Authors: Rachel Casiday and also Regina Frey department of chathamtownfc.net, Washington college St. Louis, MO 63130

Key Concepts

heat Engines Reverse warmth Engines (e.g.

You are watching: Which phase change is endothermic?

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

Introduction: heat Engines and Refrigeration

Refrigeration has permitted for great advances in our capability to store food and also othersubstances safely for long periods the time. In addition, the same modern technology that is usedto run refrigerators is also used in air conditioners, permitting people to live and workcomfortably even in unbearably hot weather. Just how does this an innovation work to create coolair when the external problems are really hot? together we chandelier see, refrigerators (and airconditioners) rely on the thermodynamic application recognized as the warm engine, too asthe molecule properties of the substance contained in the coils that the refrigerator.

One of the most vital practical applications of the principles of thermodynamics isthe heat engine (Figure 1). In the heat engine, heat is took in from a "workingsubstance" in ~ high temperature and also partially convert to work. (Heatengines are never ever 100% efficient, because the remaining warm (i.e., the heat thatis no converted come work) is exit to the surroundings, which room at a lowertemperature.) The steam engines supplied 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 heat engine, an intake of warmth causes an increase in the temperature of the working substance, permitting the working substance to perform work. In this schematic diagram, the working substance is water. At high temperature, gaseous water (steam) pushes a piston, which reasons a wheel come turn. This is the essential mechanism whereby steam-powered trains operate.

In a reverse warmth engine (Figure 2), the opposite result occurs. Job-related is convert toheat, which is released.


Figure 2

In a reverse warm engine, a job-related input is converted to a warm output. In this case, the job-related (generated by electricity) condenses gaseous water (steam) and pushes it into a heat-exchange coil. In the coil, the temperature of the water drops as it liquefies, releasing warmth to the environment.

In 1851, the Florida doctor John Gorrie to be granted the an initial U.S. Patent because that arefrigeration machine, which provides a reverse warmth engine (Figure 2) as the an initial step inits operation. Gorrie, encouraged that the cure for malaria to be cold (because outbreakswere terminated in the winter), seek to develop a device that might make ice and also cool apatient"s room in the hot Florida summer. In Dr. Gorrie"s refrigerator, air to be compressedusing a pump, which caused the temperature that the wait to increase (exchanging job-related forheat). To run this compressed air v pipes in a cold-water bathtub released the heatinto the water. The air was then permitted to expand again to atmospheric pressure, butbecause it had lost warm to the water, the temperature that the wait was reduced than beforeand could be supplied to cool the room.

Modern refrigerators run by the exact same reverse-heat-engine principle. Whereasa heat engine converts heat (from a high-temperature area) to work, a refrigeratorconverts work-related to heat. Modern refrigerators usage substances other than airas the coolant; the coolant substance changes from gas to liquid as it goes from higher tolower temperature. This change from gas to liquid is a step transition, and also the energyreleased top top this shift is largely dependent top top the intermolecular interactions ofthe substance. Hence, to know the refrigeration cycle used in modernrefrigerators, the is important to first discuss step transitions.

Questions on warm Engines and also Refrigeration

In numerous homes and also businesses, heat pumps space replacing standard heaters to heat structures by using electricity to transfer warmth to the inside of the building. Is the warm pump an instance of a warmth engine or a reverse warmth engine? Briefly, define your reasoning. Briefly, describe the process by i m sorry the heat pump transfers heat into a building. What was the "working substance" in Dr. Gorrie"s refrigerator?

Phases and also Phase Transitions

Matter have the right to exist in three different phases (physical states): solid, liquid, and gas. Aphase is a form of matter that is uniform transparent in chathamtownfc.net composition and also physicalproperties, and also that deserve to be distinguished from various other phases with which it may be incontact by these definite properties and also composition. As displayed in figure 3, a problem inthe solid phase has a definite shape and also rigidity; a substance in the fluid phase has nodefinite shape, yet has a definite volume, and a problem in the gas phase has nodefinite shape or volume, but has a shape and also volume identified by the shape and size ofthe container.


