)  H2O(l)
a)
b)
c)
d)
e)
q and w are negative.
q is positive, w is negative.
q is negative, w is positive.
q and w are both positive.
q and w are both zero.
4.
For a particular process q = 20 kJ and w = 15 kJ. Which of the following statements is true?
a)
Heat flows from the system to the surroundings.
b)
The system does work on the surroundings.
c)
E = 35 kJ
d)
All of the above are true.
e)
None of the above are true.
5.
Which of the following statements is correct?
a)
The internal energy of a system increases when more work is done by the system than heat was flowing
into the system.
b)
The internal energy of a system decreases when work is done on the system and heat is flowing into the
system.
c)
The system does work on the surroundings when an ideal gas expands against a constant external
pressure.
d)
All statements are true.
e)
All statements are false.
6.
For a particular process q = –17 kJ and w = 21 kJ. Which of the following statements is false?
a)
Heat flows from the system to the surroundings.
b)
The system does work on the surroundings.
c)
E = +4 kJ
d)
The process is exothermic.
e)
None of the above is false.
7.
One mole of an ideal gas is expanded from a volume of 1.00 liter to a volume of 4.90 liters against a constant
external pressure of 1.00 atm. How much work (in joules) is performed on the surroundings? Ignore
significant figures for this problem. (T = 300 K; 1 L·atm = 101.3 J)
a)
198 J
b)
395 J
c)
1.19  103 J
d)
496 J
e)
none of these
8.
Calculate the work for the expansion of CO2 from 1.0 to 5.8 liters against a pressure of 1.0 atm at constant
temperature.
a)
4.8 L·atm
b)
5.8 L·atm
c)
0 L·atm
d)
–4.8 L·atm
e)
–5.8 L·atm
9.
A fuel-air mixture is placed in a cylinder fitted with a piston. The original volume is 0.310-L. When the
mixture is ignited, gases are produced and 815 J of energy is released. To what volume will the gases expand
against a constant pressure of 635 mmHg, if all the energy released is converted to work to push the piston?
a)
9.32 L
b)
7.03 L
c)
9.94 L
d)
9.63 L
e)
1.59 L
10.
Which statement is true of a process in which one mole of a gas is expanded from state A to state B?
a)
When the gas expands from state A to state B, the surroundings are doing work on the system.
b)
The amount of work done in the process must be the same, regardless of the path.
c)
d)
e)
It is not possible to have more than one path for a change of state.
The final volume of the gas will depend on the path taken.
The amount of heat released in the process will depend on the path taken.
11.
Calculate the work associated with the expansion of a gas from 42.0 L to 79.0 L at a constant pressure of 19.0
atm.
a)
703 L·atm
b)
–703 L·atm
c)
–1.50  103 L·atm
d)
798 L·atm
e)
1.50  103 L·atm
22.
Of energy, work, enthalpy, and heat, how many are state functions?
a)
0
b)
1
c)
2
d)
3
e)
4
23.
Which of the following properties is (are) intensive properties?
I. mass
II. temperature
III. volume
IV. concentration
V. energy
a)
b)
c)
d)
e)
I, III, and V
II only
II and IV
III and IV
I and V
24.
For the reaction H2O(l)  H2O(g) at 298 K and 1.0 atm, H is more positive than E by 2.5 kJ/mol. This
quantity of energy can be considered to be
a)
the heat flow required to maintain a constant temperature
b)
the work done in pushing back the atmosphere
c)
the difference in the H–O bond energy in H2O(l) compared to H2O(g)
d)
the value of H itself
e)
none of these
25.
Which one of the following statements is false?
a)
The change in internal energy, E, for a process is equal to the amount of heat absorbed at constant
volume, qv.
b)
The change in enthalpy, H, for a process is equal to the amount of heat absorbed at constant pressure,
qp.
c)
A bomb calorimeter measures H directly.
d)
If qp for a process is negative, the process is exothermic.
e)
The freezing of water is an example of an exothermic reaction.
26.
C2H5OH(l) + 3O2(g)  2CO2(g) + 3H2O(l), H = –1.37  103 kJ
For the combustion of ethyl alcohol as described in the above equation, which of the following is true?
