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Principles of Technology
Ch 8 Internal Energy & Properties of Matter 3 Name_______
Key Objectives:
At the conclusion of this chapter you’ll be able to:
• Define the terms melting (fusion), freezing, boiling (vaporization), condensation,
sublimation, and deposition as they relate to phase changes.
• Define the terms heat of fusion and heat of vaporization, and solve problems
involving these quantities.
• State the factors that affect the boiling and freezing points of a liquid.
• Define the term thermodynamics, and state the three laws of thermodynamics.
8.5 LIQUIDS AND SOLIDS
Liquids and solids exist because particles of matter do not exhibit ideal behavior; they
have measurable volumes, and they exert forces on one another. When the temperature of
a gas is sufficiently low and the pressure is sufficiently high, the gas condenses to a
liquid. The motion of the liquid particles becomes more restricted (the particles cannot
move freely through space). As a result, the volume of the liquid becomes definite but it
continues to take the shape of its container.
As the temperature of a liquid is lowered, the forces of attraction between the particles
become stronger and the particles begin to arrange themselves in an orderly fashion. The
motion of the particles becomes severely restricted (largely vibrations), and the substance
is said to be in the solid phase. All true solids have crystalline structure.
ASSESSMENT QUESTION 1
All of the following are true EXCEPT:
A. The particles in liquids and solid exhibit ideal behavior.
B. When the temperature of a gas is sufficiently low and the pressure is sufficiently
high, the gas condenses to a liquid. The motion of the liquid particles becomes
more restricted (the particles cannot move freely through space)
C. As the temperature of a liquid is lowered, the forces of attraction between the
particles become stronger and the particles begin to arrange themselves in an
orderly fashion.
D. The motion of the particles becomes severely restricted (largely vibrations), and
the substance is said to be in the solid phase. All true solids have crystalline
structure.
8.6 CHANGE OF PHASE
We now take a closer look at how substances
change their phases. Phase change is dependent
on the nature of the substance and on the pressure
and temperature of its environment. Various
terms are associated with phase changes:
Reverse phase changes, melting and freezing for
example, occur at the same temperature.
Therefore, the melting point and the freezing
point of a substance refer to the same
temperature. If heat energy is being added to the sys tem, melting occurs; if it is being
removed, freezing results. Otherwise, the phases are said to coexist in dynamic
equilibrium.
During a phase change, the temperature of the substance remains constant until all of the
substance has been converted from one phase to another. This fact means that the average
kinetic energy of the particles is not changing The energy that is absorbed—or released-
during a phase change is associated with changes in the potential energy of the molecules
of the substance.
ASSESSMENT QUESTION 2
All of the following are true EXCEPT:
A. Phase change is dependent on the nature of the substance and on the pressure and
temperature of its environment. The phases are said to coexist in dynamic
equilibrium.
B. The melting point is always higher than the freezing point of a substance.
C. During a phase change, the temperature of the substance remains constant until all
of the substance has been converted from one phase to another.
D. The energy that is absorbed- or released -during a phase change is associated with
changes in the potential energy of the molecules of the substance.
Heat Energy and Change of Phase
During melting, a solid absorbs a specific amount of heat energy in order to be converted
to a liquid. This energy, known as the heat of fusion (Hp, is sometimes called a latent
(hidden) heat because its addition is not accompanied by a temperature change. Heats of
fusion are measured in kilojoules per kilogram (kJ/kg). During freezing, a liquid releases
an amount of energy equal to its heat of fusion.
Similarly, during boiling, a liquid absorbs a specific amount of heat energy in order to be
converted to a gas. This energy, known as the heat of vaporization (Hr), is also measured
in kilojoules per kilogram. During condensation, a liquid releases an amount of energy
equal to its heat of vaporization.
A table of heats of fusion and
vaporization is given below.
The table indicates that the heat of
vaporization of a substance is
significantly larger than its heat of
fusion. For this reason it is more
difficult to convert a liquid into a gas
than to convert a solid into a liquid. The
heat of vaporization for ice and the heat
of fusion of water and water vapor are
not listed: we do not vaporize ice, nor do
we melt water, nor do we freeze steam.
(We vaporize water, we melt ice and
freeze water!)
ASSESSMENT QUESTION 3
All of the following are true EXCEPT:
A. During freezing, a liquid releases an amount of energy equal to its heat of fusion.
B. During condensation, a liquid releases an amount of energy equal to its heat of
vaporization.
C. The heat of vaporization of a substance is significantly larger than its heat of
fusion. For this reason it is more difficult to convert a liquid into a gas than to
convert a solid into a liquid.
D. The heat of vaporization for ice and the heat of fusion of water vapor are needed
in calculations involving vaporizing ice, melting water or freezing steam.
We can solve problems involving heats of fusion and vaporization by using the following
relationships: Qf = mHf
Qv = mHv
PROBLEM
How much heat energy (Qf) is needed to melt 5.0 kilograms (m) of iron at its melting
point?
SOLUTION
Qf = mHf
Qf = (5.0 kg)(267 kJ/kg)
Qf = 1335 kJ
ASSESSMENT QUESTION 4
How much heat energy (Qf) is needed to melt 36 kilograms (m) of iron at its melting
point?
Qf = mHf
Qf = (36 kg)(267 kJ/kg) =
A. 560 J
B. 2300 J
C. 9600 J
D. 12000 J
PROBLEM
How many kilograms (m) of steam will release 1130 kilojoules (Qv) when the steam
condenses at the boiling point of water?
SOLUTION
We need to use the heat of vaporization of water, since the steam will release this amount
of energy when it condenses.
Qv = mHv
1130 kJ = m(2260 kJ/kg)
m = Qv / H v
m = 0.50 kg
ASSESSMENT QUESTION 5
How many kilograms (m) of steam will release 3600 kilojoules (Qv) when the steam
condenses at the boiling point of water?
