Unit 8 Question Packet
Thermochemistry
SKILLS
1.
2.
3.
4.
5.
Name …………………KEY………………………
Period ………….
Applying the heat equation when the TEMPERATURE CHANGES
Understanding ENDOTHERMIC/EXOTHERMIC CHANGES
Calculating heat during PHASE CHANGES
Interpreting HEATING/COOLING CURVES
Determining VAPOR PRESSURE/BOILING POINT USING TABLE H
SKILL #1:
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?
2. How much heat is absorbed when 100. g of water is heated from 50.0ºC to 100.ºC?
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.
4. How many joules are absorbed when 50.0 g of water are heated from 30.2 ºC to 58.6 ºC?
5. How many joules of heat are released when 50. grams of water are cooled from 70. °C to 60. °C?
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.
7. When 15.0 g of steam drops in temperature from 275 ºC to 250 ºC, how much heat energy is
released?
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?
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.
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.
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?
13. A sample of water is heated from 35° C to 45° C with 1,600 joules. What is the mass of the
sample?
14. If 720.0 g of steam at 400. ºC absorbs 800. kJ of heat energy, what will be its increase in
temperature?
15. What would be the temperature change if 15 g of water absorbed 75 J of heat?
16. If 20. grams of water vapor is cooled by removing 45 J of heat, what is its temperature change?
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?
m = 36 g
q = m C ∆T
C = 4.18 J/g•K
1200 = (36)(4.18)∆T
q = 1200 J
8 = ∆T
∆T = Tfinal - 22°C
8 = Tfinal - 22°C
30°C = Tfinal
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?
m = 15 g
q = m C ∆T
C = 4.18 J/g•K
-125 = (15)(4.18)∆T
q = -125 J
-2 = ∆T
∆T = Tfinal - 25°C
-2 = Tfinal - 25°C
23°C = Tfinal
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?
m = 18 g
q = m C ∆T
C = 4.18 J/g•K
527 = (18)(4.18)∆T
q = 527 J
7 = ∆T
∆T = 16°C - Tinitial
7 = 16°C - Tinitial
9°C = Tinitial
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?
m = 115 g
q = m C ∆T
C = 4.18 J/g•K
1,923 = (115)(4.18)∆T
q = 1,923 J
4 = ∆T
∆T = 60.°C - Tinitial
4 = 60.°C - Tinitial
56°C = Tinitial
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. [2]
(100)(4.18)(35) = 14,600 J
(b) Determine the student’s percent error for the energy content of this
peanut. [1] -14% or 14%.
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) + energy
(freezing) exothermic
c.
energy + NaCl(s)  NaCl(l)
(melting) endothermic
d.
energy + NaCl(s)  NaCl(l)
(melting) endothermic
e.
C2H3O2(g)  C2H3O2(l) + energy
(condensing) exothermic
f.
energy + C6H6(l)  C6H6(g)
(boiling) endothermic
23. Which phase change results in the release of
energy?
energy + CO2(s)  CO2(g)
(subliming) endothermic
28. Given the balanced equation:
4
24. Which change is exothermic?
(1) freezing of water
(2) melting of iron
(3) vaporization of ethanol
(4) sublimation of iodine
25. 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
26. 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.
27. Which physical changes are endothermic?
(1) melting and freezing
(2) melting and evaporating
(3) condensation and sublimation
(4) condensation and deposition
1
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
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.
SKILL #3:
Calculating heat during PHASE CHANGES
- refer to your notes & RB p. 8
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.
33. How many joules does it take to vaporize 423g of water at 100 °C?
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?
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? _______
80 deg. C
(b) In which interval or intervals are there two phases
present?
_______________________________
II and IV
(c) During which interval or intervals is all of the substance a
gas?
_______________________
I
(d) During interval IV, what happens to the kinetic energy of
the molecules in the substance?
remains constant
(e) During interval IV, what happens to the potential energy of the molecules in the substance? It decreases
(cooled down)
(f) What is the heat of vaporization for the substance?
40 J/min * 7 min = 280 J
(g) What is the heat of fusion for the substance?
40 J/min * 4 min = 160 J
(h) Why is the heat of vaporization always bigger than the heat of fusion?
It takes more energy to turn a liquid into a gas than to turn a solid into a liquid.
38. 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
a.
b.
c.
d.
What is the boiling point of the substance?
What is the freezing point of the substance?
What is the melting point of the substance?
At what temperature do the solid and liquid
phases exist at equilibrium?
e. During which segments does the kinetic energy
of the substance change?
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.
Heating Curve
120. ºC
70. ºC
70. ºC
70. ºC
Cooling Curve
110 ºC
50 ºC
50 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
Calculate the heat of fusion for Substance 1. Heat was added at a constant rate of 35 J/min.
35 J/min * 2 min = 70 J
m. Calculate the heat of vaporization for Substance 2. Heat was removed at a constant rate of 50
J/min.
50 J/min * 4 min = 200 J
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]
Heating curve: more time to boil than to melt
Cooling curve: BC is longer than DE
39. 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?
_________ 50 degrees
(b) State what is happening to the average kinetic
energy of the particles of the sample between
minute 2 and minute 6. [1]
KE remains constant
(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.
40. 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. 801°C to 1465°C
(b) Identify one line segment on the curve where the average
kinetic energy of the particles of the NaCl sample is changing.
AB or CD
(c) Identify one line segment on the curve where the NaCl sample
is in a single phase and capable of conducting electricity. CD
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.
Exothermic
(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.
from the injured leg to 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.
Energy is conserved in all chemical changes
Or
heat (energy) lost = heat (energy) gained
and
The energy released from the hot pack is equal to the energy absorbed by the surroundings.
SKILL #5:
Determining VAPOR PRESSURE/BOILING POINT USING TABLE H
42. At 65°C, which compound has a vapor
pressure of 58 kilopascals?
(1) ethanoic acid
(3) propanone
(2) ethanol
(4) water
- refer to your notes & RB p.109
48. Using your knowledge of chemistry and the
information in Reference Table H, which
statement concerning propanone and water
at 50°C is true?
43. Which liquid has the highest vapor pressure
at 75°C?
(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
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
(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.
710 (±10) mm Hg
(b) At what temperature does liquid B have the same vapor
pressure as liquid A at 70°C? Your answer must include
correct units.
114 (±2) °C
(c) Which liquid will evaporate more rapidly? Explain your
answer in terms of intermolecular forces.
A has a higher vapor pressure, so it is more likely to
evaporate and has weaker IMF