South Pasadena • Chemistry
Name
Period
Date
6 · Thermochemistry
STATION
1 •
ENDOTHERMIC/EXOTHERMIC
Determine whether each process is an Endothermic (ENDO) or Exothermic (EXO) process.
EXO
The baggie gets hotter when CaCl2 (s) is dissolved.
EXO
A hot aluminum slab cools down in a beaker of water.
EXO
Ethanol undergoes combustion.
EXO
Ethanol vapor condenses to liquid.
ENDO
The baggie gets colder when NH4Cl (s) is dissolved.
ENDO
A beaker of water is warmed when a hot aluminum slab is placed in it.
EXO
Water freezes into ice when placed in a freezer.
EXO
The size of a balloon shrinks when placed over ice.
ENDO
Ethanol liquid vaporizes when heated by a person’s hands.
ENDO
A block of ice melts in the sun.
6 · Thermochemistry
STATION 2
•
ENDOTHERMIC/EXOTHERMIC
Water is vaporized to steam.
Vapor is condensed to liquid water.
This process is [ endothermic | exothermic ] because
This process is [ endothermic | exothermic ] because
energy is [ absorbed by | released from ] the system
energy is [ absorbed by | released from ] the system
& molecular attractions are [ strengthened | weakened ]. & molecular attractions are [ strengthened |weakened ].
The value of Q is [ negative | positive ].
The value of Q is [ negative | positive ].
Chemical equation:
Heat + H2O (ℓ) " H2O (g)
Chemical equation:
H2O (g) " H2O (ℓ) + heat
Potential Energy Graph:
Potential Energy Graph:
6 · Thermochemistry
STATION
3
• SPECIFIC
HEAT
Answer each question and explain briefly.
1. Silver and sodium have specific heats 0.233 J/g·°C and 1.20 J/g·°C, respectively.
(a) 40 gram blocks of each metal at 20°C are placed in a hot stove for the same amount of time. Which metal
will have a higher temperature when they are taken out?
Silver has lower specific heat, is easier to change temp, so will change temp more than sodium
resulting in higher final temperature.
(b) Samples of each block are heated by adding 3000 J of energy. The temperatures of both blocks increase
by 120°C. Which metal has a larger mass?
Silver is easier to change temp, so with the same energy and same change in temp, silver can
heat up a larger amount/mass.
(c) 20 gram samples of each metal at 15°C are heated until they reach 50°C. Which metal required more
energy?
Sodium has higher specific heat, so is harder to change temp and will require more energy.
2. Warm samples of liquids X and Y each has a mass of 80 g and are at 25°C. When the samples are cooled so
that 200 cal are released, liquid X has a final temperature of 15°C, and liquid Y has a final temperature of
5°C. Which has a greater specific heat?
Liquid X changed a smaller amount of temp, so it’s harder to change its temperature, having a
greater specific heat.
6 · Thermochemistry
STATION
4
• CALORIMETRY
Show all work. (Identify variables, write equation, substitute values, write answer.)
1. How much heat is required to increase the temperature of a 25.0 g sample of steam from 150°C to 180°C?
(Cp = 0.497 cal/g·°C)
Q=?
m = 25.0 g
Q = m ∆T Cp = (25.0 g)(30°C)(0.497 cal/g·°C) = 373 cal
∆T = 180°C ‒ 150°C = 30°C
Cp = 0.497 cal/g·°C
2. What is the initial temperature of an 8.5 gram piece of gold if, after applying 220 J of heat, the temperature
rose to 350°C? (Cp = 0.129 J/g·°C)
Q = 220 J
m = 8.5 g
Q = m ∆T Cp
Cp = 0.129 J/g·°C
(220 J) = (8.5 g)(350°C ‒ Ti)(0.129 J/g·°C)
∆T = 350°C ‒ Ti
Ti = 149°C
6 · Thermochemistry
STATION
5
• CALORIMETRY
Show all work. (Identify variables, write equation, substitute values, write answer.)
1. What is the specific heat of mercury if 612 cal of heat is required to heat a 262 g sample of mercury from
10°C to 80°C?
Q = 612 cal
Q = m ∆T Cp
m = 262 g
(612 cal) = (262 g)(70°C)(Cp)
Cp = ?
Cp = 0.0334 cal/g·°C
∆T = 80°C ‒ 10°C = 70°C
2. What is the mass of a sample of oxygen gas, if 800 J of energy is released when it is cooled from 35°C to
15°C? (Cp = 0.918 J/g·°C)
Q = ‒ 800 J
m=?
