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Sublimation and Deposition
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Phase Changes and Refrigeration:
Thermochemistry of Heat Engines
- Heat Engines
- Reverse Heat Engines (e.g., Refrigerators)
- Phases of Matter
- Phase Transitions
- Fusion/ Freezing
- Vaporization/ Condensation
- Sublimation/ Deposition
- Breaking or Formation of Intermolecular Attractions in Phase Transitions
- Change in Enthalpy (ΔH) of Phase Transitions
- Refrigeration Cycle (Note: This section contains an
Introduction: Heat Engines and Refrigeration
Refrigeration has allowed for great advances in our ability to store food and other
substances safely for long periods of time. In addition, the same technology that is used
to run refrigerators is also used in air conditioners, allowing people to live and work
comfortably even in unbearably hot weather. How does this technology work to produce cool
air when the external conditions are very hot? As we shall see, refrigerators (and air
conditioners) rely on the thermodynamic application known as the heat engine, as well as
the molecular properties of the substance contained in the coils of the refrigerator.
One of the most important practical applications of the principles of thermodynamics is
the heat engine (Figure 1). In the heat engine, heat is absorbed from a "working
substance" at high temperature and partially converted to work. (Heat
engines are never 100% efficient, because the remaining heat (i.e., the heat that
is not converted to work) is released to the surroundings, which are at a lower
temperature.) The steam engines used to power early trains and electric generators are
heat engines in which water is the working substance.
In a heat engine, an input of heat causes an increase in the temperature of the working
In a reverse heat engine (Figure 2), the opposite effect occurs. Work is converted to
heat, which is released.
In a reverse heat engine, a work input is converted to a heat output. In this case, the
In 1851, the Florida physician John Gorrie was granted the first U.S. Patent for a
refrigeration machine, which uses a reverse heat engine (Figure 2) as the first step in
its operation. Gorrie, convinced that the cure for malaria was cold (because outbreaks
were terminated in the winter), sought to develop a machine that could make ice and cool a
patient’s room in the hot Florida summer. In Dr. Gorrie’s refrigerator, air was compressed
using a pump, which caused the temperature of the air to increase (exchanging work for
heat). Running this compressed air through pipes in a cold-water bath released the heat
into the water. The air was then allowed to expand again to atmospheric pressure, but
because it had lost heat to the water, the temperature of the air was lower than before
and could be used to cool the room.
Modern refrigerators operate by the same reverse-heat-engine principle. Whereas
a heat engine converts heat (from a high-temperature area) to work, a refrigerator
converts work to heat. Modern refrigerators use substances other than air
as the coolant; the coolant substance changes from gas to liquid as it goes from higher to
lower temperature. This change from gas to liquid is a phase transition, and the energy
released upon this transition is mainly dependent on the intermolecular interactions of
the substance. Hence, to understand the refrigeration cycle used in modern
refrigerators, it is necessary to first discuss phase transitions.
Questions on Heat Engines and Refrigeration
- In many homes and businesses, heat pumps are replacing conventional heaters to heat
buildings by using electricity to transfer heat to the inside of the building.
- Is the heat pump an example of a heat engine or a reverse heat engine? Briefly, explain
- Briefly, describe the process by which the heat pump transfers heat into a building.
- What was the "working substance" in Dr. Gorrie’s refrigerator?
Phases and Phase Transitions
Matter can exist in three different phases (physical states): solid, liquid, and gas. A
phase is a form of matter that is uniform throughout in chemical composition and physical
properties, and that can be distinguished from other phases with which it may be in
contact by these definite properties and composition. As shown in Figure 3, a substance in
the solid phase has a definite shape and rigidity; a substance in the liquid phase has no
definite shape, but has a definite volume, and a substance in the gas phase has no
definite shape or volume, but has a shape and volume determined by the shape and size of
This schematic diagram shows the differences in physical properties and particle
Molecular (Microscopic) View
One of the major differences in the three phases illustrated in Figure 3 is the number
of intermolecular interactions they contain. The particles in a solid interact with all of
their nearest neighbors (recall the discussion of bonding in solids from the tutorial
entitled " Bands,
Bonds, and Doping: How Do LED’s Wrok? "), the particles in a liquid interact with
only some of the nearby particles, and the particles in a gas ideally have no interaction
with one another. By breaking or forming intermolecular interactions, a substance can
change from one phase to another. For example, gas molecules condense to form liquids
because of the presence of attractive intermolecular forces. The stronger the attractive
forces, the greater the stability of the liquid (which leads to a higher boiling point
temperature). A transition between the phases of matter is called a phase transition. The
names of the phase transitions between solid, liquid, and gas are shown in Figure 4.
This diagram shows the names of the phase transitions between solids, liquids, and
Phase transitions are a type of chemical reaction. Most of the chemical reactions
studied in Chem 151 and 152 involve the breaking or forming of bonds within molecules;
phase transitions involve the breaking or forming of intermolecular forces (attractive
interactions between molecules). Hence, as with other chemical reactions, it is necessary
to discuss the energy that is absorbed or given off during the breaking or forming of
intermolecular interactions in a phase transition.
