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

Key Concepts

warmth Engines Reverse warmth Engines (e.g.

You are watching: Which of the following phase changes is endothermic?

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

Introduction: warmth Engines and Refrigeration

Refrigeration has enabled for great advances in our capacity to keep food and also othersubstances for sure for lengthy periods of time. In addition, the same an innovation that is usedto operation refrigerators is additionally used in wait conditioners, allowing people come live and also workcomfortably also in unbearably hot weather. How does this an innovation work to create coolair once the external problems are an extremely hot? together we chandelier see, refrigerators (and airconditioners) count on the thermodynamic application well-known as the warmth engine, also asthe molecular properties that the substance included in the coils that the refrigerator.

One of the most important practical applications the the ethics of thermodynamics isthe warm engine (Figure 1). In the warm engine, warm is soaked up from a "workingsubstance" in ~ high temperature and also partially converted to work. (Heatengines are never 100% efficient, because the remaining warm (i.e., the warm thatis no converted to work) is exit to the surroundings, which space at a lowertemperature.) The vapor engines used to power beforehand trains and also electric generators areheat engines in i m sorry water is the functioning substance.


Figure 1

In a warm engine, an input of warm causes rise in the temperature the the functioning substance, enabling the working substance to do work. In this schematic diagram, the functioning substance is water. In ~ high temperature, gas water (steam) pushes a piston, which causes a wheel to turn. This is the necessary mechanism by which steam-powered trains operate.

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


Figure 2

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

In 1851, the Florida medical professional John Gorrie was granted the an initial U.S. Patent for arefrigeration machine, which supplies a reverse warm engine (Figure 2) together the first step inits operation. Gorrie, encouraged that the cure because that malaria to be cold (because outbreakswere terminated in the winter), search to construct a maker that might make ice and cool apatient"s room in the warm Florida summer. In Dr. Gorrie"s refrigerator, air to be compressedusing a pump, which caused the temperature that the waiting to increase (exchanging work forheat). Running this compressed air through pipes in a cold-water bath released the heatinto the water. The air was then permitted to increase again to atmospheric pressure, butbecause it had lost heat to the water, the temperature the the air was lower than beforeand can be used to cool the room.

Modern refrigerators operate by the same reverse-heat-engine principle. Whereasa warm engine converts warm (from a high-temperature area) to work, a refrigeratorconverts occupational to heat. Modern refrigerators usage substances various other than airas the coolant; the coolant substance changes from gas to liquid as it go from greater tolower temperature. This change from gas to liquid is a phase transition, and also the energyreleased upon this change is largely dependent top top the intermolecular interaction ofthe substance. Hence, to understand the refrigeration cycle provided in modernrefrigerators, the is necessary to very first discuss phase transitions.

Questions on warmth Engines and Refrigeration

In many homes and also businesses, warmth pumps room replacing standard heaters to heat buildings by using power to transfer heat to the inside of the building. Is the warmth pump an example of a warm engine or a reverse warmth engine? Briefly, explain your reasoning. Briefly, define the procedure by which the warm pump transfers heat into a building. What was the "working substance" in Dr. Gorrie"s refrigerator?

Phases and also Phase Transitions

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


Figure 3

This schematic diagram reflects the differences in physical properties and also particle arrangement between a substance in the solid, liquid, and gas phases. In a solid, the particles space densely packed in a strict configuration, giving the problem a identify shape and size. In a liquid, the particles room close together yet may move with respect to one another, providing the problem a identify volume but a fluid shape. In a gas, the particles might occupy the entire volume of the container, so that their shape and also volume room both defined by the container.

Molecular (Microscopic) View

One of the significant differences in the 3 phases illustrated in number 3 is the numberof intermolecular interaction they contain. The corpuscle in a solid communicate with every oftheir nearest neighbors (recall the conversation of bonding in solids from the tutorialentitled "Bands,Bonds, and Doping: exactly how Do LED"s Wrok?"), the corpuscle in a liquid interact withonly some of the nearby particles, and the particles in a gas ideally have no interactionwith one another. By breaking or forming intermolecular interactions, a problem canchange native one step to another. For example, gas molecules condense to type liquidsbecause of the visibility of attractive intermolecular forces. The more powerful the attractiveforces, the better the security of the liquid (which leader to a higher boiling pointtemperature). A change between the phases of issue is referred to as a phase transition. Thenames of the step transitions in between solid, liquid, and gas are shown in number 4.


Figure 4

This diagram shows the surname of the step transitions in between solids, liquids, and also gases. The arrow to the ideal of the diagram demonstrates the these three phases have various enthalpies: gas has the highest enthalpy, liquid has actually an intermediate enthalpy, and also solid has the lowest enthalpy. Hence, each of the phase transitions shown in this figure requires a adjust in the enthalpy of the substance.

