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Chapter 6
Thermochemistry
QUESTION
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:
1) –24.8 J.
2) 14.8 J.
3) 55.2 J.
4) –15.2 J.
5) none of these.
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CRS Question, 6–2
ANSWER
4)
–15.2 J.
Section 6.1 The Nature of Energy (p. 229)
When q = 0, then the internal energy equals the
work.
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CRS Question, 6–3
QUESTION
Of energy, work, enthalpy, and heat, how many
are state functions?
1) 0
2) 1
3) 2
4) 3
5) 4
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CRS Question, 6–4
ANSWER
3)
2
Section 6.1 The Nature of Energy (p. 229)
The original experiment determining that heat
was not a state function was conducted during
the boring of cannons. It was found that if the
drill was dull, heat could be produced indefinitely
without boring deeper into the metal.
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CRS Question, 6–5
QUESTION
For a particular process q = 20 kJ and
w = 15 kJ. Which of the following statements is
true?
1) Heat flows from the system to the
surroundings.
2) The system does work on the surroundings.
3) E = 35 kJ.
4) All of these are true.
5) None of these are true.
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CRS Question, 6–6
ANSWER
3)
E = 35 kJ.
Section 6.1 The Nature of Energy (p. 229)
Internal energy = q + w. The heat and work both
have a positive sign indicating energy flowed
from the system to the surroundings.
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CRS Question, 6–7
QUESTION
Which of the following statements is correct?
1) The internal energy of a system increases when more
work is done by the system than heat was flowing into
the system.
2) The internal energy of a system decreases when work
is done on the system and heat is flowing into the
system.
3) The system does work on the surroundings when an
ideal gas expands against a constant external
pressure.
4) All statements are true.
5) All statements are false.
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CRS Question, 6–8
ANSWER
3)
The system does work on the surroundings
when an ideal gas expands against a
constant external pressure.
Section 6.1 The Nature of Energy (p. 229)
The gas molecules of the atmosphere
have an average distance from each other.
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CRS Question, 6–9
ANSWER (continued)
A gas from a reaction expanding into the
atmosphere must force all the gas molecules
nearby closer together. This is the work the gas
does on the atmosphere.
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CRS Question, 6–10
QUESTION
Which one of the following statements is false?
1) The change in internal energy, E, for a process is
equal to the amount of heat absorbed at constant
volume, qv.
2) The change in enthalpy, H, for a process is equal to
the amount of heat absorbed at constant pressure, qp.
3) A bomb calorimeter measures H directly.
4) If qp for a process is negative, the process is
exothermic.
5) The freezing of water is an example of an exothermic
reaction.
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CRS Question, 6–11
ANSWER
3)
A bomb calorimeter measures H directly.
Section 6.2 Enthalpy and Calorimetry (p. 235)
Enthalpy is heat at constant pressure. The
pressure can change drastically inside of a
bomb calorimeter.
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CRS Question, 6–12
QUESTION
You take 200. g of a solid at 30.0C and let it
melt in 400. g of water. The water temperature
decreases from 85.1C to 30.0C. Calculate the
heat of fusion of this solid.
1) 125 J/g
2) 285 J/g
3) 461 J/g
4) 518 J/g
5) cannot without the heat capacity of the solid
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CRS Question, 6–13
ANSWER
3)
461 J/g
Section 6.2 Enthalpy and Calorimetry (p. 235)
The heat of fusion of the ice is found indirectly
from the heat loss of the water.
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CRS Question, 6–14
QUESTION
For a particular process q = –17 kJ and w = 21
kJ. Which of the following statements is false?
1) Heat flows from the system to the
surroundings.
2) The system does work on the surroundings.
3) E = +4 kJ
4) The process is exothermic.
5) None of these is false.
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CRS Question, 6–15
ANSWER
3)
E = +4 kJ
Section 6.1 The Nature of Energy (p. 229)
For a thermodynamic process we are only
interested in the change of energy, E. E is the
energy of a particular state.
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CRS Question, 6–16
QUESTION
In the lab, you mix two solutions (each originally
at the same temperature) and the temperature
of the resulting solution decreases. Which of the
following is true?
1) The chemical reaction is releasing energy.
2) The energy released is equal to s  m  T.
3) The chemical reaction is absorbing energy.
4) The chemical reaction is exothermic.
5) More than one of these.
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CRS Question, 6–17
ANSWER
3)
The chemical reaction is absorbing energy.
Section 6.2 Enthalpy and Calorimetry (p. 235)
The temperature drops because the reactants
are absorbing energy from the solution faster
than the surroundings can replace it.
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CRS Question, 6–18
QUESTION
For the reaction H2O(l)  H2O(g) at 298 K, 1.0 atm,
H is more positive than E by 2.5 kJ/mol. This
quantity of energy can be considered to be:
1) the heat flow required to maintain a constant
temperature.
2) the work done in pushing back the atmosphere.
3) the difference in the H—O bond energy in
H2O(l) compared to H2O(g).
4) the value of H itself.
5) none of these
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CRS Question, 6–19
ANSWER
2)
the work done in pushing back the
atmosphere.
Section 6.2 Enthalpy and Calorimetry (p. 235)
The gas molecules need to use some energy to
move atmospheric gas molecules out of their
way.
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CRS Question, 6–20
QUESTION
If 5.0 kJ of energy is added to a 15.5-g sample
of water at 10.C, the water is:
1) boiling.
2) completely vaporized.
3) frozen solid.
4) decomposed.
5) still a liquid.
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CRS Question, 6–21
ANSWER
5)
still a liquid.