I. The reaction is exothermic.
II. The enthalpy change would be different if gaseous water was produced.
III. The reaction is not an oxidation–reduction one.
IV. The products of the reaction occupy a larger volume than the reactants.
a)
b)
c)
d)
e)
I, II
I, II, III
I, III, IV
III, IV
only I
45.
If a student performs an endothermic reaction in a calorimeter, how does the calculated value of H differ
from the actual value if the heat exchanged with the calorimeter is not taken into account?
a)
Hcalc would be more negative because the calorimeter always absorbs heat from the reaction.
b)
Hcalc would be less negative because the calorimeter would absorb heat from the reaction.
c)
Hcalc would be more positive because the reaction absorbs heat from the calorimeter.
d)
Hcalc would be less positive because the reaction absorbs heat from the calorimeter.
e)
Hclac would equal the actual value because the calorimeter does not absorb heat.
46.
A bomb calorimeter has a heat capacity of 2.47 kJ/K. When a 0.111-g sample of ethylene (C2H4) was burned
in this calorimeter, the temperature increased by 2.26 K. Calculate the energy of combustion for one mole of
ethylene.
a)
–5.29 kJ/mol
b)
–50.3 kJ/mol
c)
–624 kJ/mol
d)
–0.274 kJ/mol
e)
–1.41  103 kJ/mol
47.
Consider the reaction:
C2 H5 OH(l ) + 3O2 (g )  2CO 2 (g ) + 3H 2 O(l ), H  1.37 × 10 3 kJ
When a 24.8-g sample of ethyl alcohol (molar mass = 46.07 g/mol) is burned, how much energy is released as
heat?
a)
b)
c)
d)
e)
60.
0.538 kJ
0.737 kJ
7.37  102 kJ
3.40  104 kJ
1.86 kJ
Consider the following processes:
2A  (1/2)B + C
H1 = 5 kJ/mol
(3/2)B + 4C  2A + C + 3D
H2 = –15 kJ/mol
E + 4A  C
H3 = 10 kJ/mol
Calculate H for:
a)
b)
c)
d)
e)
0 kJ/mol
10 kJ/mol
–10 kJ/mol
–20 kJ/mol
20 kJ/mol
C  E + 3D
61.
Consider the following processes:
H (kJ/mol)
3B  2C + D
–125.
(1/2)A  B
150
E+AD
350
Calculate H for: B  E + 2C
a)
b)
c)
d)
e)
62.
325 kJ/mol
525 kJ/mol
–175 kJ/mol
–325 kJ/mol
none of these
Consider the following numbered processes:
1. A  2B
2.
BC+D
3. E  2D
H for the process A  2C + E is
a)
b)
c)
d)
e)
63.
H1 + H2 + H3
H1 + H2
H1 + H2 – H3
H1 + 2H2 – H3
H1 + 2H2 + H3
At 25°C, the following heats of reaction are known:
H (kJ/mol)
2ClF + O2  Cl2O + F2O
167.4
2ClF3 + 2O2  Cl2O + 3F2O
341.4
2F2 + O2  2F2O
–43.4
At the same temperature, calculate H for the reaction: ClF + F2 → ClF3
a)
b)
c)
d)
e)
–217.5 kJ/mol
–130.2 kJ/mol
+217.5 kJ/mol
–108.7 kJ/mol
none of these
64.
Calculate H° for the reaction C4H4(g) + 2H2(g)  C4H8(g), using the following data:
H°combustion for C4H4(g) = –2341 kJ/mol
H°combustion for H2(g) = –286 kJ/mol
H°combustion for C4H8(g) = –2755 kJ/mol
a)
b)
c)
d)
e)
–128 kJ
–158 kJ
128 kJ
158 kJ
none of these
68.
The heat of formation of Fe2O3(s) is -826.0 kJ/mol. Calculate the heat of the reaction
4Fe(s) + 3O2 (g )  2Fe2 O3 (s) when a 27.42-g sample of iron is reacted.
a)
–101.4 kJ
b)
–202.8 kJ
c)
–405.5 kJ
d)
–811 kJ
e)
–1.132  104 kJ
69.