Phase transitions space a kind of chemical reaction. Most of the chemical reactionsstudied in Chem 151 and also 152 indicate the breaking or developing of bonds within molecules;phase transitions involve the break or developing of intermolecular forces (attractiveinteractions in between molecules). Hence, just like other altoalsimce.org reactions, that is necessaryto discuss the power that is took in or given off during the break or developing ofintermolecular interactions in a step transition.

Phase transitions entailing the break of intermolecular attractions (i.e.,fusion (melting), vaporization, and also sublimation) call for an entry of power to overcomethe attractive forces 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 the the intermolecular attractions between molecules, and also therefore the lot ofenergy required to get rid of these attractive pressures (as well as the quantity of energyreleased once the attractions room formed) relies on the molecule properties that thesubstance. Generally, the more polar a molecule is, the stronger the attractiveforces in between molecules are. Hence, more polar molecules frequently require moreenergy to get over the intermolecular attractions in an endothermic step transition, andrelease much more energy by creating intermolecular attractions during an exothermic phasetransition.

Thermodynamic (Macroscopic) View

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

Phase transitions are accompanied by transforms in enthalpy and also entropy. In this tutorial,we will problem ourselves mostly with changes in enthaply. The energy change involved inbreaking or forming intermolecular attractions is generally supplied or exit in theform the heat. Adding heat reasons intermolecular attractions to it is in broken.How walk this occur? heat is a carry of energy to molecules, leading to the molecules toincrease their movement as defined by the kinetic theory of gases (discussed in thetutorial entitled, "GasLaws conserve Lives: The altoalsimce.org Behind Airbags"), and also thereby weakening theintermolecular pressures holding the molecules in place. Likewise, molecule loseheat to type intermolecular attractions; when heat is lost, the molecule moveslower and also therefore deserve to interact more with other surrounding molecules.

Because phase changes generally occur at consistent pressure (i.e., in areaction vessel open up to the atmosphere), the heat deserve to be defined by a readjust in enthalpy(ΔH=qp=nCp ΔT, whereby n is thenumber of mole of the substance and Cp is the molar warmth capacity at constantpressure). For phase transitions involving the break of intermolecularattractions, warm 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 of the vertical arrow to the best of figure 4. Hence, fusion,vaporization, and sublimation are all endothermic phase transitions. For phasetransitions involving the developing of intermolecular attractions, warm is released and ΔH is negative, since the system is going indigenous a higher-enthalpyphase to a lower-enthalpy phase, as displayed in figure 4. Hence, freezing,condensation, and also deposition space all exothermic phase transitions. The direction of theenthalpy adjust for each of the phase-transition processes called in number 4 is shown inTable 1, below.

Phase change Direction the ΔH

Fusion (Melting) (solid to liquid)

ΔH>0; enthalpy rises (endothermic process)
Vaporization (liquid to gas) ΔH>0; enthalpy boosts (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 decreases (exothermic process)
Deposition (gas to solid) ΔH0; enthalpy decreases (exothermic process)

Table 1

This table shows the sign of the enthalpy change for each of the step transitions explained above. Recall that endothermic processes have actually a confident enthalpy change, and exothermic processes have a an unfavorable enthalpy change.

As with other chemical reactions, due to the fact that enthalpy is a state function, ΔH because that phase transitions have the right to be included or subtracted according toHess"s law. (Recall indigenous Chem 112 and also the arrival to the experiment that,according to Hess"s law, once chemical reaction are included or subtracted to accomplish a netreaction, the corresponding ΔH"s are included or subtracted toobtain the ΔH for the net reaction.)

The enthalpy change of step transitions can additionally be provided to explain differences inmelting points and also boiling point out of substances. A given substance has actually a characteristicrange of temperatures at which the undergoes each of the step transitions (at a givenpressure). This temperatures are named for the phase transition that wake up at thetemperature (e.g., melt point). In general, the higher the enthalpychange for a phase change is (the much more heat required for one endothermic transition, orreleased for an exothermic transition), the better the temperature is at which thesubstance undergoes the phase transition. Because that example, liquids with strongintermolecular attractions require more heat come vaporize than liquids v weakintermolecular attractions; therefore, the boiling suggest (vaporization point) for theseliquids will be greater than because that the liquids v weaker intermolecular attractions.

Questions ~ above Phases and also Phase Transitions

A student steps the melting points that two typical household crystalline solids: sodium chloride (NaCl) and sucrose (C12H22O11). She finds that the melting suggest of sodium chloride is much greater than the melting point of sucrose. Briefly, define why the melting allude for NaCl is greater than for C12H22O11, in regards to the form of attractive pressures in the solids and also your molecular expertise of phase transitions. once you ar your finger into a glass that water instantly after including an ice cube, and also again five minutes later, you find that the water feeling cooler after several of the ice has begun to melt. Briefly, define this phenomenon in terms of your thermodynamic expertise of step changes.


