Authors: Rachel Casiday and Regina Frey department of lungemine.com, Washington university St. Louis, MO 63130

Key Concepts

heat Engines Reverse warmth Engines (e.g.

You are watching: Which change of phase is exothermic

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

Introduction: heat Engines and Refrigeration

Refrigeration has allowed for an excellent advances in our capacity to store food and also othersubstances safe for long periods of time. In addition, the same modern technology that is usedto operation refrigerators is also used in wait conditioners, enabling people to live and workcomfortably also in unbearably warm weather. Exactly how does this an innovation work to produce coolair as soon as the external problems are really hot? as we chandelier see, refrigerators (and airconditioners) rely on the thermodynamic application recognized as the heat engine, too asthe molecule properties the the substance included in the coils that the refrigerator.

One that the most essential practical applications of the principles of thermodynamics isthe heat engine (Figure 1). In the warm engine, heat is soaked up from a "workingsubstance" in ~ high temperature and partially converted to work. (Heatengines are never ever 100% efficient, since the remaining heat (i.e., the warm thatis not converted come work) is exit to the surroundings, which are at a lowertemperature.) The heavy steam engines offered to power early on trains and also 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 the the functioning substance, enabling the working substance to carry out work. In this schematic diagram, the working substance is water. At high temperature, gaseous water (steam) pushes a piston, which causes a wheel come turn. This is the essential mechanism whereby steam-powered trains operate.

In a reverse warmth engine (Figure 2), the opposite effect occurs. Work is converted toheat, which is released.


Figure 2

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

In 1851, the Florida doctor John Gorrie was granted the very first U.S. Patent for arefrigeration machine, which uses a reverse warm engine (Figure 2) together the an initial step inits operation. Gorrie, convinced that the cure for malaria to be cold (because outbreakswere terminated in the winter), seek to construct a an equipment that might make ice and cool apatient"s room in the hot Florida summer. In Dr. Gorrie"s refrigerator, air was compressedusing a pump, which led to the temperature of the waiting to boost (exchanging occupational forheat). To run this compressed air v pipes in a cold-water bath released the heatinto the water. The air was then enabled to expand again to atmospheric pressure, butbecause it had actually lost warm to the water, the temperature the the wait was lower than beforeand can be offered to cool the room.

Modern refrigerators run by the same reverse-heat-engine principle. Whereasa heat engine converts heat (from a high-temperature area) come work, a refrigeratorconverts work to heat. Modern refrigerators use substances various other than airas the coolant; the coolant substance changes from gas come liquid as it goes from higher tolower temperature. This readjust from gas to fluid is a step transition, and the energyreleased upon this shift is largely dependent on the intermolecular interaction ofthe substance. Hence, to recognize the refrigeration cycle offered in modernrefrigerators, the is important to very first discuss step transitions.

Questions on warm Engines and Refrigeration

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

Phases and Phase Transitions

Matter deserve to exist in three different phases (physical states): solid, liquid, and also gas. Aphase is a type of matter that is uniform transparent in chemical composition and physicalproperties, and that deserve to be differentiated from various other phases through which it might be incontact by these definite properties and composition. As shown in figure 3, a problem inthe solid phase has a definite shape and also rigidity; a substance in the liquid phase has nodefinite shape, however has a identify volume, and also a problem in the gas phase has nodefinite shape or volume, yet has a shape and volume determined by the shape and size ofthe container.


Figure 3

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

Molecular (Microscopic) View

One that the major differences in the three phases illustrated in figure 3 is the numberof intermolecular interactions 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 also Doping: exactly how Do LED"s Wrok?"), the particles in a liquid communicate withonly several of the adjacent particles, and the corpuscle in a gas ideally have actually no interactionwith one another. By breaking or forming intermolecular interactions, a problem canchange from one step to another. Because that example, gas molecules condense to kind liquidsbecause that the visibility of attractive intermolecular forces. The more powerful the attractiveforces, the better the stability of the liquid (which leads to a higher boiling pointtemperature). A change between the phases of matter is referred to as a phase transition. Thenames that the step transitions in between solid, liquid, and also gas are shown in number 4.


Figure 4

This diagram reflects the name of the phase transitions in between solids, liquids, and gases. The arrowhead to the best of the diagram demonstrates the these three phases have different enthalpies: gas has actually the greatest enthalpy, liquid has an intermediary enthalpy, and also solid has the lowest enthalpy. Hence, each of the phase transitions presented in this figure involves a adjust in the enthalpy of the substance.

Phase transitions room a type of chemical reaction. Most of the lungemine.com reactionsstudied in Chem 151 and also 152 show off the breaking or forming of bonds in ~ molecules;phase transitions involve the breaking or creating of intermolecular forces (attractiveinteractions in between molecules). Hence, similar to other chemical reactions, the is necessaryto discuss the power that is soaked up or offered off throughout the break or forming ofintermolecular interactions in a phase transition.