Phase transitions involving the breaking of intermolecular attractions (i.e.,
fusion (melting), vaporization, and sublimation) require an input of energy to overcome
the attractive forces between the particles of the substance. Phase transitions involving
the formation of intermolecular attractions (i.e., freezing, condensation, and
deposition) release energy as the particles adopt a lower-energy conformation. The
strength of the intermolecular attractions between molecules, and therefore the amount of
energy required to overcome these attractive forces (as well as the amount of energy
released when the attractions are formed) depends on the molecular properties of the
substance. Generally, the more polar a molecule is, the stronger the attractive
forces between molecules are. Hence, more polar molecules typically require more
energy to overcome the intermolecular attractions in an endothermic phase transition, and
release more energy by forming intermolecular attractions during an exothermic phase
Thermodynamic (Macroscopic) View
In addition to the microscopic, molecular view presented above, we can describe phase
transitions in terms of macroscopic, thermodynamic properties. It is important to bear in
mind that the microscopic and macroscopic views are interdependent; i.e., the
thermodynamic properties, such as enthalpy and temperature, of a substance are dependent
on the molecular behavior of the substance.
Phase transitions are accompanied by changes in enthalpy and entropy. In this tutorial,
we will concern ourselves mainly with changes in enthaply. The energy change involved in
breaking or forming intermolecular attractions is primarily supplied or released in the
form of heat. Adding heat causes intermolecular attractions to be broken.
How does this occur? Heat is a transfer of energy to molecules, causing the molecules to
increase their motion as described by the kinetic theory of gases (discussed in the
tutorial entitled, " Gas
Laws Save Lives: The Chemistry Behind Airbags "), and thereby weakening the
intermolecular forces holding the molecules in place. Likewise, molecules lose
heat to form intermolecular attractions; when heat is lost, the molecules move
slower and therefore can interact more with other nearby molecules.
Because phase changes generally occur at constant pressure (i.e., in a
reaction vessel open to the atmosphere), the heat can be described by a change in enthalpy
number of moles of the substance and Cp is the molar heat capacity at constant
pressure). For phase transitions involving the breaking of intermolecular
attractions, heat is added and ΔH is positive, because the
system is going from a lower-enthalpy phase to a higher-enthalpy phase, as shown
by the direction of the vertical arrow to the right of Figure 4. Hence, fusion,
vaporization, and sublimation are all endothermic phase transitions. For phase
transitions involving the forming of intermolecular attractions, heat is released and ΔH is negative, because the system is going from a higher-enthalpy
phase to a lower-enthalpy phase, as shown in Figure 4. Hence, freezing,
condensation, and deposition are all exothermic phase transitions. The direction of the
enthalpy change for each of the phase-transition processes named in Figure 4 is shown in
Table 1, below.
Direction of ΔH
This table shows the sign of the enthalpy change for each of the phase
As with other chemical reactions, because enthalpy is a state function, ΔH for phase transitions can be added or subtracted according to
Hess’s law. (Recall from Chem 112 and the Introduction to the experiment that,
according to Hess’s law, when chemical reactions are added or subtracted to achieve a net
reaction, the corresponding ΔH’s are added or subtracted to
obtain the ΔH for that net reaction.)
The enthalpy change of phase transitions can also be used to explain differences in
melting points and boiling points of substances. A given substance has a characteristic
range of temperatures at which it undergoes each of the phase transitions (at a given
pressure). These temperatures are named for the phase transition that occurs at the
temperature (e.g., melting point). In general, the greater the enthalpy
change for a phase transition is (the more heat required for an endothermic transition, or
released for an exothermic transition), the greater the temperature is at which the
substance undergoes the phase transition. For example, liquids with strong
intermolecular attractions require more heat to vaporize than liquids with weak
intermolecular attractions; therefore, the boiling point (vaporization point) for these
liquids will be higher than for the liquids with weaker intermolecular attractions.
Questions on Phases and Phase Transitions
- A student measures the melting points of two common household crystalline solids: sodium
chloride (NaCl) and sucrose (C12H22O11). She finds that
the melting point of sodium chloride is much higher than the melting point of sucrose.
Briefly, explain why the melting point for NaCl is higher than for C12H22O11,
in terms of the type of attractive forces in the solids and your molecular understanding
of phase transitions.
- When you place your finger into a glass of water immediately after adding an ice cube,
and again five minutes later, you find that the water feels cooler after some of the ice
has begun to melt. Briefly, explain this phenomenon in terms of your thermodynamic
understanding of phase changes.
Now, we shall use our understanding of heat engines and phase transitions to explain
how refrigerators work. The enthalpy changes associated with phase transitions may be used
by a heat engine (Figure 1) to do work and to transfer heat between (1) the substance
undergoing a phase transition and (2) its surrounding environment. In a heat engine, a
"working substance" absorbs heat at a high temperature and converts part of this
heat to work. In a secondary process, the rest of the heat is released to the surroundings
at a lower temperature, because the heat engine is not 100% efficient.