Figure 3

This schematic diagram mirrors the distinctions in physical properties and particle arrangement between a problem in the solid, liquid, and also gas phases. In a solid, the particles space densely pack in a strictly configuration, giving the problem a definite shape and size. In a liquid, the particles space close together yet may relocate with respect come one another, providing the substance a definite volume however a fluid shape. In a gas, the particles may occupy the whole volume that the container, so that their shape and also volume are both characterized by the container.

Molecular (Microscopic) View

One of the significant differences in the three phases depicted in figure 3 is the numberof intermolecular interaction they contain. The corpuscle in a solid interact with every oftheir nearest next-door neighbors (recall the discussion of bonding in solids native the tutorialentitled "Bands,Bonds, and Doping: exactly how Do LED"s Wrok?"), the corpuscle in a liquid communicate withonly few of the adjacent particles, and also the corpuscle in a gas ideally have actually no interactionwith one another. By breaking or forming intermolecular interactions, a problem canchange indigenous one phase to another. For example, gas molecules condense to form liquidsbecause that the visibility of attractive intermolecular forces. The more powerful the attractiveforces, the higher the stability of the fluid (which leader to a greater boiling pointtemperature). A transition between the phases of issue is referred to as a phase transition. Thenames of the phase transitions in between solid, liquid, and also gas are displayed in number 4.


Figure 4

This diagram shows the name of the step transitions between solids, liquids, and gases. The arrowhead to the best of the chart demonstrates that these 3 phases have different enthalpies: gas has actually the highest possible enthalpy, liquid has actually an intermediate enthalpy, and solid has actually the lowest enthalpy. Hence, each of the phase transitions presented in this figure involves a change in the enthalpy the the substance.

Phase transitions are a kind of chathamtownfc.net reaction. Many of the chemical reactionsstudied in Chem 151 and 152 indicate the breaking or developing of bonds in ~ molecules;phase transitions show off the breaking or forming of intermolecular forces (attractiveinteractions between molecules). Hence, as with other chemical reactions, that is necessaryto talk about the power that is soaked up or offered off throughout the break or developing ofintermolecular interaction in a step transition.

Phase transitions entailing the break of intermolecular attractions (i.e.,fusion (melting), vaporization, and also sublimation) call for an input of power to overcomethe attractive forces in between the corpuscle of the substance. Phase transitions involvingthe formation of intermolecular attractions (i.e., freezing, condensation, anddeposition) release power as the particles take on a lower-energy conformation. Thestrength of the intermolecular attractions between molecules, and therefore the quantity ofenergy required to conquer these attractive pressures (as well as the lot of energyreleased as soon as the attractions are formed) depends on the molecular properties of thesubstance. Generally, the an ext polar a molecule is, the more powerful the attractiveforces in between molecules are. Hence, more polar molecules frequently require moreenergy to get over the intermolecular attractions in one endothermic step transition, andrelease much more energy by forming intermolecular attractions throughout an exothermic phasetransition.

Thermodynamic (Macroscopic) View

In enhancement to the microscopic, molecular see presented above, us can explain phasetransitions in terms of macroscopic, thermodynamic properties. The is necessary to be affected by each other inmind that the microscopic and macroscopic views space interdependent; i.e., thethermodynamic properties, such together enthalpy and also temperature, the a substance are dependenton the molecular behavior of the substance.

Phase transitions are accompanied by changes in enthalpy and also entropy. In this tutorial,we will concern ourselves mostly with transforms in enthaply. The energy adjust involved inbreaking or creating intermolecular attractions is mostly supplied or exit in theform the heat. Adding heat causes intermolecular attractions to be broken.How does this occur? warm is a deliver of power to molecules, resulting in the molecule toincrease their motion as defined by the kinetic concept of gases (discussed in thetutorial entitled, "GasLaws conserve Lives: The chathamtownfc.net Behind Airbags"), and thereby weakening theintermolecular pressures holding the molecule in place. Likewise, molecules loseheat to kind intermolecular attractions; when warm is lost, the molecules moveslower and therefore can interact much more with other surrounding molecules.