Section 6.2 Enthalpy and Calorimetry (p. 235)
q = s  m  T. q = 5.0 kJ, s = 4.184 J/g  C
and m = 15.5 g. Solving for T shows that the
water was not raised to the 100C necessary for
boiling.
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CRS Question, 6–22
QUESTION
Given the equation S(s) + O2(g)  SO2(g),
H = –296 kJ, which of the following
statement(s) is (are) true?
a.
b.
c.
The reaction is exothermic.
When 0.500 mole sulfur is reacted,
148 kJ of energy is released.
When 32.0 g of sulfur are burned,
5
2.96  10 J of energy is released.
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CRS Question, 6–23
QUESTION (continued)
1)
2)
3)
4)
5)
All are true.
None is true.
a and b are true.
a and c are true.
Only b is true.
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CRS Question, 6–24
ANSWER
1)
All are true.
Section 6.2 Enthalpy and Calorimetry (p. 235)
The minus sign indicates that the reaction is
exothermic.
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CRS Question, 6–25
QUESTION
What is the specific heat capacity of gold if it
requires 48.8 J to raise the temperature of
15 grams of gold 25C?
1) 29 J/gC
2) 0.13 J/gC
3) 79 J/gC
4) 0.011 J/gC
5) none of these
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CRS Question, 6–26
ANSWER
2)
0.13 J/gC
Section 6.2 Enthalpy and Calorimetry (p. 235)
235
A common mistake is attempting to solve for the
heat capacity instead of specific heat capacity.
Using and keeping track of the units of measure
takes care of this type of problem.
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CRS Question, 6–27
QUESTION
The heat of formation of Fe2O3(s) is
–826 kJ/mol. Calculate the heat of the reaction
4Fe(s) + 3O2(g)  2Fe2O3(s) when a 55.8-g
sample of iron is reacted.
1) –206 kJ
2) –413 kJ
3) –826 kJ
4) –1650 kJ
3
5) –3.30  10 kJ
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CRS Question, 6–28
ANSWER
2)
–413 kJ
Section 6.2 Enthalpy and Calorimetry (p. 235)
Don’t forget to convert grams to moles in this
problem. Enthalpy is always in units of kJ/mol.
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CRS Question, 6–29
QUESTION
When 0.157 mol NH3 is reacted with excess
HCl, 6.91 kJ of energy is released as heat. What
is H for this reaction per mole of NH3
consumed?
1) –22.7 J
2) –1.08 kJ
3) –44.0 kJ
4) +22.7 J
5) +44.0 kJ
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CRS Question, 6–30
ANSWER
3)
44.0 kJ
–44.0
Section 6.2 Enthalpy and Calorimetry (p. 235)
A common mistake is use the wrong sign for
the enthalpy. Careful reading of the problem
will indicate the sign that must be used.
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CRS Question, 6–31
ANSWER (continued)
Phrases like “heat is released”, “heat is given
off” or “heat is emitted” indicate that a minus
sign is used. Phrases like “heat is absorbed” or
“heat is added” indicate that a plus sign is used.
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CRS Question, 6–32
QUESTION
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?
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CRS Question, 6–33
QUESTION (continued)
1)
2)
3)
4)
5)
Hcalc would be more negative because the
calorimeter always absorbs heat from the reaction.
Hcalc would be less negative because the
calorimeter would absorb heat from the reaction.
Hcalc would be more positive because the
reaction absorbs heat from the calorimeter.
Hcalc would be less positive because the reaction
absorbs heat from the calorimeter.
Hcalc would equal the actual value because the
calorimeter does not absorb heat.
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CRS Question, 6–34
ANSWER
4)
Hcalc would be less positive because the
reaction absorbs heat from the calorimeter.
Section 6.2 Enthalpy and Calorimetry (p. 235)
Most calorimeters are at least partly made of
metal and metals have high thermal
conductivity, so ignoring the heat loss to the
calorimeter will lead to poor accuracy in your
results.
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CRS Question, 6–35
QUESTION
Consider the following numbered processes:
a.
b.
c.
A  2B
BC+D
E  2D
H for the process A  2C + E is
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CRS Question, 6–36
QUESTION
1)
2)
3)
4)
5)
H1 + H2 + H3
H1 + H2
H1 + H2 – H3
H1 + 2H2 – H3
H1 + 2H2 + H3
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CRS Question, 6–37
ANSWER
4)
H1 + 2H2 – H3
Section 6.3 Hess’s Law (p. 242)
The final equation is found by summing the
reactions as follows: I + 2(II) – III. The minus
sign means the reaction is reversed.
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CRS Question, 6–38
QUESTION
Consider the following standard heats of formation:
P4O10(s) = –3110 kJ/mol
H2O(l) = –286 kJ/mol
H3PO4(s) = –1279 kJ/mol
Calculate the change in enthalpy for the following
process:
P4O10(s) + 6H2O(l)  4H3PO4(s)
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CRS Question, 6–39
ANSWER
–290 kJ
Section 6.4 Standard Enthalpies of Formation
(p. 246)
Remember to multiply the heat of formation of
each compound by its coefficient.
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CRS Question, 6–40
QUESTION
Which of the following is not being considered
as an energy source for the future?
1) Ethanol
2) Methanol
3) Seed oil
4) Shale oil
5) Carbon dioxide
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CRS Question, 6–41
ANSWER
5)
Carbon dioxide
Section 6.6 New Sources of Energy (p. 256)
Carbon dioxide is a very stable product of most
combustion reactions. It will not react further to
produce more useful heat.
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CRS Question, 6–42