2.6 Changes of State

Report
Chapter 2 Energy and Matter
2.6
Changes of State
Copyright © 2009 by Pearson Education, Inc.
1
Melting and Freezing
A substance
• is melting while it changes from a solid to a liquid.
• is freezing while it changes from a liquid to a solid.
• such as water has a freezing (melting) point of 0 °C.
2
Calculations Using Heat of Fusion
The heat of fusion
• is the amount of heat released when 1 gram of
liquid freezes (at its freezing point).
• is the amount of heat needed to melt 1 gram of a
solid (at its melting point).
• for water (at 0 °C) is
80. cal
1 g water
3
Calculation Using Heat of Fusion
The heat needed to freeze (or melt) a specific mass
of water (or ice) is calculated using the heat of
fusion.
Heat = g water (ice) x 80. cal
1 g water (ice)
Example: How much heat in cal is needed to melt
15. g of ice?
15. g ice
x
80. cal
1 g ice
= 1200 cal
4
F
Study Tip: Using Heat of
Fusion
5
Learning Check
A. How many calories are needed to melt 5.0 g of
ice at 0 °C?
1) 80. cal
2) 4.0 x 102 cal
3) 0 cal
B. How many calories are released when 25 g of
water at 0 °C freezes?
1) 80. cal
2) 0 cal
3) 2.0 x 103 cal
6
Solution
A. How many calories are needed to melt 5.00 g of ice
at 0 °C?
2) 4.0 x 102 cal
5.0 g x 80. cal
1g
B. How many calories are released when 25.0 g of
water at 0 °C freezes?
3) 2.0 x 103 cal 25 g x 80. cal
1g
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Sublimation
Sublimation
• occurs when particles
change directly from solid
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.
Copyright © 2009 by Pearson Education, Inc.
8
Evaporation and Condensation
Water
• evaporates when
molecules on the
surface gain sufficient
energy to form a gas.
• condenses when gas
molecules lose energy
and form a liquid.
Copyright © 2009 by Pearson Education, Inc.
9
Boiling
At boiling,
• all the water molecules
acquire enough energy
to form a gas.
• bubbles appear
throughout the liquid.
Copyright © 2009 by Pearson Education, Inc.
10
Heat of Vaporization
The heat of vaporization is the amount of heat
• absorbed to vaporize 1 g of a liquid to gas at the
boiling point.
• released when 1 g of a gas condenses to liquid at
the boiling point.
Boiling Point of Water = 100 °C
Heat of Vaporization (water)
=
540 cal
1 g water
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Learning Check
How many kilocalories (kcal) are released when 50.0 g
of steam from a volcano condenses at 100 °C?
1) 27 kcal
2) 540 kcal
3) 2700 kcal
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Solution
How many kilocalories (kcal) are released when 50.0 g
of steam in a volcano condenses at 100 °C?
1) 27 kcal
50.0 g steam x
540 cal
1 g steam
x 1 kcal = 27 kcal
1000 cal
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Summary of Changes of State
14
Heating Curve
A heating curve
• illustrates the changes
of state as a solid is
heated.
• uses sloped lines to
show an increase in
temperature.
• uses plateaus (flat
lines) to indicate a
change of state.
Copyright © 2009 by Pearson Education, Inc.
15
Learning Check
A. A flat 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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Solution
A. A flat line on a heating curve represents
2) a constant temperature.
3) a change of state.
(note that this is happening as energy is being put into the
system. The energy is being used to change state instead of
increase the temperature.)
B. A sloped line on a heating curve represents
1) a temperature change.
17
Cooling Curve
A cooling curve
• illustrates the changes
of state as a gas is
cooled.
• uses sloped lines to
indicate a decrease in
temperature.
• uses plateaus (flat
lines) to indicate a
change of state.
Copyright © 2009 by Pearson Education, Inc.
18
Learning Check
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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Solution
Use the cooling curve for water to answer each.
A. Water condenses at a temperature of
3) 100 °C.
B. At a temperature of 0 °C, liquid water
1) freezes.
C. At 40 °C, water is a
2) liquid.
D. When water freezes, heat is
1) removed.
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Combined Heat Calculations
To reduce a fever, an infant is packed in 250. g of ice. If
the ice (at 0 °C) melts and warms to body temperature
(37.0 °C), how many calories are removed from the body?
Step 1: Diagram the changes
 T = 37.0 °C - 0°C = 37.0 °C
37 °C
temperature increase
0 °C
solid
liquid
melting
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Combined Heat Calculations
(continued)
Step 2: Calculate the heat to melt ice (fusion)
250. g ice x 80. cal
= 20 000 cal
1 g ice
Step 3: Calculate the heat to warm the water from
0 °C to 37.0 °C
250. g x 37.0 °C x 1.00 cal = 9 250 cal
g °C
Total: Step 2 + Step 3
= 29 000 cal
(rounded to 2 SF)
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