Now, we shall use our expertise of warmth engines and also phase transitions come explainhow refrigerators work. The enthalpy changes linked with phase transitions may be usedby a warmth engine (Figure 1) to carry out work and also to transfer heat in between (1) the substanceundergoing a phase shift and (2) its bordering environment. In a heat engine, a"working substance" absorbs heat at a high temperature and converts component of thisheat come work. In a second process, the rest of the warm is released to the surroundingsat a lower temperature, due to the fact that the heat engine is no 100% efficient.

As displayed in number 2, a refrigerator deserve to be thought of as a warm engine in reverse. Thecooling effect in a frozen fridge is achieved by a bicycle of condensation and vaporizationof the nontoxic link CCl2F2 (Freon-12). As shown inFigure 5, the refrigerator contains (1) one electrically-powered compressor the does workon Freon gas, and also (2) a series of coils that enable heat to it is in released outside (on theback of) the frozen refrigerator or absorbed from inside the refrigerator together Freon passes throughthese coils.


Figure 5

This is a schematic diagram of the significant functional materials of a refrigerator. The significant features encompass a compressor comprise Freon (CCl2F2) gas, an outside heat-exchange coil (on the outside ago of the refrigerator) in which the Freon passes and condenses, an development valve, and also a heat-exchange coil inside the insulated compartment that the frozen refrigerator (blue) in which the Freon is vaporized, soaking up heat from within the frozen fridge (and thus lowering the temperature).

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


Figure 6

This diagram reflects the major steps in the refrigeration cycle. For a description of each step (indicated through the green numbers), watch the numbered steps below. In this figure, blue dots represent Freon gas, and also solid blue locations represent liquid Freon. Little arrows indicate the direction that heat flow into or the end of the frozen fridge coils.

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


external of the refrigerator, the electrically-run compressor does work on the Freon gas, boosting the push of the gas. Together the pressure of the gas increases, for this reason does the temperature (as guess by the ideal-gas law). Next, this high-pressure, high-temperature gas beginning the coil top top the outside of the refrigerator. Heat (q) operation from the high-temperature gas come the lower-temperature waiting of the room bordering the coil. This warm loss causes the high-pressure gas to condense come liquid, as activity of the Freon molecules decreases and also intermolecular attractions room formed. Hence, the work-related done ~ above the gas by the compressor (causing one exothermic phase change in the gas) is converted to heat provided off in the air in the room behind the refrigerator. If you have ever before felt the coils top top the earlier of the refrigerator, you have actually experienced the heat offered off throughout the condensation of Freon. Next, the liquid Freon in the outside coil passes v an expansion valve into a coil inside the insulated compartment the the refrigerator. Now, the fluid is at a low press (as a an outcome of the expansion) and is lower in temperature (cooler) 보다 the neighboring air (i.e., the air within the refrigerator). Since heat is transferred from locations of greater temperature to locations of lower temperature, warm is absorbed (from inside the refrigerator) through the fluid Freon, resulting in the temperature within the refrigerator to it is in reduced. The soaked up heat begins to break the intermolecular attractions the the fluid Freon, allowing the endothermic vaporization procedure to occur. When all of the Freon alters to gas, the cycle deserve to start over. The cycle described above does no run continuously, however rather is regulated by athermostat. Once the temperature within the frozen fridge rises above the collection temperature,the thermostat starts the compressor. As soon as the refrigerator has actually been cooled below the settemperature, the compressor is turn off. This control mechanism allows the refrigeratorto conserve power by only running as lot as is important to keep the refrigeratorat the wanted temperature.

Questions top top Refrigeration

exactly how would the efficiency of a refrigerator be influenced if the food within the frozen fridge is packed really tightly and very close come the interior coils, so the there is no air flow to the internal coils? Briefly, define your reasoning. Ammonia (NH3) was one of the early refrigerants used before Freon. That is no longer used in family refrigerators, due to the fact that of the toxicity of ammonia must there be a leak. The boiling suggest of NH3 is similar to that of Freon. based upon molecular framework only, i m sorry substance, ammonia or Freon, would you expect to have a bigger enthalpy adjust of vaporization (ΔHvap)? Briefly, define your answer. based upon your prize to component (a), which substance, ammonia or Freon, would certainly you intend to be a far better refrigerant? Briefly, explain your answer.


Refrigerators are basically heat engines functioning in reverse. Whereas a heat engineconverts warmth to work, reverse warmth engines transform work to heat. In the refrigerator, theheat the is generated is moved to the outside 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 also liquid phases the Freon. Work-related isdone top top the Freon by a compressor, and also the Freon then releases warm to the air external ofthe frozen refrigerator (as it undergoes 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 experience the endothermic vaporization native the liquid phase to the gaseousphase. This endothermic procedure causes the Freon to absorb warmth from the air within therefrigerator, cooling the refrigerator.

Additional Links:


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

Petrucci and also Harwood. General altoalsimce.org, 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 approve from the Howard HughesMedical Institute, through the Undergraduate organic Sciences education and learning program, GrantHHMI# 71199-502008 come Washington University.

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Revised January 2001.