Which of the following does not have a standard enthalpy of formation equal to zero at 25°C and 1.0 atm?
a)
F2(g)
b)
Al(s)
c)
H2O(l)
d)
H2(g)
e)
They all have a standard enthalpy equal to zero.
70.
Given the following two reactions at 298 K and 1 atm, which of the statements is true?
1. N2(g) + O2(g)  2NO(g)
H1
2.
a)
b)
c)
d)
e)
NO(g) +
1
2
O2(g)  NO2(g)
ΔHf° for NO2(g) = H2
ΔHf° for NO(g) = H1
ΔHf° = H2
ΔHf° for NO2(g) = H2 +
none of these
H2
1
2
H1
71.
Given:
Cu2O(s) +
1
2
O2(g)  2CuO(s)
H° = –144 kJ
Cu2O(s)  Cu(s) + CuO(s)
H° = +11 kJ
Calculate the standard enthalpy of formation of CuO(s).
a)
b)
c)
d)
e)
72.
–166 kJ
–299 kJ
+299 kJ
+155 kJ
–155 kJ
Using the following data, calculate the standard heat of formation of the compound ICl in kJ/mol:
H° (kJ/mol)
Cl2(g)  2Cl(g)
242.3
I2(g)  2I(g)
151.0
ICl(g)  I(g) + Cl(g)
211.3
I2(s)  I2(g)
62.8
a)
b)
c)
d)
e)
–211 kJ/mol
–14.6 kJ/mol
16.8 kJ/mol
245 kJ/mol
439 kJ/mol
80.
This fossil fuel was formed from the remains of plants that were buried and exposed to high pressure and heat
over time.
a)
coal
b)
natural gas
c)
diesel fuel
d)
propane
e)
gasoline
81.
The coal with the highest energy available per unit burned is
a)
Lignite.
b)
Subbituminous.
c)
Bituminous.
d)
Anthracite.
e)
They are equal in energy value.
82.
All of the following statements about the greenhouse effect are true except:
a)
It occurs only on earth.
b)
The molecules H2O and CO2 play an important role in retaining the atmosphere's heat.
c)
Low humidity allows efficient radiation of heat back into space.
d)
The carbon dioxide content of the atmosphere is quite stable.
e)
A and D
83.
Which of the following is both a greenhouse gas and a fuel?
a)
carbon dioxide
b)
coal
c)
freon
d)
methane
e)
nitrogen
84.
One of the main advantages of hydrogen as a fuel is that:
a)
The only product of hydrogen combustion is water.
b)
It exists as a free gas.
c)
It can be economically supplied by the world's oceans.
d)
Plants can economically produce the hydrogen needed.
e)
It contains a large amount of energy per unit volume of hydrogen gas.
85.
Which of the following is not being considered as an energy source for the future?
a)
ethanol
b)
methanol
c)
seed oil
d)
shale oil
e)
carbon dioxide
86.
The combustion of hydrogen gas releases 286 kJ per mol of hydrogen. If 10.0 L of hydrogen at STP was
burned to produce electricity, how long would it power a 100-watt (W) light bulb? Assume no energy is lost to
the surroundings. (1 W = 1 J/s)
a)
21.3 min
b)
1.48 days
c)
1.28 hr
d)
7.94 hr
e)
3.55 hr
Spontaneity, Entropy, and Free Energy
1.
For which process is S negative?
a)
evaporation of 1 mol of CCl4(l)
b)
mixing 5 mL ethanol with 25 mL water
c)
compressing 1 mol Ne at constant temperature from 1.5 L to 0.5 L
d)
raising the temperature of 100 g Cu from 275 K to 295 K
e)
grinding a large crystal of KCl to powder
2.
Ten identical coins are shaken vigorously in a cup and then poured out onto a table top. Which of the
following distributions has the highest probability of occurrence? (T = Tails, H = Heads)
a)
T10H0
b)
T8H2
c)
T7H3
d)
T5H5
e)
T4H6
3.
If two pyramid-shaped dice (with numbers 1 through 4 on the sides) were tossed, which outcome has the
highest entropy?
a)
The sum of the dice is 3.
b)
The sum of the dice is 4.
c)
The sum of the dice is 5.
d)
The sum of the dice is 6.
e)
The sum of the dice is 7.