Phase transitions including the break of intermolecular attractions (i.e.,fusion (melting), vaporization, and sublimation) require an intake of energy to overcomethe attractive forces in between the corpuscle of the substance. Step transitions involvingthe formation of intermolecular attractions (i.e., freezing, condensation, anddeposition) release energy as the particles take on a lower-energy conformation. Thestrength that the intermolecular attractions between molecules, and therefore the amount ofenergy forced to get rid of these attractive forces (as well together the quantity of energyreleased when the attractions are formed) counts on the molecular properties that thesubstance. Generally, the more polar a molecule is, the more powerful the attractiveforces in between molecules are. Hence, more polar molecules commonly require moreenergy to conquer the intermolecular attractions in an endothermic phase transition, andrelease much more energy by forming intermolecular attractions throughout an exothermic phasetransition.

Thermodynamic (Macroscopic) View

In enhancement to the microscopic, molecular view presented above, us can describe phasetransitions in regards to macroscopic, thermodynamic properties. The is essential to be afflicted with inmind that the microscopic and macroscopic views are interdependent; i.e., thethermodynamic properties, such as enthalpy and temperature, of a substance are dependenton the molecular habits of the substance.

Phase transitions space accompanied by alters in enthalpy and also entropy. In this tutorial,we will worry ourselves largely with changes in enthaply. The energy change involved inbreaking or creating intermolecular attractions is primarily supplied or exit in theform the heat. Adding heat causes intermolecular attractions to be broken.How does this occur? heat is a carry of power to molecules, causing the molecules toincrease their motion as explained by the kinetic concept of gases (discussed in thetutorial entitled, "GasLaws conserve Lives: The lungemine.com Behind Airbags"), and thereby weakening theintermolecular forces holding the molecules in place. Likewise, molecules loseheat to type intermolecular attractions; when warm is lost, the molecule moveslower and also therefore deserve to interact an ext with other adjacent molecules.

Because phase transforms generally happen at constant pressure (i.e., in areaction vessel open to the atmosphere), the heat can be defined by a readjust in enthalpy(ΔH=qp=nCp ΔT, wherein n is thenumber of mole of the substance and Cp is the molar warmth capacity at constantpressure). For phase transitions entailing the breaking of intermolecularattractions, warmth is added and ΔH is positive, since thesystem is going indigenous a lower-enthalpy step to a higher-enthalpy phase, as shownby the direction of the vertical arrow to the appropriate of figure 4. Hence, fusion,vaporization, and sublimation space all endothermic step transitions. For phasetransitions involving the forming of intermolecular attractions, warmth is released and ΔH is negative, because the device is going indigenous a higher-enthalpyphase to a lower-enthalpy phase, as shown in figure 4. Hence, freezing,condensation, and deposition are all exothermic step transitions. The direction of theenthalpy change for every of the phase-transition processes called in figure 4 is displayed inTable 1, below.

Phase transition Direction of ΔH

Fusion (Melting) (solid to liquid)

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

Table 1

This table mirrors the authorize of the enthalpy change for each of the step transitions described above. Recall that endothermic processes have actually a hopeful enthalpy change, and also exothermic processes have actually a negative enthalpy change.

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

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

Questions top top Phases and Phase Transitions

A student actions the melt points of two usual household crystalline solids: sodium chloride (NaCl) and also sucrose (C12H22O11). She finds the the melting allude of sodium chloride is much greater than the melting allude of sucrose. Briefly, describe why the melting suggest for NaCl is higher than for C12H22O11, in terms of the type of attractive pressures in the solids and also your molecular understanding of phase transitions. as soon as you ar your finger right into a glass of water automatically after including an ice cube, and again five minutes later, you discover that the water feels cooler after some of the ice cream has begun to melt. Briefly, explain this phenomenon in regards to your thermodynamic knowledge of step changes.


Now, we shall usage our expertise of warm engines and phase transitions to explainhow refrigerators work. The enthalpy changes connected with step transitions might be usedby a heat engine (Figure 1) to do work and also to carry heat between (1) the substanceundergoing a phase change and (2) its bordering environment. In a warmth engine, a"working substance" absorbs warmth at a high temperature and also converts component of thisheat come work. In a secondary process, the remainder of the heat is exit to the surroundingsat a reduced temperature, because the warm engine is no 100% efficient.

As presented in figure 2, a refrigerator deserve to be assumed of as a heat engine in reverse. Thecooling impact in a frozen refrigerator is completed by a bicycle of condensation and also vaporizationof the nontoxic compound CCl2F2 (Freon-12). As displayed inFigure 5, the refrigerator includes (1) one electrically-powered compressor that does workon Freon gas, and (2) a series of coils that allow heat to it is in released exterior (on theback of) the frozen fridge or took in from inside the refrigerator as Freon passes throughthese coils.


