Unit 8 Question Packet
Thermochemistry
SKILLS
1. Applying the heat equation when the
TEMPERATURE CHANGES
2. Understanding ENDOTHERMIC/EXOTHERMIC
CHANGES
SKILL #1:
Name ………………………………………………
Period ………….
3. Calculating heat during PHASE CHANGES
4. Interpreting HEATING/COOLING CURVES
5. Determining VAPOR PRESSURE/BOILING POINT
USING TABLE H
Applying the heat equation when the TEMPERATURE CHANGES - refer to your notes & RB p. 7
1. How much energy must be absorbed by
20.0 g of water to increase its
temperature from 283 ºC to 303ºC?
6. A 75-gram sample of water vapor is cooled
from 150. ºC to 105 ºC. Determine the
amount of heat removed from the sample
during this time.
2. How much heat is absorbed when 100. g
of water is heated from 50.0ºC to 100.ºC?
7. When 15.0 g of steam drops in temperature
from 275 ºC to 250 ºC, how much heat
energy is released?
3. When 15 grams of ammonia is heated
from 25 ºC to 45 ºC, how much energy is
absorbed? The specific heat capacity
for ammonia is 4.71 J/g  ºC.
8. If it takes 41.72 joules to heat a piece of gold
with a mass of 18.69 g from 10. ºC to 27 ºC,
what is the specific heat of the gold?
4. How many joules are absorbed when
50.0 g of water are heated from 30.2 ºC
to 58.6 ºC?
9. Find the specific heat of copper if it takes 849
J of energy to change the temperature of
95.4g of copper from 25° C to 48° C.
5. How many joules of heat are released
when 50. grams of water are cooled from
70. °C to 60. °C?
10. What is the specific heat of silver if 0.024 kJ of
energy is required to change a 15-gram
sample of silver from 22° C to 32° C?
11. A certain mass of water was heated with
41,840 J, raising its temperature from 22 ºC to
28.5 ºC. Find the mass of the water.
16. If 20. grams of water vapor is cooled by
removing 45 J of heat, what is its
temperature change?
12. The temperature of a sample of water
changes from 10. °C to 20. °C when the
water absorbs 420 joules of heat. What is the
mass of the sample?
17. A 36-gram sample of water has an initial
temperature of 22°C. What is the final
temperature of the sample after it
absorbs1200 joules of heat energy?
13. A sample of water is heated from 35° C to
45° C with 1,600 joules. What is the mass of
the sample?
18. A 15-gram sample of water has an initial
temperature of 25°C. What is the final
temperature of the sample after it loses 125
joules of heat energy?
14. If 720.0 g of steam at 400. ºC absorbs 800. kJ
of heat energy, what will be its increase in
temperature?
19. An 18-gram sample of water has a
temperature of 16°C after it has absorbed
527 joules of heat energy. What was the
initial temperature of the sample?
15. What would be the temperature change if
15 g of water absorbed 75 J of heat?
20. A 115-gram sample of water has a
temperature of 60.°C after it has absorbed
1,923 joules of heat energy. What was the
initial temperature of the sample?
21. A student performed an experiment to determine the total amount of energy stored in a peanut. The
accepted value for the energy content of a peanut is 30.2 kilojoules per gram. The student measured
100.0 grams of water into a metal can and placed the can on a ring stand, as shown in the diagram
below. The peanut was attached to a wire suspended under the can. The initial temperature of the
water was recorded as 22.0°C. The peanut was ignited and allowed to burn. When the peanut
finished burning, the final water temperature was recorded as 57.0°C. The student’s experimental
value for the energy content of this peanut was 25.9 kilojoules per gram.
(a) Calculate the total amount of heat absorbed by the water. Your
response must include both a correct numerical setup and the
calculated result.
(b) Determine the student’s percent error for the energy content of this
peanut.
SKILL #2:
understanding ENDOTHERMIC/EXOTHERMIC CHANGES
- refer to your notes & RB p. 4
22. Under each arrow, write “endothermic” or “exothermic” to indicate if the change releases or
absorbs heat. Also, write in “energy” on the correct side of the arrow.
a.
b.
Au(l)  Au(s)
NaCl(s)  NaCl(l)
c.
d.
NaCl(s)  NaCl(l)
e.
C2H3O2(g)  C2H3O2(l)
f.
C6H6(l)  C6H6(g)
CO2(s)  CO2(g)
23. Which phase change results in the release of
energy?