4.
A two-bulbed flask contains 5 particles. What is the probability of finding all 5 particles on the left side?
a)
2.50%
b)
2.24%
c)
3.13%
d)
0.20%
e)
6.25%
5.
Which of the following shows a decrease in entropy?
a)
precipitation
b)
gaseous reactants forming a liquid
c)
a burning piece of wood
d)
melting ice
e)
two of these
6.
Which of the following result(s) in an increase in the entropy of the system?
I.
II.
Br2(g)  Br2(l)
III.
NaBr(s)  Na (aq) + Br (aq)
IV.
O2(298 K)  O2(373 K)
+
–
V.
NH3(1 atm, 298 K)  NH3(3 atm, 298 K)
a)
b)
c)
d)
e)
I
II, V
I, III, IV
I, II, III, IV
I, II, III, V
10.
A 100-mL sample of water is placed in a coffee cup calorimeter. When 1.0 g of an ionic solid is added, the
temperature decreases from 21.5°C to 20.8°C as the solid dissolves. For the dissolving of the solid
a)
H < 0
b)
Suniv > 0
c)
Ssys< 0
d)
Ssurr > 0
e)
none of these
11.
A chemical reaction is most likely to be spontaneous if it is accompanied by
a)
increasing energy and increasing entropy
b)
lowering energy and increasing entropy
c)
increasing energy and decreasing entropy
d)
lowering energy and decreasing entropy
e)
none of these (A-D)
12.
Assume that the enthalpy of fusion of ice is 6020 J/mol and does not vary appreciably over the temperature
range 270-290 K. If one mole of ice at 0°C is melted by heat supplied from surroundings at 286 K, what is the
entropy change in the surroundings, in J/K?
a)
22.1
b)
21.0
c)
0.0
d)
–21.0
e)
–22.1
13.
If the change in entropy of the surroundings for a process at 451 K and constant pressure is -326 J/K, what is
the heat flow absorbed by for the system?
a)
326 kJ
b)
1.38 kJ
c)
–147 kJ
d)
125 kJ
e)
147 kJ
29.
Substance X has a heat of vaporization of 46.3 kJ/mol at its normal boiling point (423°C). For the process X(l)
 X(g) at 1 atm and 423°C calculate the value of S.
a)
0
b)
66.5 J/K mol
c)
109 J/K mol
d)
–66.5 J/K mol
e)
–109 J/K mol
30.
Substance X has a heat of vaporization of 54.1 kJ/mol at its normal boiling point (423°C). For the process X(l)
 X(g) at 1 atm and 423°C calculate the value of G.
a)
0J
b)
77.7 J
c)
128 J
d)
–77.7 J
e)
–128 J
31.
he enthalpy of vaporization of isopropanol is 44.00 kJ/mol at its boiling point (82.3°C). Calculate the value
of S when 1.00 mole of isopropanol is vaporized at 82.3°C and 1.00 atm.
a)
0
b)
5.35  102 J/K mol
c)
d)
e)
1.24  102 J/K mol
–1.24  102 J/K mol
–5.35  102 J/K mol
32.
S is _______ for exothermic reactions and ______ for endothermic reactions.
a)
favorable, unfavorable
b)
unfavorable, favorable
c)
favorable, favorable
d)
unfavorable, unfavorable
e)
cannot tell
33.
For the process CHCl3(s)  CHCl3(l), H° = 9.15 kJ/mol and S° = 43.9 J/mol/K. What is the melting point
of chloroform?
a)
–65 °C
b)
208 °C
c)
129 °C
d)
65 °C
e)
–129 °C
Given that Hvap is 54.5 kJ/mol, and the boiling point is 83.4°C, 1 atm, if one mole of this substance is
vaporized at 1 atm, calculate S.
a)
–153 J/K mol
b)
153 J/K mol
c)
653 J/K mol
d)
–653 J/K mol
e)
0
Use the following to answer questions 36-37:
34.
Consider two perfectly insulated vessels. Vessel #1 initially contains an ice cube at 0°C and water at 0°C. Vessel #2
initially contains an ice cube at 0°C and a saltwater solution at 0°C. In each vessel, consider the "system" to be the
ice, and the "surroundings" to be the liquid.
