Authors: Rachel Casiday and Regina Frey room of snucongo.org, Washington university St. Louis, MO 63130

Key Concepts

warmth Engines Reverse warmth Engines (e.g.

You are watching: Which of the following phase changes is an exothermic change?

, Refrigerators) Phases of issue Solid liquid Gas step Transitions Fusion/ freeze Vaporization/ Condensation Sublimation/ Deposition breaking or formation of Intermolecular Attractions in phase Transitions readjust in Enthalpy (ΔH) of step Transitions Refrigeration cycle (Note: This section includes an animation.)

Introduction: warm Engines and also Refrigeration

Refrigeration has enabled for an excellent advances in our ability to save food and othersubstances safe for lengthy periods that time. In addition, the same technology that is usedto run refrigerators is likewise used in waiting conditioners, permitting people to live and also workcomfortably even in unbearably hot weather. Exactly how does this technology work to develop coolair as soon as the external problems are really hot? together we shall see, refrigerators (and airconditioners) depend on the thermodynamic application recognized as the warm engine, too asthe molecule properties the the substance contained in the coils the the refrigerator.

One that the most important practical applications the the values of thermodynamics isthe warmth engine (Figure 1). In the warm engine, warmth is took in from a "workingsubstance" at high temperature and also partially convert to work. (Heatengines are never ever 100% efficient, because the remaining warmth (i.e., the heat thatis not converted to work) is exit to the surroundings, which space at a lowertemperature.) The steam engines provided to power at an early stage trains and electric generators areheat engines in i beg your pardon water is the working substance.


Figure 1

In a warmth engine, an intake of heat causes rise in the temperature the the functioning substance, enabling the working substance to perform work. In this schematic diagram, the working substance is water. At high temperature, gas water (steam) pushes a piston, which causes a wheel come turn. This is the essential mechanism by which steam-powered trains operate.

In a reverse warm engine (Figure 2), the opposite result occurs. Occupational is converted toheat, which is released.


Figure 2

In a reverse warmth engine, a work-related input is convert to a heat output. In this case, the job-related (generated by electricity) condenses gaseous water (steam) and also pushes it right into a heat-exchange coil. In the coil, the temperature that the water drops together it liquefies, releasing warmth to the environment.

In 1851, the Florida doctor John Gorrie to be granted the first U.S. Patent for arefrigeration machine, which supplies a reverse warm engine (Figure 2) together the first step inits operation. Gorrie, persuaded that the cure for malaria was cold (because outbreakswere terminated in the winter), seek to build a maker that can make ice and also cool apatient"s room in the warm Florida summer. In Dr. Gorrie"s refrigerator, air was compressedusing a pump, which led to the temperature of the wait to increase (exchanging work-related forheat). Running this compressed air v pipes in a cold-water bathtub released the heatinto the water. The air was then allowed to increase again come atmospheric pressure, butbecause it had lost warmth to the water, the temperature that the waiting was reduced than beforeand could be offered to cool the room.

Modern refrigerators operate by the same reverse-heat-engine principle. Whereasa heat engine converts warmth (from a high-temperature area) to work, a refrigeratorconverts work to heat. Modern refrigerators usage substances various other than airas the coolant; the coolant substance alters from gas to liquid as it go from higher tolower temperature. This change from gas to liquid is a step transition, and also the energyreleased ~ above this transition is mostly dependent ~ above the intermolecular interaction ofthe substance. Hence, to recognize the refrigeration cycle provided in modernrefrigerators, the is important to first discuss step transitions.

Questions on warmth Engines and also Refrigeration

In numerous homes and also businesses, warmth pumps room replacing traditional heaters come heat structures by using electrical power to transfer heat to the within of the building. Is the warm pump an instance of a warmth engine or a reverse warm engine? Briefly, define your reasoning. Briefly, define the procedure by i m sorry the warm pump transfers warmth into a building. What to be the "working substance" in Dr. Gorrie"s refrigerator?

Phases and also Phase Transitions

Matter have the right to exist in three various phases (physical states): solid, liquid, and gas. Aphase is a type of issue that is uniform throughout in snucongo.org composition and also physicalproperties, and that can be differentiated from various other phases v which it might be incontact by these definite properties and composition. As presented in number 3, a problem inthe solid phase has a identify shape and rigidity; a substance in the fluid phase has actually nodefinite shape, yet has a identify volume, and a problem in the gas phase has actually nodefinite shape or volume, however has a shape and also volume figured out by the shape and also size ofthe container.