Because phase alters generally happen at continuous pressure (i.e., in areaction vessel open up to the atmosphere), the heat deserve to be defined by a adjust in enthalpy(ΔH=qp=nCp ΔT, wherein n is thenumber of moles of the substance and Cp is the molar heat capacity in ~ constantpressure). For phase transitions including the break of intermolecularattractions, heat is added and ΔH is positive, due to the fact that thesystem is going from a lower-enthalpy phase to a higher-enthalpy phase, together shownby the direction that the vertical arrowhead to the ideal of figure 4. Hence, fusion,vaporization, and sublimation are all endothermic step transitions. For phasetransitions including the developing of intermolecular attractions, warmth is released and also ΔH is negative, due to the fact that the system is going native a higher-enthalpyphase to a lower-enthalpy phase, as presented in number 4. Hence, freezing,condensation, and deposition are all exothermic step transitions. The direction the theenthalpy change for every of the phase-transition processes named in number 4 is shown inTable 1, below.

Phase transition Direction the ΔH

Fusion (Melting) (solid come liquid)

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

Table 1

This table reflects the sign of the enthalpy readjust for each of the phase transitions explained above. Recall that endothermic processes have a optimistic enthalpy change, and also exothermic processes have actually a an adverse enthalpy change.

As with other chemical reactions, because enthalpy is a state function, ΔH for phase transitions deserve to be included or subtracted follow toHess"s law. (Recall native Chem 112 and also the introduction to the experiment that,according to Hess"s law, as soon as chemical reactions are included or subtracted to achieve a netreaction, the equivalent ΔH"s are included or subtracted toobtain the ΔH for that net reaction.)

The enthalpy adjust of step transitions can also be used to explain differences inmelting points and boiling points of substances. A given substance has a characteristicrange of temperatures at which it undergoes every of the step transitions (at a givenpressure). This temperatures are called for the phase change that occurs at thetemperature (e.g., melting point). In general, the greater the enthalpychange for a phase shift is (the more heat forced for one endothermic transition, orreleased for an exothermic transition), the higher the temperature is at which thesubstance undergoes the step transition. Because that example, liquids v strongintermolecular attractions require much more heat come vaporize 보다 liquids v weakintermolecular attractions; therefore, the boiling suggest (vaporization point) because that theseliquids will be greater than because that the liquids through weaker intermolecular attractions.

Questions on Phases and Phase Transitions

A student measures the melt points the two common household crystalline solids: sodium chloride (NaCl) and also sucrose (C12H22O11). She finds the the melting suggest of sodium chloride is much greater than the melting allude of sucrose. Briefly, explain why the melting allude for NaCl is higher than because that C12H22O11, in terms of the form of attractive forces in the solids and your molecular understanding of step transitions. as soon as you ar your finger right into a glass the water immediately after including an ice cream cube, and again 5 minutes later, you find that the water feeling cooler after few of the ice cream has started to melt. Briefly, define this phenomenon in terms of your thermodynamic expertise of step changes.


Now, we shall use our understanding of warmth engines and also phase transitions to explainhow refrigerators work. The enthalpy changes associated with step transitions may be usedby a heat engine (Figure 1) to carry out work and to deliver heat in between (1) the substanceundergoing a phase shift and (2) its surrounding environment. In a heat engine, a"working substance" absorbs heat at a high temperature and also converts component of thisheat come work. In a an additional process, the rest of the warm is exit to the surroundingsat a reduced temperature, because the warmth engine is no 100% efficient.

As shown in figure 2, a refrigerator can be assumed of together a warmth engine in reverse. Thecooling result in a frozen fridge is achieved by a bike of condensation and also vaporizationof the nontoxic compound CCl2F2 (Freon-12). As displayed inFigure 5, the refrigerator has (1) an electrically-powered compressor the does workon Freon gas, and (2) a collection of coils that allow heat to be released outside (on theback of) the refrigerator or soaked up from within the refrigerator together Freon overcome throughthese coils.


Figure 5

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

Figure 6 (below) traces the phase transitions that Freon and their associatedheat-exchange events that occur during the refrigeration cycle. The measures of therefrigeration cycle space described listed below the figure. (The number in the figure correspondto the numbered procedures below.)