36.
Determine the sign of Ssys, Ssurr, and Suniv for the contents of
Vessel #1.
Ssys
a)
b)
c)
d)
e)
37.
0
+
+
+
+
Ssurr
Suniv
0
–
+
–
0
0
0
+
+
+
Determine the sign of Ssys, Ssurr, and Suniv for the system (ice/saltwater) in
Vessel #2.
Ssys
Ssurr
Suniv
a)
b)
c)
d)
e)
0
+
+
+
+
0
–
+
–
0
0
0
+
+
+
38.
The melting point of water is 0°C at 1 atm pressure because under these conditions:
a)
S for the process H2O(s)  H2O(l) is positive.
b)
S and Ssurr for the process H2O(s)  H2O(l) are both positive.
c)
S and Ssurr for the process H2O(s)  H2O(l) are equal in magnitude and opposite in sign.
d)
G is positive for the process H2O(s)  H2O(l).
e)
None of these is correct.
41.
For a spontaneous exothermic process, which of the following must be true?
a)
G must be positive.
b)
S must be positive.
c)
S must be negative.
d)
Two of the above must be true.
e)
None of the above (A-C) must be true.
42.
A mixture of hydrogen and chlorine remains unreacted until it is exposed to ultraviolet light from a burning
magnesium strip. Then the following reaction occurs very rapidly:
H2(g) + Cl2(g)  2HCl(g)
G = –45.54 kJ
H = –44.12 kJ
S = –4.76 J/K
Which of the following is consistent with this information?
a)
b)
c)
d)
e)
The reactants are thermodynamically more stable than the products.
The reaction has a small equilibrium constant.
The ultraviolet light raises the temperature of the system and makes the reaction more favorable.
The negative value for S slows down the reaction.
The reaction is spontaneous, but the reactants are kinetically stable.
43.
For a particular chemical reaction H = 6.8 kJ and S = –29 J/K. Under what temperature condition is the
reaction spontaneous?
a)
When T < –234 K.
b)
When T < 234 K.
c)
The reaction is spontaneous at all temperatures.
d)
The reaction is not spontaneous at any temperature.
e)
When T > 234 K.
45.
Consider the freezing of liquid water at –10°C. For this process what are the signs for H, S, and G?
H
S
G
a)
+
–
0
b)
+
–
–
c)
–
+
0
d)
–
+
–
e)
–
–
–
For the process of a certain liquid vaporizing at 1 atm, H vap = 66.8 kJ/mol and S vap= 74.1 J/mol K.
Assuming these values are independent of T, what is the normal boiling point of this liquid?
a)
901 °C
b)
1174 °C
c)
628 °C
d)
0.901 °C
e)
none of these
Use the following to answer questions 53-55:
46.
The following questions refer to the following reaction at constant 25°C and 1 atm.
2Fe(s) + (3/2)O2(g) + 3H2O(l)  2Fe(OH)3(s) H = –789 kJ/mol
Substance
S° (J/mol K)
Fe(OH)3(s)
107
Fe(s)
27
O2(g)
205
H2O(l)
70
53.
Determine Ssurr for the reaction (in kJ/mol K)
a)
3.14
b)
0.937
c)
0.378
d)
1.31
e)
2.65
54.
Determine Suniv for the reaction (in kJ/mol K)
a)
0.23
b)
2.3
c)
0.36
d)
2.8
e)
3.6
55.
What must be true about G for this reaction?
a)
G = H
b)
G = 0
c)
G > 0
d)
G < 0
e)
G = Suniv
56.
Given the following data, calculate the normal boiling point for formic acid (HCOOH).
S°(J/mol K)
Hf°(kJ/mol)
HCOOH(l)
-410.
130.0
HCOOH(g)
-363
250.9
a)
b)
c)
d)
e)
57.
0.39 °C
389 °C
662 °C
279 °C
116 °C
The following reaction takes place at 120°C: H2O(l)  H2O(g)
Which of the following must be true?
a)
b)
c)
d)
e)
The reaction is not spontaneous.
The reaction is spontaneous.
G = 0
G < 0
Two of these.
65.