Figure 3

This schematic diagram reflects the distinctions in physical properties and also particle plan between a problem in the solid, liquid, and also gas phases. In a solid, the particles are densely pack in a strict configuration, giving the substance a identify shape and size. In a liquid, the particles space close together however may move with respect to one another, giving the substance a definite volume yet a liquid shape. In a gas, the particles may occupy the entire volume the the container, so the their shape and also volume space both identified by the container.

Molecular (Microscopic) View

One that the significant differences in the 3 phases depicted in number 3 is the numberof intermolecular interaction they contain. The particles in a solid connect with all oftheir nearest next-door neighbors (recall the conversation of bonding in solids from the tutorialentitled "Bands,Bonds, and Doping: how Do LED"s Wrok?"), the corpuscle in a liquid connect withonly some of the adjacent particles, and the particles in a gas ideally have no interactionwith one another. By break or developing intermolecular interactions, a problem canchange native one phase to another. Because that example, gas molecules condensation to kind liquidsbecause that the presence of attractive intermolecular forces. The more powerful the attractiveforces, the better the security of the fluid (which leader to a higher boiling pointtemperature). A transition between the phases of issue is referred to as a phase transition. Thenames of the phase transitions between solid, liquid, and also gas are presented in number 4.


Figure 4

This diagram shows the name of the phase transitions in between solids, liquids, and gases. The arrowhead to the appropriate of the diagram demonstrates the these 3 phases have different enthalpies: gas has actually the highest enthalpy, liquid has actually an intermediary enthalpy, and also solid has the shortest enthalpy. Hence, every of the step transitions presented in this figure requires a change in the enthalpy of the substance.

Phase transitions room a form of snucongo.org reaction. Many of the chemical reactionsstudied in Chem 151 and 152 involve the breaking or creating of bonds within molecules;phase transitions indicate the break or creating of intermolecular forces (attractiveinteractions between molecules). Hence, similar to other snucongo.org reactions, that is necessaryto talk about the power that is soaked up or provided off during the breaking or developing ofintermolecular interactions in a phase transition.

Phase transitions entailing the break of intermolecular attractions (i.e.,fusion (melting), vaporization, and sublimation) call for an intake of energy to overcomethe attractive forces in between the particles of the substance. Step transitions involvingthe formation of intermolecular attractions (i.e., freezing, condensation, anddeposition) release power as the particles embrace a lower-energy conformation. Thestrength that the intermolecular attractions between molecules, and therefore the amount ofenergy required to get rid of these attractive pressures (as well together the amount of energyreleased as soon as the attractions space formed) depends on the molecule properties that thesubstance. Generally, the much more polar a molecule is, the stronger the attractiveforces between molecules are. Hence, an ext polar molecules commonly require moreenergy to overcome the intermolecular attractions in one endothermic step transition, andrelease an ext energy by forming intermolecular attractions throughout an exothermic phasetransition.

Thermodynamic (Macroscopic) View

In addition to the microscopic, molecular check out presented above, us can define phasetransitions in terms of macroscopic, thermodynamic properties. The is necessary to bear inmind that the microscopic and macroscopic views room interdependent; i.e., thethermodynamic properties, such together enthalpy and temperature, that a substance room dependenton the molecular habits of the substance.

Phase transitions are accompanied by changes in enthalpy and entropy. In this tutorial,we will concern ourselves mainly with transforms in enthaply. The energy change involved inbreaking or forming intermolecular attractions is generally supplied or exit in theform that heat. Adding heat reasons intermolecular attractions to it is in broken.How does this occur? heat is a transfer of power to molecules, resulting in the molecules toincrease their activity as explained by the kinetic theory of gases (discussed in thetutorial entitled, "GasLaws save Lives: The snucongo.org Behind Airbags"), and also thereby weakening theintermolecular forces holding the molecule in place. Likewise, molecule loseheat to form intermolecular attractions; when warmth is lost, the molecule moveslower and also therefore can interact much more with other nearby molecules.

Because phase changes generally take place at consistent pressure (i.e., in areaction vessel open up to the atmosphere), the heat can be defined by a readjust in enthalpy(ΔH=qp=nCp ΔT, where n is thenumber of moles of the substance and also Cp is the molar heat capacity in ~ constantpressure). For step transitions entailing the break of intermolecularattractions, warmth is included and ΔH is positive, since thesystem is going indigenous a lower-enthalpy step to a higher-enthalpy phase, as shownby the direction the the vertical arrowhead to the best of number 4. Hence, fusion,vaporization, and sublimation room all endothermic step transitions. For phasetransitions including the creating of intermolecular attractions, warm is released and ΔH is negative, due to the fact that the mechanism is going native a higher-enthalpyphase to a lower-enthalpy phase, as presented in number 4. Hence, freezing,condensation, and also deposition room all exothermic step transitions. The direction of theenthalpy adjust for every of the phase-transition processes named in figure 4 is presented inTable 1, below.

