South Pasadena • Chemistry
Name
Period
Date
6 · Thermochemistry
CHANGES IN STATES
WORKSHEET
1. Write a chemical equation for the following changes (include “heat”). Indicate whether each is endothermic
or exothermic.
(a) The melting of ice.
Heat + H2O (s)  H2O (ℓ)
endo | exo
(b)
The condensation of steam.
H2O (g)  H2O (ℓ) + heat
(c)
The vaporization of liquid water.
Heat + H2O (ℓ)  H2O (g)
(d)
The freezing of liquid water.
H2O (ℓ)  H2O (s) + heat
endo | exo
endo | exo
endo | exo
2. For water, ∆Hfus = 333 J/g, ∆Hvap = 2260 J/g.
(a)
Find the value for Q when 15.0 g water freezes.
VARIABLES:
(b)
(c)
(d)
WORK:
ANSWER:
Q=?
Q = m ∆H = (15.0 g)(‒333 J/g) = 4995 J
m = 15.0 g
∆H = ‒ 333 J/g
How many grams of water are converted to steam when 15,000 J of heat is absorbed?
‒4995 J
VARIABLES:
ANSWER:
WORK:
Q = 15000 J
Q = m ∆H
m=?g
15000 J = (m)(2260 J/g)
∆H =+2260 J/g m = 6.6 g
How much energy is released when 50.0 g of water vapor condenses into liquid?
6.6 g
VARIABLES:
ANSWER:
WORK:
Q=?
Q = m ∆H
m = 15.0 g
Q = (50.0 g)(‒2260 J/g) = ‒113,000 J
∆H =‒2260 J/g
How much energy is absorbed when 1.80 moles of ice melts?
113,000 J
VARIABLES:
WORK:
ANSWER:
Q=?
Q = m ∆H
Q = (32.4 g)(333 J/g) = 10,800 J
10,800 J
18.01
g

m = 1.80 mol
 1 mol 
= 32.4 g
∆H = 333 J/g
3. What is the heat of fusion of aluminum, in J/g, if 5970 J of energy is required to melt a 15.0 gram sample?
VARIABLES:
WORK:
ANSWER:
Q = 5970 J
Q = m ∆H
398 J/g
m = 15.0 g
(5970 J) = (15.0 g)(∆Hfus)
∆H = ?
∆Hfus = 398 J/g
4. How many grams of methanol can be vaporized if it is heated with 1674 J of energy? ∆Hvap = 1105 J/g
VARIABLES:
WORK:
Q = 1674 J
Q = m ∆H
m=?g
1674 J = (m)(1105 J/g)
∆H = 1105 J/g m = 1.515 g
ANSWER:
1.515 g
5. The heat of vaporization, ∆Hvap, of acetone, C3H6O, is 31,300 J/mol. (Pay attention to the units. Because it’s
given per “mole”, our formula becomes Q = n ∆H, where n is number of moles.)
(a) Calculate the number of moles if we have a 30.0 gram sample of acetone.
Molar mass = 3(12.01) + 6(1.008) + 1(16.00) = 58.08 g/mol
1 mol 
30.0 g 
58.08 g = 0.517 mol
(b) How much energy is required to vaporize the 30.0 gram sample of acetone?
VARIABLES:
WORK:
ANSWER:
Q=?
Q = n ∆H
16,200 J
n = 0.517 mol Q = (0.517 mol)(31,300 J/g) = 16,200 J
∆H = 31,300
J/mol
6. The heat of fusion, ∆Hfus, of lead is 1213 J/mol. How much energy is required to melt a 320 gram piece of
lead?
VARIABLES:
WORK:
ANSWER:
Q=?
Q = n ∆H
Q = (1.54 mol)(1213 J/g) = 1870 J
1870 J
1 mol 
n = 320 g
207.2 g
= 1.54 mol
∆H = 1213 J/mol
7. It takes 3000 J to melt a 20-g block of solid A, and 4500 J to melt a 20-g block of solid B. Which has a
greater heat of fusion, ∆Hfus? Explain without using math.
Both blocks have the same mass. However, it takes more energy to melt the block of solid B, so solid B has
a greater heat of fusion.
8. The heats of vaporization for ethanol and water are 201 cal/g and 540 cal/g, respectively. If 2000 calories of
heat are added to a sample of each substance, which substance will produce more vapor? Explain briefly.
The heat of vaporization for ethanol is lower than that for water, so it is easy to vaporize ethanol. With the
same amount of energy, ethanol will produce more vapor.