South Pasadena • Honors Chemistry
Name
6 • Thermochemistry
Period
6.1
1. Determine whether each process is endothermic or
exothermic. Explain briefly. (Assume that the
selection that is italicized is the system)
(a) Propane gas (C3H8) undergoes combustion.
The process is exothermic, since combustion
reactions release lots of energy.
(b) Water droplets form on the surface of the cup
containing of a cold drink.
The process is exothermic, since water vapor
releases heat into the cold drink, condensing
into liquid.
(c) A balloon shrinks when it is placed in a freezer.
The process is exothermic, since heat leaves
the warmer balloon into the cooler freezer.
(d) A solution of NaCl increases in temperature
when it is heated by a Bunsen burner.
The process is endothermic, since the
solution absorbs heat from the Bunsen
burner.
2. Answer each question. Explain briefly.
Date
PROBLEMS
–
HEAT
(c) Metal blocks X and Y both have the same
mass. When both are heated with 300 J of
energy, the temperature of block X increases
10°C and that of block Y increases 30°C.
Which has a greater specific heat?
Block X has a smaller change in temperature
(i.e. it’s harder to change temp), so it has a
greater specific heat.
(d) Metal blocks A and B both have the same
mass. It requires 50 J to heat block A from
20°C to 45°C, whereas it requires 80 J to heat
block B from 20°C to 45°C. Which has a
greater specific heat?
Block B requires more energy to change its
temperature (i.e. it’s harder to change
temp), and has a greater specific heat.
3. Calculate the energy required for a 25.0 g piece of
lead to be heated from 40°C to 95°C.
(Clead = 0.0308 cal/g·°C)
q=?
m = 25.0 g
Clead = 0.0308 cal/g·°C
∆T = 95°C ‒ 40°C = 55°C
(a) A bar of copper ((Ccopper = 0.385 J/g·°C) and a
q = (m)(C)(∆T)
bar of aluminum (Caluminum = 0.897 J/g·°C) have
= (25.0 g)( 0.0308 cal/g·°C)(55°C) = 42.4 cal
the same mass. Which requires more energy to
4. The specific heat of copper metal is 0.385 J/g·°C.
increase the temperature from 20°C to 30 °C?
Mass of a pre-1982 penny is 3.1 grams. How much
Al has a higher specific heat, so it is harder,
heat is required to heat a penny from 20.0°C to
and requires more energy, to increase its
100.0°C?
temperature.
q=?
m = 3.1 g
(b) A bar of copper (Ccopper = 0.385 J/g·°C) and a
Ccopper = 0.385 J/g·°C
bar of aluminum (Caluminum = 0.897 J/g·°C) have
∆T = 100.0°C ‒ 20.0°C = 80.0°C
the same mass. If 2000 J of heat are transferred
q = (m)(C)(∆T)
to each bar, which will see the greater increase
= (3.1 g)(0.385 J/g·°C)(80.0°C) = 95 J
in temperature?
Cu has a lower specific heat, so it’s easier to
increase its temperature.
5. What is the mass of a piece of aluminum if 120. J is 8. What is the final temperature when a balloon filled
released when it is cooled from 60.0°C to 30.0°C?
with 0.250 g of helium gas at 25.0°C releases 19.5 J
(Caluminum = 0.897 J/g·°C)
of energy? (Chelium = 5.19 J/g·°C)
q = ‒120. J
m=?g
Caluminum = 0.897 J/g·°C
∆T = 30.0°C ‒ 60.0°C = ‒30.0°C
q = ‒19.5 J
Chelium = 5.19 J/g·°C
q
‒19.5 J
=
m · C (0.250 g)(5.19 J/g·°C)
= ‒15.0°C = Tf ‒ 25.0°C
∆T =
q
‒120. J
m=
=
= 4.46 g
C · ∆T (0.897 J/g·°C)(‒30.0°C)
6. How many degrees celsius does a 1.50 gram block
of ice increase when 30.0 cal of heat energy is
added? (Cice = 0.502 cal/g·°C)
q = 30.0 cal
Cice = 0.502 cal/g·°C
∆T =
m = 1.50 g
∆T = ?°C
q
30.0 cal
=
= 39.8°C
m · C (1.50 g)(0.502 cal/g·°C)
7. What is the specific heat of a diamond if 4.50 J of
heat is required to heat a 0.720 g piece of diamond
from 10.0°C to 22.3°C?