Phase shift Direction the ΔH

Fusion (Melting) (solid come liquid)

ΔH>0; enthalpy increases (endothermic process)
Vaporization (liquid to gas) ΔH>0; enthalpy boosts (endothermic process)
Sublimation (solid come gas) ΔH>0; enthalpy boosts (endothermic process)
Freezing (liquid to solid) ΔH0; enthalpy decreases (exothermic process)
Condensation (gas to liquid) ΔH0; enthalpy reduce (exothermic process)
Deposition (gas come solid) ΔH0; enthalpy to reduce (exothermic process)

Table 1

This table mirrors the sign of the enthalpy change for each of the phase transitions defined above. Recall that endothermic processes have actually a positive enthalpy change, and also exothermic processes have actually a an adverse enthalpy change.

As with various other chemical reactions, because enthalpy is a state function, ΔH for phase transitions have the right to be added or subtracted follow toHess"s law. (Recall from Chem 112 and the advent to the experiment that,according come Hess"s law, as soon as chemical reactions are included or subtracted to accomplish a netreaction, the corresponding ΔH"s are included or subtracted toobtain the ΔH for that net reaction.)

The enthalpy adjust of step transitions can additionally be provided to define differences inmelting points and also boiling point out of substances. A given substance has actually a characteristicrange of temperatures at which that undergoes each of the step transitions (at a givenpressure). These temperatures are named for the phase transition that occurs at thetemperature (e.g., melting point). In general, the higher the enthalpychange for a phase transition is (the much more heat compelled for one endothermic transition, orreleased for an exothermic transition), the higher the temperature is in ~ which thesubstance undergoes the step transition. For example, liquids through strongintermolecular attractions require an ext heat to vaporize 보다 liquids through weakintermolecular attractions; therefore, the boiling point (vaporization point) for theseliquids will be higher than for the liquids with weaker intermolecular attractions.

Questions on Phases and also Phase Transitions

A student procedures the melt points that two common household crystalline solids: sodium chloride (NaCl) and sucrose (C12H22O11). She finds that the melting allude of sodium chloride is much greater than the melting point of sucrose. Briefly, explain why the melting suggest for NaCl is greater than for C12H22O11, in regards to the type of attractive forces in the solids and also your molecular understanding of step transitions. when you place your finger into a glass that water automatically after adding an ice cream cube, and also again 5 minutes later, you find that the water feeling cooler after some of the ice has begun to melt. Briefly, define this phenomenon in regards to your thermodynamic knowledge of step changes.


Now, we shall use our understanding of heat engines and phase transitions come explainhow refrigerators work. The enthalpy changes associated with phase transitions may be usedby a warm engine (Figure 1) to execute work and to move heat between (1) the substanceundergoing a phase transition and (2) its surrounding environment. In a heat engine, a"working substance" absorbs warmth at a high temperature and converts component of thisheat to work. In a secondary process, the remainder of the warmth is exit to the surroundingsat a lower temperature, since the warm engine is no 100% efficient.

As shown in figure 2, a refrigerator can be thought of as a warmth engine in reverse. Thecooling impact in a frozen fridge is completed by a cycle of condensation and vaporizationof the nontoxic compound CCl2F2 (Freon-12). As shown inFigure 5, the refrigerator includes (1) one electrically-powered compressor that does workon Freon gas, and also (2) a series of coils that allow heat to it is in released outside (on theback of) the frozen fridge or soaked up from within the refrigerator together Freon overcome throughthese coils.


Figure 5

This is a schematic chart of the major functional contents of a refrigerator. The significant features incorporate a compressor comprise Freon (CCl2F2) gas, an outside heat-exchange coil (on the outside earlier of the refrigerator) in i beg your pardon the Freon passes and condenses, an expansion valve, and also a heat-exchange coil within the insulated compartment of the frozen fridge (blue) in which the Freon is vaporized, absorbing heat from inside the frozen refrigerator (and hence lowering that is temperature).

Figure 6 (below) traces the phase transitions the Freon and their associatedheat-exchange occasions that occur throughout the refrigeration cycle. The procedures of therefrigeration cycle are described below the figure. (The number in the number correspondto the numbered steps below.)