Figure 5

This is a schematic diagram of the major functional materials of a refrigerator. The major features incorporate a compressor comprise Freon (CCl2F2) gas, an outside heat-exchange coil (on the outside back 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 refrigerator (blue) in i m sorry the Freon is vaporized, absorbing heat from within the frozen refrigerator (and hence lowering the temperature).

Figure 6 (below) traces the step transitions of Freon and also their associatedheat-exchange occasions that occur during the refrigeration cycle. The measures of therefrigeration cycle space described below the figure. (The number in the number correspondto the numbered actions below.)


Figure 6

This diagram reflects the significant steps in the refrigeration cycle. For a summary of each action (indicated through the eco-friendly numbers), view the numbered measures below. In this figure, blue dots stand for Freon gas, and also solid blue locations represent fluid Freon. Little arrows suggest the direction that heat flow into or the end of the refrigerator coils.

Please click the pink button listed below to watch a QuickTime movie mirroring an computer 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 see the movie.


outside of the refrigerator, the electrically-run compressor does work on the Freon gas, raising the pressure of the gas. As the press of the gas increases, so does its temperature (as suspect by the ideal-gas law). Next, this high-pressure, high-temperature gas beginning the coil top top the external of the refrigerator. Heat (q) flows from the high-temperature gas to the lower-temperature wait of the room bordering the coil. This warmth loss reasons the high-pressure gas come condense to liquid, as motion of the Freon molecule decreases and intermolecular attractions are formed. Hence, the occupational done on the gas by the compressor (causing an exothermic phase transition in the gas) is converted to heat offered off in the wait in the room behind the refrigerator. If girlfriend have ever before felt the coils ~ above the earlier of the refrigerator, you have actually experienced the heat offered off throughout the condensation of Freon. Next, the liquid Freon in the exterior coil passes with an development valve right into a coil within the insulated compartment that the refrigerator. Now, the fluid is at a low push (as a an outcome of the expansion) and also is lower in temperature (cooler) than the bordering air (i.e., the air inside the refrigerator). Since warm is moved from locations of higher temperature to locations of reduced temperature, warm is took in (from within the refrigerator) by the liquid Freon, bring about the temperature inside the refrigerator to it is in reduced. The soaked up heat starts to break the intermolecular attractions the the fluid Freon, enabling the endothermic vaporization procedure to occur. When every one of the Freon changes to gas, the cycle deserve to start over. The cycle described above does not run continuously, however rather is regulated by athermostat. As soon as the temperature within the frozen fridge rises over the set temperature,the thermostat beginning the compressor. Once the refrigerator has actually been cooled below the settemperature, the compressor is turn off. This manage mechanism enables the refrigeratorto conserve electrical power by just running as much as is crucial to keep the refrigeratorat the desired temperature.

Questions top top Refrigeration

how would the performance of a frozen refrigerator be impacted if the food inside the frozen fridge is packed an extremely tightly and really close to the interior coils, so that there is no air circulation to the inner coils? Briefly, define your reasoning. Ammonia (NH3) was among the at an early stage refrigerants used before Freon. It is no longer used in family refrigerators, since of the toxicity of ammonia should there be a leak. The boiling suggest of NH3 is similar to that of Freon. based on molecular framework only, which substance, ammonia or Freon, would you expect to have a bigger enthalpy adjust of vaporization (ΔHvap)? Briefly, define your answer. based on your price to component (a), which substance, ammonia or Freon, would certainly you intend to be a better refrigerant? Briefly, explain your answer.


Refrigerators are essentially heat engines functioning in reverse. Whereas a warmth engineconverts warm to work, reverse heat engines transform work come heat. In the refrigerator, theheat that is created is transferred to the exterior of the refrigerator. To cool therefrigerator, a "working substance", or "coolant", such as Freon isrequired.The refrigerator works by a bike of compressing and also expanding the Freon,combined through phase transitions between the gaseous and liquid phases of Freon. Occupational isdone ~ above the Freon by a compressor, and the Freon climate releases heat to the air outside ofthe frozen fridge (as it undergoes the exothermic condensation from a gas to a liquid). Toregenerate the gas Freon because that compression, the Freon passes v an inner coil,where it undergoes the endothermic vaporization native the fluid phase to the gaseousphase. This endothermic procedure causes the Freon come absorb warmth from the air within therefrigerator, cooling the refrigerator.

Additional Links:


Brown, Lemay, and also Bursten. lungemine.com: The main Science, 7th ed., p. 395-98.

Petrucci and also Harwood. General lungemine.com, 7th ed., p. 435, 699-701, 714-15.


The authors thank Dewey Holten, Michelle Gilbertson, Jody Proctor and CarolynHerman for countless helpfulsuggestions in the composing of this tutorial.

The advancement of this tutorial was sustained by a give from the Howard HughesMedical Institute, with the Undergraduate biological Sciences education program, GrantHHMI# 71199-502008 to Washington University.

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Revised January 2001.