25. Which change is exothermic?
(1) freezing of water
(2) melting of iron
(3) vaporization of ethanol
(4) sublimation of iodine
24. Given the balanced equation representing
a reaction:
Cu + S  CuS + energy
Which statement explains why the energy
term is written to the right of the arrow?
(1) The compound CuS is composed of two
metals.
(2) The compound CuS is composed of two
nonmetals.
(3) Energy is absorbed as the bonds in CuS
form.
(4) Energy is released as the bonds in CuS
form.
26. An iron bar at 325 K is placed in a sample of
water. The iron bar gains energy from the
water if the temperature of the water is
(1) 65 K
(3) 65°C
(2) 45 K
(4) 45°C
27. Which physical changes are endothermic?
(1) melting and freezing
(2) melting and evaporating
(3) condensation and sublimation
(4) condensation and deposition
28. Given the balanced equation:
29. Systems in nature tend to undergo changes
toward
(1) lower energy and lower entropy
(2) lower energy and higher entropy
(3) higher energy and lower entropy
(4) higher energy and higher entropy
SKILL #3:
Calculating heat during PHASE CHANGES
32. At a pressure of 101.3 kilopascals and a
temperature of 373 K, heat is removed from
a sample of water vapor, causing the
sample to change from the gaseous phase
to the liquid phase. This phase change is
represented by the equation below.
H2O(g) → H2O(l) + heat
Determine the total amount of heat released
by 5.00 grams of water vapor during this
phase change.
30. Even though the process is endothermic,
snow can sublime. Which tendency in nature
accounts for this phase change?
(1) a tendency toward greater entropy
(2) a tendency toward greater energy
(3) a tendency toward less entropy
(4) a tendency toward less energy
31. Which transfer of energy occurs when ice
cubes are placed in water that has a
temperature of 45°C?
(1) Chemical energy is transferred from the
ice to the water.
(2) Chemical energy is transferred from the
water to the ice.
(3) Thermal energy is transferred from the ice
to the water.
(4) Thermal energy is transferred from the
water to the ice.
- refer to your notes & RB p. 8
34. How many kilojoules of heat must be
removed to freeze 35 g of water at 0 °C?
35. How many joules are required to melt 225 g
of ice at 0° C?
33. How many joules does it take to vaporize
423g of water at 100 °C?
36. In which equation does the term "heat"
represent heat of fusion?
(1) NaCl(s) + heat NaCl(l)
(2) NaOH(aq) + HCl(aq) 
NaCl(aq) + H2O(l) + heat
(3) H2O(l) + heat  H2O(g)
(4) H2O(l) + HCl(g) 
H3O+(aq) + Cl-(aq) + heat
SKILL #4:
Interpreting HEATING/COOLING CURVES
- refer to your notes & RB p. 6
37. Refer to the graph below, which shows a cooling curve for a substance in a gaseous phase that was cooled
during a 24-minute period. The substance gave off heat at a constant rate of 40 J/min during the entire time.
The 24-minute period has been divided into five time intervals, labeled I, II, III, IV, and V.
(a) What is the boiling point of the substance? _______
(b) In which interval or intervals are there two phases
present?
_______________________________
(c) During which interval or intervals is all of the substance a
gas?
_______________________
(d) During interval IV, what happens to the kinetic energy of
the molecules in the substance?
(e) During interval IV, what happens to the potential energy of the molecules in the substance?
(f) What is the heat of vaporization for the substance?
(g) What is the heat of fusion for the substance?
(h) Why is the heat of vaporization always bigger than the heat of fusion?
38. A 100.0-gram sample of NaCl(s) has an initial temperature of 0°C. A chemist measures the
temperature of the sample as it is heated. Heat is not added at a constant rate. The heating curve
for the sample is shown below.
(a) Determine the temperature range over which the entire NaCl
sample is a liquid.
(b) Identify one line segment on the curve where the average
kinetic energy of the particles of the NaCl sample is changing.
(c) Identify one line segment on the curve where the NaCl sample
is in a single phase and capable of conducting electricity.
39. Base your answers to the following questions on the heating curve and cooling curve below.
Heating Curve for Substance 1
Cooling Curve for Substance 2
Heating Curve
120. ºC
Cooling Curve
110 ºC
b. What is the freezing point of the substance?
70. ºC
50 ºC
c. What is the melting point of the substance?
70. ºC
50 ºC
d. At what temperature do the solid and liquid
phases exist at equilibrium?
e. During which segments does the kinetic energy
of the substance change?