Figure 6

This diagram reflects the major steps in the refrigeration cycle. Because that a summary of each action (indicated by the eco-friendly numbers), view the numbered actions below. In this figure, blue dots represent Freon gas, and solid blue areas represent fluid Freon. Little arrows indicate the direction the heat circulation into or out of the refrigerator coils.

Please click on the pink button below to see a QuickTime movie reflecting an computer animation of the refrigeration cycle shown in the figure above and described below. Click the blue button below to download QuickTime 4.0 to watch the movie.


exterior of the refrigerator, the electrically-run compressor does occupational on the Freon gas, enhancing the press of the gas. As the push of the gas increases, therefore does its temperature (as guess by the ideal-gas law). Next, this high-pressure, high-temperature gas beginning the coil ~ above the external of the refrigerator. Heat (q) flows from the high-temperature gas come the lower-temperature wait of the room neighboring the coil. This heat loss reasons the high-pressure gas to condense to liquid, as activity of the Freon molecules decreases and intermolecular attractions are formed. Hence, the work done on the gas by the compressor (causing one exothermic phase change in the gas) is convert to heat offered off in the wait in the room behind the refrigerator. If friend have ever felt the coils top top the ago of the refrigerator, you have actually experienced the heat provided off throughout the condensation the Freon. Next, the fluid Freon in the external coil passes through an expansion valve right into a coil inside the insulated compartment the the refrigerator. Now, the fluid is in ~ a low press (as a an outcome of the expansion) and also is lower in temperature (cooler) than the surrounding air (i.e., the air within the refrigerator). Since heat is moved from areas of greater temperature to areas of lower temperature, warm is took in (from within the refrigerator) by the fluid Freon, causing the temperature inside the frozen fridge to it is in reduced. The absorbed heat starts to break the intermolecular attractions the the fluid Freon, enabling the endothermic vaporization process to occur. When every one of the Freon alters to gas, the cycle have the right to start over. The bike described over does no run continuously, yet rather is controlled by athermostat. When the temperature inside the refrigerator rises over the set temperature,the thermostat beginning the compressor. As soon as the refrigerator has actually been cooled below the settemperature, the compressor is turn off. This control mechanism permits the refrigeratorto conserve electrical power by just running as much as is important to keep the refrigeratorat the desired temperature.

Questions top top Refrigeration

just how would the performance of a frozen fridge be influenced if the food inside the frozen refrigerator is packed really tightly and really close come the inner coils, so the there is no air circulation to the internal coils? Briefly, define your reasoning. Ammonia (NH3) was among the early on refrigerants used prior to Freon. The is no longer used in household refrigerators, since of the toxicity that ammonia have to there it is in a leak. The boiling suggest of NH3 is similar to the of Freon. based on molecular framework only, i m sorry substance, ammonia or Freon, would certainly you intend to have actually a larger enthalpy readjust of vaporization (ΔHvap)? Briefly, define your answer. based on your price to component (a), i m sorry substance, ammonia or Freon, would certainly you suppose to be a much better refrigerant? Briefly, describe your answer.


Refrigerators are essentially heat engines working in reverse. Whereas a warm engineconverts heat to work, reverse warm engines transform work to heat. In the refrigerator, theheat that is produced is moved to the outside of the refrigerator. To cool therefrigerator, a "working substance", or "coolant", such as Freon isrequired.The refrigerator functions by a bicycle of compressing and expanding the Freon,combined v phase transitions in between the gaseous and also liquid phases that Freon. Work-related isdone on the Freon by a compressor, and the Freon climate releases heat to the air external ofthe refrigerator (as it experience the exothermic condensation native a gas to a liquid). Toregenerate the gas Freon because that compression, the Freon passes v an internal coil,where it undergoes the endothermic vaporization from the liquid phase come the gaseousphase. This endothermic process causes the Freon to absorb warmth from the air inside therefrigerator, cooling the refrigerator.

Additional Links:


Brown, Lemay, and also Bursten. snucongo.org: The central Science, 7th ed., p. 395-98.

Petrucci and Harwood. General snucongo.org, 7th ed., p. 435, 699-701, 714-15.


The authors give thanks to Dewey Holten, Michelle Gilbertson, Jody Proctor and CarolynHerman for many helpfulsuggestions in the creating of this tutorial.

The advance of this accuse was supported by a grant from the Howard HughesMedical Institute, with the Undergraduate organic Sciences education program, GrantHHMI# 71199-502008 to Washington University.

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Revised January 2001.