Chapter 10 Lecture
Change of State
Basic Chemistry
Fifth Edition
Matter undergoes a
change of state when
it is converted from
one state to another
at a constant
temperature.
Chapter 10 Bonding and
Properties of Solids and Liquids
10.7 Changes of State
Learning Goal Describe
the changes of state
between solids, liquids,
and gases; calculate the
energy involved.
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Figure 10.4 Changes of state
include melting and freezing, boiling
and condensation, sublimation and
deposition.
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Heat of Fusion
Melting and Freezing
A substance
• changes from a solid to a liquid at its
melting point (mp).
• changes from a liquid to a solid at its
freezing point (fp).
• Water has a freezing (melting) point of 0
°C.
The heat of fusion is the energy that must be
added to convert exactly 1 g of solid to a
liquid at its melting point.
For H2O, 334 J of heat is needed to melt 1 g
of ice at its melting point (0 °C).
H2O(s) + 334 J/g
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Heat of Fusion, H2O
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H2O(l)
Endothermic
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Heat of Fusion, H2O
The heat of fusion is also the quantity of heat
that must be removed to freeze 1 g of water
at its freezing point (0 °C).
The heat of fusion for water can be used as a
conversion factor.
1 g H2O = 334 J
For H2O, 334 J of heat is removed to freeze
1 g of ice at its melting point (0 °C).
H2O(l)
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H2O(s) + 334 J/g
When calculating the heat needed to melt (or
freeze) water, we use the following equation:
Exothermic
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Calculations Using Heat of
Fusion
Example of a Calculation Using
Heat of Fusion
How much heat in joules is needed to melt 15.0 g of
ice (H2O)?
STEP 1 List the grams of substance and change
of state.
Given 15.0 g of H2O(s)
Need number of joules to melt ice to H2O(l)
STEP 2 Write the plan to convert grams to heat.
grams of ice H2O(s)
joules (to melt)
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Example of a Calculation Using
Heat of Fusion (continued)
STEP 3 Write the heat conversion factors and
metric factors if needed.
1 g of H2O(s l)
= 334 J
334 J
and
1 g H2O
g of H2O
334 J
STEP 4 Set up the problem with factors.
Heat to melt ice at 0 °C
15.0 g ice x
334 J
= 5010 J
1 g ice
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Sublimation
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Learning Check
How many kilojoules are released when 25.0 g of
water at 0 °C freezes?
1) 0.335 kJ
2) 0 kJ
3) 8.35 kJ
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Evaporation and Condensation
Sublimation
• occurs when a solid
changes directly to a gas
• is typical of dry ice, which
sublimes at -78 C
• takes place in frost-free
refrigerators
• is used to prepare freezedried foods for long-term
storage
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Water
• evaporates when
molecules of water on
the surface gain
sufficient energy to
form a gas
• condenses when
water molecules in a
gas lose energy and
form a liquid
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Boiling of Water
Heat of Vaporization
At boiling,
• all the water
molecules acquire
the energy to form a
gas (vaporize)
• bubbles of water
vapor appear
throughout the liquid
The heat of vaporization is
the amount of heat
• absorbed to change 1 g of
liquid to gas at the boiling
point
• released when 1 g of gas
changes to liquid at the
boiling point
boiling point of H2O = 100 °C
heat of vaporization (water)
2260 J
1 g H2O
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Heats of Vaporization and Fusion
of Polar and Nonpolar Compounds
Heats of Fusion and Vaporization
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Calculations Using Heat of
Vaporization
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Learning Check
How many kilojoules (kJ) are released when 50.0 g
of steam from a volcano condenses at 100 °C?
1) 113 kJ
2) 2260 kJ
3) 113 000 kJ
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Heating Curve
Learning Check
A heating curve
• illustrates the
changes of state as a
solid is heated
• uses sloped lines to
show an increase in
temperature
• uses plateaus
(horizontal lines) to
indicate a change of
state
A. A plateau (horizontal line) on a heating curve
represents
1) a temperature change
2) a constant temperature
3) a change of state
B. A sloped line on a heating curve represents
1) a temperature change
2) a constant temperature
3) a change of state
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Cooling Curve
Learning Check
A cooling curve
• illustrates the
changes of state as a
gas is cooled
• uses sloped lines to
indicate a decrease
in temperature
• uses plateaus
(horizontal lines) to
indicate a change of
state
Use the cooling curve for water to answer each.
A. Water condenses at a temperature of
1) 0 °C
2) 50 °C
3) 100 °C
B. At a temperature of 0 °C, liquid water
1) freezes
2) melts
3) changes to a gas
C. At 40 °C, water is a
1) solid
2) liquid
3) gas
D. When water freezes, heat is
1) removed
2) added
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Example of Combining Heat
Calculations
To reduce a fever, an infant is packed in 250. g of
ice H2O(s). If the ice (at 0 °C) melts and warms to
body temperature (37.0 °C), how many joules are
removed?
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Example of Combining Heat
Calculations (continued)
STEP 1 1 List the grams of substance and change
of state.
Given 250. g of ice H2O(s); H2O(l) water at 37.0 °C
Need joules to melt H2O(s) at 0 °C and warm to 37.0 °C
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Example of Combining Heat
Calculations (continued)
STEP 2 Write the plan to convert grams to heat.
Total heat = joules to melt ice at 0 °C and to warm
liquid water from 0 °C to 37 °C. For several
changes, we can draw a heating diagram.
37.0 °C
Example of Combining Heat
Calculations (continued)
STEP 3 Write heat conversion factors and
metric factors if needed.
1 g of H2O(s
l)
= 334 J
334 J
and
1 g H 2O
1 g H 2O
334 J
temperature increase
S
melting
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L
0 °C
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Example of Combining Heat
Calculations (continued)
STEP 4 Set up problem with factors.
T = 37.0 °C – 0 °C = 37.0 °C
Heat to melt the water at 0 °C
250. g H2O x 334 J
= 83 500 J
1 g H 2O
Heat to warm the water from 0 °C to 37 °C
250. g x 37.0 °C x 4.184 J = 38 700 J
g °C
Total: 83 500 J + 38 700 J = 122 200 J
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SH of H2O = 4.184 J/g °C
4.184 J
and
g °C
g °C
4.184 J
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Learning Check
When a volcano erupts, 175 g of steam at 100 °C is
released. How many kilojoules are lost when the
steam condenses, cools, freezes, and cools to -5 °C
(SH of H2O(s) = 2.03 J/g °C)?
1) 396 kJ
2) 529 kJ
3) 133 kJ
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