In which reaction is S° expected to be positive?
a)
I2(g)  I2(s)
b)
H2O(l) H2O(s)
c)
CH3OH(g) + 3 2 O2(g) CO2(g) + 2H2O(l)
d)
2O2(g) + 2SO(g)  2SO3(g)
e)
none of these
66.
For the dissociation reaction of the acid HF:
HF(aq)
H+(aq) + F–(aq)
S is observed to be negative. The best explanation is:
H = 44.0 kJ/mol
S = 0.119 kJ/mol K
a)
b)
c)
d)
e)
67.
This is the expected result since each HF molecule produces two ions when it dissociates.
Hydration of the ions produces the negative value of S.
The reaction is expected to be exothermic and thus S should be negative.
The reaction is expected to be endothermic and thus S should be negative.
None of these can explain the negative value of S.
Consider the dissociation of hydrogen:
H2(g)
2H(g)
One would expect that this reaction:
a)
b)
c)
d)
e)
68.
will be spontaneous at any temperature
will be spontaneous at high temperatures
will be spontaneous at low temperatures
will not be spontaneous at any temperature
will never happen
When a stable diatomic molecule spontaneously forms from its atoms, what are the signs of H°, S°, and
G°?
H
S
G
a)
+
+
+
b)
+
–
–
c)
–
+
+
d)
–
–
+
e)
–
–
–
1983
CO(g) + 2 H2(g) <===> CH3OH(l)
For this reaction, H° = -128.1 kilojoules
Hf° (kJ mol¯1) Gf° (kJ mol¯1) S° (J mol¯1 K¯1)
CO(g)
-110.5
-173.3
+197.9
CH3OH(l)
-238.6
-166.2
+126.8
The data in the table above were determined at 25 °C.
(a) Calculate G° for the reaction above at 25 °C.
(b) Calculate Keq for the reaction above at 25 °C.
(c) Calculate S° for the reaction above at 25 °C.
(d) In the table above, there are no data for H2. What are the values of Hf°, Gf°, and of the
absolute entropy, S°, for H2 at 25 °C?
1989
Br2 (l) ---> Br2 (g)
At 25 °C the equilibrium constant, Kp, for the reaction above is 0.281 atmosphere.
(a) What is G°298 for this reaction?
(b) It takes 193 joules to vaporize 1.00 gram of Br2(l) at 25 °C and 1.00 atmosphere pressure.
What are the values of H°298 and of S°298 for this reaction?
(c) Calculate the normal boiling point of bromine. Assume that H° and S° remain constant
as the temperature is changed.
(d) What is the equilibrium vapor pressure of bromine at 25 °C ?
2011
3. Answer the following questions about glucose, C6H12O6 , an important biochemical energy source.
(a) Write the empirical formula of glucose.
In many organisms, glucose is oxidized to carbon dioxide and water, as represented by the following
equation.
C6H12O6(s) + 6 O2(g) → 6 CO2(g) + 6 H2O(l)
A 2.50 g sample of glucose and an excess of O2(g) were placed in a calorimeter. After the reaction was
initiated and proceeded to completion, the total heat released by the reaction was calculated to be 39.0 kJ.
(b) Calculate the value of ΔH°, in kJ mol−1, for the combustion of glucose.
(c) When oxygen is not available, glucose can be oxidized by fermentation. In that process, ethanol and
carbon dioxide are produced, as represented by the following equation.
C6H12O6(s) → 2 C2H5OH(l) + 2 CO2(g) ΔH° = −68.0 kJ mol−1 at 298 K
The value of the equilibrium constant, Kp , for the reaction at 298 K is 8.9 × 1039.
(i) Calculate the value of the standard free-energy change, ΔG°, for the reaction at 298 K. Include units
with your answer.
(ii) Calculate the value of the standard entropy change, ΔS°, in J K−1 mol−1, for the reaction at 298 K.
(iii) Indicate whether the equilibrium constant for the fermentation reaction increases, decreases, or
remains the same if the temperature is increased. Justify your answer.
(d) Using your answer for part (b) and the information provided in part (c), calculate the value of ΔH° for
the following reaction.
C2H5OH(l) + 3 O2(g) → 2 CO2(g) + 3 H2O(l)