Figure 6

This diagram reflects the significant steps in the refrigeration cycle. Because that a summary of each step (indicated by the environment-friendly numbers), see the numbered procedures below. In this figure, blue dots represent Freon gas, and solid blue areas represent liquid Freon. Small arrows suggest the direction of heat flow into or out of the frozen fridge coils.

Please click the pink button below to check out a QuickTime movie showing an animation of the refrigeration cycle displayed in the figure over and described below. Click the blue button listed below to download QuickTime 4.0 to check out the movie.


outside of the refrigerator, the electrically-run compressor does occupational on the Freon gas, enhancing the push of the gas. As the press of the gas increases, for this reason does that temperature (as predicted by the ideal-gas law). Next, this high-pressure, high-temperature gas beginning the coil ~ above the outside of the refrigerator. Heat (q) flows from the high-temperature gas come the lower-temperature air of the room neighboring the coil. This warmth loss causes the high-pressure gas come condense to liquid, as motion of the Freon molecules decreases and also intermolecular attractions room formed. Hence, the work-related done top top the gas by the compressor (causing an exothermic phase transition in the gas) is converted to heat provided off in the air in the room behind the refrigerator. If girlfriend have ever felt the coils ~ above the earlier of the refrigerator, you have actually experienced the heat offered off during the condensation the Freon. Next, the fluid Freon in the external coil passes v an development valve into a coil inside the insulated compartment the the refrigerator. Now, the liquid is at a low push (as a result of the expansion) and also is reduced in temperature (cooler) than the bordering air (i.e., the air inside the refrigerator). Since warm is moved from areas of greater temperature to areas of reduced temperature, warm is soaked up (from inside the refrigerator) through the liquid Freon, leading to the temperature within the refrigerator to it is in reduced. The absorbed heat starts to rest the intermolecular attractions of the liquid Freon, enabling the endothermic vaporization procedure to occur. When all of the Freon transforms to gas, the cycle can start over. The cycle described above does no run continuously, but rather is regulated by athermostat. Once the temperature within the refrigerator rises over the set temperature,the thermostat beginning the compressor. When the refrigerator has been cooled listed below the settemperature, the compressor is turned off. This regulate mechanism permits the refrigeratorto conserve electricity by only running as lot as is crucial to save the refrigeratorat the wanted temperature.

Questions ~ above Refrigeration

exactly how would the efficiency of a frozen refrigerator be influenced if the food inside the frozen fridge is packed an extremely tightly and an extremely close to the interior coils, so that there is no air circulation to the internal coils? Briefly, define your reasoning. Ammonia (NH3) was one of the at an early stage refrigerants used before Freon. The is no much longer used in family members refrigerators, because of the toxicity of ammonia should there be a leak. The boiling point of NH3 is comparable to that of Freon. based on molecular structure only, which substance, ammonia or Freon, would you mean to have actually a larger enthalpy adjust of vaporization (ΔHvap)? Briefly, define your answer. based on your answer to part (a), i beg your pardon substance, ammonia or Freon, would you intend to be a better refrigerant? Briefly, explain your answer.


Refrigerators are essentially heat engines working in reverse. Whereas a heat engineconverts heat to work, reverse heat engines convert work come heat. In the refrigerator, theheat that is created is transferred to the exterior of the refrigerator. Come cool therefrigerator, a "working substance", or "coolant", such together Freon isrequired.The refrigerator functions by a bike of compressing and also expanding the Freon,combined v phase transitions between the gaseous and liquid phases that Freon. Job-related isdone top top the Freon by a compressor, and the Freon climate releases warmth to the air external ofthe frozen fridge (as it undergoes the exothermic condensation indigenous a gas come a liquid). Toregenerate the gas Freon for compression, the Freon passes through an internal coil,where it experience the endothermic vaporization indigenous the fluid phase come the gaseousphase. This endothermic process causes the Freon come absorb heat from the air within therefrigerator, cooling the refrigerator.

Additional Links:


Brown, Lemay, and Bursten. chathamtownfc.net: The main Science, 7th ed., p. 395-98.

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


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

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

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Revised January 2001.