Q = m ∆T Cp
Cp = 0.918 J/g·°C
(‒800 J) = (m)(‒20°C)(0.918 J/g·°C)
∆T = 15°C ‒ 35°C = ‒20°C
m = 43.6 g
6 · Thermochemistry
STATION
6 • SPECIFIC
HEAT
LAB
An experiment was carried out to determine the specific heat of a slab of an unknown metal. Record the data.
• A 175-mL sample of water was placed in a Styrofoam cup, and the temperature of 25.5°C was recorded.
• The metal slap was carefully taken out of a beaker of boiling water at 100.0°C and submerged into the water
in the Styrofoam cup. The temperature of the water with the metal slab rose to 28.7°C.
• The mass of the dry metal slab was found to be 86.8 g.
Volume of Water
175 mL
Mass of Metal
86.8 g
Initial Temp of Water
25.5°C
Initial Temp of Metal
100.0°C
Final Temp of Water
28.7°C
Final Temp of Metal
28.7°C
Calculate the heat absorbed by the water. The specific heat of water is 4.18 J/g·°C. Water’s density is 1.00 g/mL.
1 g ⎞
Q = m ∆T Cp = (175 mL)⎛⎜
(28.7°C ‒ 25.5°C)(4.18 J/g·°C) = 2341 J
⎝1 mL⎟⎠
Calculate the specific heat of the metal.
Q = m ∆T Cp
(‒2341 J) = (86.8 g)(28.7°C ‒ 100.0°C)(Cp)
Cp = 0.378 J/g·°C
Determine the identity of the unknown metal and calculate % error of the experiment.
| 0.378 - 0.387 |
Zn
% Error =
× 100% = 2.3%
0.387
Metal
Al
Fe
Mg
Zn
Cp (J/g·°C)
0.897
0.450
1.02
0.387
6 · Thermochemistry
STATION
7
•
HEATS
OF
FUSION/VAPORIZATION
Answer each question and explain briefly.
1. For water, ∆Hfus = 333 J/g and ∆Hvap = 2260 J/g. Does it require more energy to melt 50.0g of ice or vaporize
50.0 g of water?
Since ∆Hvap > ∆Hfus, it takes more energy (it’s harder) to vaporize water than melt ice.
2. 80.0 gram pieces of solid gold and silver were placed in separate hot ovens at their respective melting points.
When they were taken out, all of the gold had melted, while only half of the silver half melted. Which metal
has a higher value for ∆Hfus?
Silver melted less, so it’s harder to melt silver than gold, and it has a higher ∆Hfus.
3. 5500 cal of heat was applied to samples of water and ethanol at their respective boiling points. At exactly
five
minutes, both samples were completely vaporized. Which sample had the greater mass?
(∆Hvap for water = 540 cal/g, ethanol = 200 cal/g)
Since ethanol has a lower ∆Hvap, it’s easier to vaporize ethanol than water, so with the same
energy, more mass of ethanol was vaporized
6 · Thermochemistry
STATION 8
•
CHANGES
IN
STATES
Show all work. (Identify variables, write equation, substitute values, write answer.)
1. What is the value of Q when 31.6 grams of water freezes at 0°C? (∆Hfus = 333 J/g)
Q=?
Q = m ∆Hfus = (31.6 g)(333 J/g) = 10500 J
m = 31.6 g
∆Hfus = 333 J/g
2. How many grams of CO2 can be vaporized when 500 cal is applied at its boiling point, -57°C?
(∆Hvap = 83.2 cal/g)
Q = 500 cal
Q = m ∆Hvap
m=?
(500 cal) = m (83.2 cal/g)
∆Hvap = 83.2 cal/g
m = 6.01 g
3.
What is the value of ∆Hvap of ammonia in J/g if 18,500 J of heat is required to vaporize a 13.5 g sample?
Q = 18,500 J
Q = m ∆Hvap
m = 13.5 g
(18,500 J) = (13.5 g)∆Hvap
∆Hvap = ?
∆Hvap = 1370 J/g
6 · Thermochemistry
STATION 9
•
HEATING
CURVE
Sketch the graph when 40.0 g of ice at -30°C is heated to steam at 140°C. Label the axes (with units), and the
Freezing and Boiling points. Label the segments (a) through (e). Identify the state(s) of matter of each segment.
140°C
BP=100°C
FP=0°C
‒30°C
(g)
gas
(d)
liquid/gas
(c)
liquid
(b)
solid/liquid
(a)
solid
In which segments: (circle)
• are molecular attractions weakening?
• are the kinetic energy increasing?
• are the processes endothermic?
• are the processes exothermic?
• do you use Q = m (∆T) Cp to calculate the heat?
• do you use Q = m ∆H to calculate the heat?
a
a
a
a
a
a
b
b
b
b
b
b
c
c
c
c
c
c
d
d
d
d
d
d
e
e
e
e
e
e