As shown in Figure 2, a refrigerator can be thought of as a heat engine in reverse. The
cooling effect in a refrigerator is achieved by a cycle of condensation and vaporization
of the nontoxic compound CCl2F2 (Freon-12). As shown in
Figure 5, the refrigerator contains (1) an electrically-powered compressor that does work
on Freon gas, and (2) a series of coils that allow heat to be released outside (on the
back of) the refrigerator or absorbed from inside the refrigerator as Freon passes through
This is a schematic diagram of the major functional components of a refrigerator. The
Figure 6 (below) traces the phase transitions of Freon and their associated
heat-exchange events that occur during the refrigeration cycle. The steps of the
refrigeration cycle are described below the figure. (The numbers in the figure correspond
to the numbered steps below.)
This diagram shows the major steps in the refrigeration cycle.
Please click on the pink button below to view a QuickTime movie showing an animation of
- Outside of the refrigerator, the electrically-run compressor does work on the
Freon gas, increasing the pressure of the gas. As the pressure of the gas
increases, so does its temperature (as predicted by the ideal-gas law).
- Next, this high-pressure, high-temperature gas enters the coil on the outside of the
- Heat (q) flows from the high-temperature gas to the lower-temperature air of the
room surrounding the coil. This heat loss causes the high-pressure gas to condense
to liquid, as motion of the Freon molecules decreases and intermolecular
attractions are formed. Hence, the work done on the gas by the compressor (causing
an exothermic phase transition in the gas) is converted to heat given off in the air in
the room behind the refrigerator. If you have ever felt the coils on the back of
the refrigerator, you have experienced the heat given off during the condensation of
- Next, the liquid Freon in the external coil passes through an expansion valve into a
coil inside the insulated compartment of the refrigerator. Now, the liquid is at a
low pressure (as a result of the expansion) and is lower in temperature (cooler) than the
surrounding air (i.e., the air inside the refrigerator).
- Since heat is transferred from areas of greater temperature to areas of lower
temperature, heat is absorbed (from inside the refrigerator) by the liquid Freon, causing
the temperature inside the refrigerator to be reduced. The absorbed heat begins
to break the intermolecular attractions of the liquid Freon, allowing the endothermic
vaporization process to occur.
- When all of the Freon changes to gas, the cycle can start over.
The cycle described above does not run continuously, but rather is controlled by a
thermostat. When the temperature inside the refrigerator rises above the set temperature,
the thermostat starts the compressor. Once the refrigerator has been cooled below the set
temperature, the compressor is turned off. This control mechanism allows the refrigerator
to conserve electricity by only running as much as is necessary to keep the refrigerator
at the desired temperature.
Questions on Refrigeration
- How would the efficiency of a refrigerator be affected if the food inside the
refrigerator is packed very tightly and very close to the internal coils, so that there is
no air flow to the internal coils? Briefly, explain your reasoning.
- Ammonia (NH3) was one of the early refrigerants used before Freon. It is no
longer used in household refrigerators, because of the toxicity of ammonia should there be
a leak. The boiling point of NH3 is similar to that of Freon.
- Based on molecular structure only, which substance, ammonia or Freon, would you expect
to have a larger enthalpy change of vaporization (ΔHvap)?
Briefly, explain your answer.
- Based on your answer to part (a), which substance, ammonia or Freon, would you expect to
be a better refrigerant? Briefly, explain your answer.
Refrigerators are essentially heat engines working in reverse. Whereas a heat engine
converts heat to work, reverse heat engines convert work to heat. In the refrigerator, the
heat that is generated is transferred to the outside of the refrigerator. To cool the
refrigerator, a "working substance", or "coolant", such as Freon is
required.The refrigerator works by a cycle of compressing and expanding the Freon,
combined with phase transitions between the gaseous and liquid phases of Freon. Work is
done on the Freon by a compressor, and the Freon then releases heat to the air outside of
the refrigerator (as it undergoes the exothermic condensation from a gas to a liquid). To
regenerate the gaseous Freon for compression, the Freon passes through an internal coil,
where it undergoes the endothermic vaporization from the liquid phase to the gaseous
phase. This endothermic process causes the Freon to absorb heat from the air inside the
refrigerator, cooling the refrigerator.
- For more explanation about how
refrigerators work , see this site from "How Stuff Works," by Marshall Brian.
- This site on Dr. John Gorrie
gives his biography and explains how his refrigerator design worked. It also includes
photographs of the first patented refrigerator!
- This site from Eastern Illinois University has explanations and schematics of heat engines .
- This site has an essay on thermodynamics by
Isaac Asimov .
Brown, Lemay, and Bursten. Chemistry: The Central Science, 7th ed., p. 395-98.
Petrucci and Harwood. General Chemistry, 7th ed., p. 435, 699-701, 714-15.
The authors thank Dewey Holten, Michelle Gilbertson, Jody Proctor and Carolyn
Herman for many helpful
suggestions in the writing of this tutorial.
The development of this tutorial was supported by a grant from the Howard Hughes
Medical Institute, through the Undergraduate Biological Sciences Education program, Grant
HHMI# 71199-502008 to Washington University.
Copyright 1999, Washington University, All Rights Reserved.
Revised January 2001.