70. ºC
50 ºC
0 to 2 min
4 to 7 min
11 to 12 min
(KE increases – temp. ↑)
2 to 4 min
7 to 11 min
2 to 4 min
7 to 11 min
(PE increases – space
btwn particles ↑)
0 to 2 min
4 to 7 min
11 to 12 min
11 to 12 min
A to B
C to D
E to F
(KE decreases – temp.)
B to C
D to E
B to C
D to E
(PE decreases – space
btwn particles )
A to B
C to D
E to F
A to B
0 to 2 min
E to F
4 to 7 min
C to D
a. What is the boiling point of the substance?
f. During which segments does the kinetic energy of
the substance remain the same?
g. During which segments does the potential
energy of the substance change?
h. During which segments does the potential energy
of the substance remain the same?
i. During which segment does the substance exist
only as a gas?
j. During which segment does the substance exist
only as a solid?
k. During which segment does the substance exist
only in the liquid phase?
l.
Calculate the heat of fusion for Substance 1. Heat was added at a constant rate of 35 J/min.
m. Calculate the heat of vaporization for Substance 2. Heat was removed at a constant rate of 50
J/min.
n. How do both the heating curve and cooling curve above illustrate that the heat of vaporization is
greater than the heat of fusion? [1]
40. The graph below shows a compound being cooled at a constant rate starting in the liquid phase at
75°C and ending at 15°C.
(a) What is the freezing point of the compound, in
degrees Celsius?
_________
(b) State what is happening to the average kinetic
energy of the particles of the sample between
minute 2 and minute 6. [1]
(c) A different experiment was conducted with
another sample of the same compound starting
in the solid phase. The sample was heated at a
constant rate from 15°C to 75°C. On the graph
to the left, draw the resulting heating curve.
41. A hot pack contains chemicals that can be activated to produce heat. A cold pack contains
chemicals that feel cold when activated.
(a) Based on energy flow, state the type of chemical change that occurs in a hot pack.
(b) A cold pack is placed on an injured leg. Indicate the direction of the flow of energy between the
leg and the cold pack.
(c) What is the Law of Conservation of Energy? Describe how the Law of Conservation of Energy
applies to the chemical reaction that occurs in the hot pack.
SKILL #5:
Determining VAPOR PRESSURE/BOILING POINT USING TABLE H
- refer to your notes & RB p.109
42. At 65°C, which compound has a vapor
pressure of 58 kilopascals?
(1) ethanoic acid
(3) propanone
(2) ethanol
(4) water
44. Which liquid has the lowest vapor pressure at
65°C?
(1) ethanoic acid
(3) propanone
(2) ethanol
(4) water
43. Which liquid has the highest vapor pressure
at 75°C?
(1) ethanoic acid
(3) propanone
(2) ethanol
(4) water
45. At which temperature is the vapor pressure
of ethanol equal to the vapor pressure of
propanone at 35°C?
(1) 35°C
(3) 82°C
(2) 60.°C
(4) 95°C
46. The boiling point of a liquid is the
temperature at which the vapor pressure of
the liquid is equal to the pressure on the
surface of the liquid. What is the boiling point
of propanone if the pressure on its surface is
48 kilopascals?
(1) 25°C
(3) 35°C
(2) 30.°C
(4) 40.°C
47. According to Reference Table H, what is the
boiling point of ethanoic acid at 80 kPa?
(1) 28°C
(3) 111°C
(2) 100°C
(4) 125°C
48. Using your knowledge of chemistry and the
information in Reference Table H, which
statement concerning propanone and water
at 50°C is true?
(1) Propanone has a higher vapor pressure
and stronger intermolecular forces than
water.
(2) Propanone has a higher vapor pressure
and weaker intermolecular forces than
water.
(3) Propanone has a lower vapor pressure
and stronger intermolecular forces than
water.
(4) Propanone has a lower vapor pressure
and weaker intermolecular forces than
water.
49. As the pressure on the surface of a liquid
decreases, the temperature at which the
liquid will boil
(1) decreases
(2) increases
(3) remains the same
50. As the temperature of a liquid increases, its
vapor pressure
(1) decreases
(2) increases
(3) remains the same
51. The vapor pressure of a liquid is 0.92 atm at
60°C. The normal boiling point of the liquid
could be
(1) 35°C
(3) 55°C
(2) 45°C
(4) 65°C
52. Base your answers to the questions below on the graph, which shows the vapor pressure curves for
liquids A and B.
(a) What is the vapor pressure of liquid A at 70°C? Your answer
must include correct units.
(b) At what temperature does liquid B have the same vapor
pressure as liquid A at 70°C? Your answer must include
correct units.
(c) Which liquid will evaporate more rapidly? Explain your
answer in terms of intermolecular forces.