q = 4.50 J
m = 0.720 g
Cdiamond = ? J/g·°C
∆T = 22.3°C ‒ 10.0°C = 12.3°C
C=
m = 0.250 g
∆T = Tf ‒ 25.0°C
Tf = 10.0°C
9. How much heat energy is released when 250. mL
of hot water at 90.0°C cools to room temperature,
20.0°C. The density of water is 1.00 g/mL? (Cwater
= 4.18 J/g·°C)
q=?J
1g 
m = 250. mL
1
 mL = 250. g
Cwater = 4.18 J/g·°C
∆T = 20.0°C ‒ 90.0°C = ‒70.0°C
q = (m)(C)(∆T) = (250. g)(4.18 J/g·°C)(‒70.0°C)
= ‒73,200 J
q
4.50 J
=
= 0.508 J/g·°C
m · ∆T (0.720 g)(12.3°C)
10. A 110. g sample of sample of copper (Ccopper = 0.385 J/g·°C) is heated to 82.4°C and then placed in an
insulated container of water (Cwater = 4.18 J/g·°C) at 22.3°C. The final temperature of the water and copper is
24.9°C. What is the volume of water? (Dwater = 1 g/mL)
qcopper = ?
mcopper = 110. g
Ccopper = 0.385 J/g·°C
∆Tcopper = 24.9°C ‒ 82.4°C = ‒57.5°C
qwater = ?
mwater = ? g
Cwater = 4.18 J/g·°C
∆Twater = 24.9°C ‒ 22.3°C = 2.6°C
‒qcopper = +qwater
‒(mcopper)(Ccopper)(∆Tcopper) = +(mwater)(Cwater)(∆Twater)
‒(110. g)(0.385 J/g·°C)(‒57.5°C) = +(mwater)(4.18 J/g·°C)(2.6°C)
mwater = 224 g
Vwater = 224 g 
1 mL
 1 g  = 224 mL
11. When 55.0 g NH3 (g) is burned according to the following reaction, 946 kJ energy is released. Find the value
of ∆Hrxn.
4 NH3 (g) + 5 O2 (g)  4 NO (g) + 6 H2O (ℓ)
nrxn = 55.0 g NH3 
1 mol NH3   1 mol rxn 
17.03 g NH3 4 mol NH3 0.807 mol rxn
∆Hrxn =
q
‒946 kJ
=
= ‒1170 kJ/mol
nrxn 0.807 mol rxn
12. What mass of oxygen must be burned to release 267 kJ of heat?
CH4 (g) + 2 O2 (g)  CO2 (g) + 2 H2O (g)
∆Hrxn = ‒802.3 kJ/mol
‒267 kJ 
1 mol rxn  2 mol O2  32.00 g O2
‒802.3 kJ 1 mol rxn  1 mol O2  = 21.3 g O2
13. If a bottle of nail polish remover contains 177 mL of acetone, C3H6O, how much heat is released by its
complete combustion? (dacetone = 0.788 g/mL)
C3H6O (ℓ) + 4 O2 (g)  3 CO2 (g) + 3 H2O (g) + 1790 kJ
0.788 g C3H6O  1 mol C3H6O   1 mol rxn   ‒1790 kJ 
= ‒4300 kJ
 1 mL C3H6O  58.08 g C3H6O 1 mol C3H6O 1 mol rxn
177 mL C3H6O
14. When 1.00 g propanol (C3H7OH) is burned, ‒16.8 kJ of energy is released. Find the ∆Hrxn.
2 C3H7OH (ℓ) + 9 O2 (g)  6 CO2 (g) + 8 H2O (ℓ)
nrxn = 1.00 g C3H7OH 
1 mol C3H7OH   1 mol rxn 
60.09 g C3H7OH 2 mol C3H7OH = 0.00832 mol rxn
∆Hrxn =
q
‒16.8 kJ
=
= ‒2020 kJ/mol
nrxn 0.00832 mol rxn