Qv = mHv
3600 kJ = m(2260 kJ/kg)
m = Qv / H v
m = 3600 kJ / (2260 kJ/kg)
A. 0.50 kg
B. 1.6 kg
C. 9.0 kg
D. 12 kg
Factors Affecting Boiling and Freezing
The boiling and freezing temperatures of a substance are affected by pressure. An
increase in pressure raises the boiling point of a substance. Generally, the freezing point
is also raised by an increase in pressure. Water, however, is a substance that behaves
irregularly because the density of liquid water is greater than the density of ice.
Increasing the pressure lowers the freezing point of water.
When a substance such as salt is dissolved in water, the salt interferes with the water’s
ability to freeze and boil. As a consequence, the water-salt solution has a higher boiling
point and a lower freezing point than pure water has. The degree to which the boiling
point is elevated and the freezing point is depressed depends on the number of salt
particles dissolved in the water.
ASSESSMENT QUESTION 6
All of the following are true EXCEPT:
A. An increase in pressure raises the boiling point of a substance. Generally, the
freezing point is also raised by an increase in pressure.
B. Water is a substance that behaves irregularly because the density of liquid water is
greater than the density of ice. Increasing the pressure lowers the freezing point of
water.
C. When a substance such as salt is dissolved in water, the salt interferes with the
water’s ability to freeze and boil and the water-salt solution has a higher boiling
point and a lower freezing point than pure water has.
D. The degree to which the boiling point is elevated and the freezing point is
depressed depends on the size of salt particles dissolved in the water.
8.7 THE LAWS OF THERMODYNAMICS
Thermodynamics is the study of the relationship among heat, work, and energy in the
universe. The laws of thermodynamics are based on our experiences in observing nature.
Thermodynamics has many applications in disciplines ranging from physics and
engineering to biology and medicine.
The First Law of Thermodynamics
The first law of thermodynamics is a restatement of the law of conservation of energy. It
states that the change in the internal energy of a system (∆U) is equal to the heat (Q) that
the system absorbs (or releases) minus the work (W) it does (or has done on it). In
symbolic form the first law is written as follows:
ΔU= Q-W
ASSESSMENT QUESTION 7
All of the following are true EXCEPT:
A. Thermodynamics is the study of the relationship among heat, work, and energy in
the universe.
B. Thermodynamics does not involve other science disciplines such as physics,
engineering, biology and medicine.
C. The first law of thermodynamics is a restatement of the law of conservation of
energy.
D. The first law of thermodynamics states that the change in the internal energy of a
system (∆U) is equal to the heat (Q) that the system absorbs (or releases) minus
the work (W) it does (or has done on it).
PROBLEM
What is the change in the internal energy of a system (∆U) if the heat that the system
absorbs is 5500 J (Q) and the work performed on the system is 3300 J (W)?
SOLUTION
ΔU= Q-W
ΔU= 5500 J -3300 J
ΔU= 2200 J
ASSESSMENT QUESTION 8
What is the change in the internal energy of a system (∆U) if the heat that the system
absorbs is 5800 J (Q) and the work performed on the system is 1200 J (W)?
ΔU= Q-W
ΔU= 5500 J -3300 J
A. 1700 J
B. 2500 J
C. 7000 J
D. 69600 J
The Second Law of Thermodynamics
The second law is a result of the work of the French physicist Nicolas Carnot with heat
engines. This law states that heat cannot flow from a colder object to a warmer one
without work being done on the system. For example, refrigerators must be run by motors
in order to withdraw heat from objects placed inside them.
Another consequence of the second law is that heat can never be converted completely
into work. In other words, no heat engine can be 100 percent efficient. Some of the heat
absorbed by the engine must be lost in the random motions of its molecules. The quantity
known as entropy is a measure of this disorder.
ASSESSMENT QUESTION 9
All of the following are true EXCEPT:
A. The second law of Thermodynamics states that heat cannot flow from a colder
object to a warmer one without work being done on the system.
B. Modern refrigerators do not need motors or a power source to withdraw heat from
objects placed inside them.
C. A consequence of the second law is that heat can never be converted completely
into work. No heat engine can be 100 percent efficient.
D. Some of the heat absorbed by the engine must be lost in the random motions of its
molecules. The quantity known as entropy is a measure of this disorder.
The Third Law of Thermodynamics
The third law is tied to the second law in the following way: The efficiency of a heat
engine depends on its operating temperatures; the engine would reach 100 percent
efficiency only if its “cold” temperature were absolute zero (0 K). Since such an engine
cannot be completely efficient, it follows that a temperature of absolute zero cannot be
reached. Although temperatures extremely close to absolute zero have been achieved, the
remaining gap will never be bridged.
ASSESSMENT QUESTION 10
All of the following are true EXCEPT:
A. The efficiency of a heat engine depends on its operating temperatures.
B. A heat engine would reach 100 percent efficiency only if its “cold” temperature
were below absolute zero (0 K) such as -150 K.
C. Since such an engine cannot be completely efficient, it follows that a temperature
of absolute zero cannot be reached.
D. Although temperatures extremely close to absolute zero have been achieved, the
remaining gap will never be bridged.
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Conclusion
The liquid and solid phases of matter result from the presence of attractive forces
between molecules.
Changes of phase occur at specific temperatures and pressures, and heat energy is
absorbed or released during these changes.
The laws of thermodynamics relate internal energy to heat energy and work.
The first law is equivalent to the law of conservation of energy.
The second law expresses the impossibility of converting heat energy completely
into useful work. It also introduces the concept of entropy, the disorder factor.
The third law states the impossibility of attaining a temperature of absolute zero.