Authors: Rachel Casiday and also Regina Frey department of moment-g.com, Washington college St. Louis, MO 63130

Key Concepts

warm Engines Reverse warm Engines (e.g.

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

Introduction: warm Engines and Refrigeration

Refrigeration has allowed for an excellent advances in our capability to keep food and also othersubstances safe for lengthy periods of time. In addition, the same modern technology that is usedto run refrigerators is also used in wait conditioners, permitting people to live and also workcomfortably also in unbearably warm weather. How does this an innovation work to create coolair once the external problems are very hot? as we shall see, refrigerators (and airconditioners) depend on the thermodynamic application well-known as the warmth engine, too asthe molecular properties that the substance included in the coils that the refrigerator.

One that the most vital practical applications of the values of thermodynamics isthe heat engine (Figure 1). In the warmth engine, warmth is absorbed from a "workingsubstance" in ~ high temperature and partially converted to work. (Heatengines are never ever 100% efficient, since the remaining warmth (i.e., the warmth thatis not converted to work) is released to the surroundings, which room at a lowertemperature.) The vapor engines offered to power early on trains and electric generators areheat engines in i m sorry water is the functioning substance.


Figure 1

In a heat engine, an input of warmth causes rise in the temperature that the working substance, enabling the functioning substance to execute work. In this schematic diagram, the functioning substance is water. In ~ high temperature, gaseous water (steam) pushes a piston, which causes a wheel come turn. This is the necessary mechanism through which steam-powered trains operate.

In a reverse warmth engine (Figure 2), the opposite impact occurs. Work-related is converted toheat, i beg your pardon is released.


Figure 2

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

In 1851, the Florida medical professional John Gorrie was granted the first U.S. Patent for arefrigeration machine, which uses a reverse warm engine (Figure 2) as the first step inits operation. Gorrie, convinced that the cure because that malaria was cold (because outbreakswere terminated in the winter), sought to develop a maker that can 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 resulted in the temperature the the waiting to increase (exchanging job-related forheat). To run this compressed air with pipes in a cold-water bathtub released the heatinto the water. The air was then permitted to increase again to atmospheric pressure, butbecause it had lost warmth to the water, the temperature that the air was reduced than beforeand could be provided to cool the room.

Modern refrigerators operate by the very same reverse-heat-engine principle. Whereasa warm engine converts warm (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 together it goes from higher tolower temperature. This adjust from gas to fluid is a step transition, and also the energyreleased top top this transition is largely dependent top top the intermolecular interactions ofthe substance. Hence, to recognize the refrigeration cycle used in modernrefrigerators, it is crucial to an initial discuss phase transitions.

Questions on warmth Engines and Refrigeration

In many homes and businesses, heat pumps space replacing typical heaters to heat buildings by using electrical energy to transfer heat to the inside of the building. Is the heat pump an instance of a warm engine or a reverse warm engine? Briefly, define your reasoning. Briefly, describe the procedure by i m sorry the warm pump transfers warm into a building. What was the "working substance" in Dr. Gorrie"s refrigerator?

Phases and Phase Transitions

Matter can exist in three various phases (physical states): solid, liquid, and also gas. Aphase is a form of issue that is uniform transparent in chemistry composition and also physicalproperties, and that have the right to be identified from various other phases with which it might be incontact by this definite properties and also composition. As presented in number 3, a substance inthe hard phase has actually a identify shape and rigidity; a substance in the liquid phase has nodefinite shape, however has a identify volume, and a substance in the gas phase has actually nodefinite form or volume, however has a shape and volume identified by the shape and size ofthe container.


Figure 3

This schematic diagram reflects the distinctions in physics 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 rigid configuration, offering the substance a definite shape and size. In a liquid, the particles space close together however may relocate with respect come one another, giving the substance a definite volume yet a liquid shape. In a gas, the particles may occupy the whole volume of the container, so that their shape and volume are both characterized by the container.

Molecular (Microscopic) View

One the the major differences in the 3 phases shown in figure 3 is the numberof intermolecular interactions they contain. The particles in a solid interact with every oftheir nearest neighbors (recall the conversation of bonding in solids native the tutorialentitled "Bands,Bonds, and also Doping: just how Do LED"s Wrok?"), the particles in a liquid connect withonly few of the nearby particles, and the particles in a gas ideally have no interactionwith one another. By breaking or developing intermolecular interactions, a substance canchange indigenous one step to another. Because that example, gas molecules condensation to form liquidsbecause that the existence of attractive intermolecular forces. The stronger the attractiveforces, the higher the stability of the liquid (which leader to a greater boiling pointtemperature). A change between the phases of matter is dubbed a phase transition. Thenames the the phase transitions in between solid, liquid, and gas are presented in number 4.


Figure 4

This diagram mirrors the surname of the step transitions in between solids, liquids, and also gases. The arrow to the appropriate of the diagram demonstrates the these three phases have different enthalpies: gas has the greatest enthalpy, liquid has an intermediary enthalpy, and also solid has the lowest enthalpy. Hence, each of the phase transitions displayed in this figure involves a adjust in the enthalpy the the substance.

Phase transitions room a form of chemistry reaction. Many of the chemistry reactionsstudied in Chem 151 and also 152 show off the break or developing of bonds in ~ molecules;phase transitions indicate the break or developing of intermolecular pressures (attractiveinteractions in between molecules). Hence, as with other chemistry reactions, that is necessaryto comment on the energy that is absorbed or offered off during the break or developing ofintermolecular interaction in a step transition.

Phase transitions involving the breaking of intermolecular attractions (i.e.,fusion (melting), vaporization, and also sublimation) require an entry of energy to overcomethe attractive forces between the corpuscle of the substance. Phase transitions involvingthe formation of intermolecular attractions (i.e., freezing, condensation, anddeposition) release energy as the particles embrace a lower-energy conformation. Thestrength that the intermolecular attractions between molecules, and also therefore the quantity ofenergy forced to get rid of these attractive forces (as well as the amount of energyreleased once the attractions are formed) counts on the molecular properties the thesubstance. Generally, the more polar a molecule is, the stronger the attractiveforces in between molecules are. Hence, an ext polar molecules frequently require moreenergy to get over the intermolecular attractions in one endothermic phase transition, andrelease more energy by forming intermolecular attractions throughout an exothermic phasetransition.

Thermodynamic (Macroscopic) View

In enhancement to the microscopic, molecular check out presented above, us can explain phasetransitions in regards to macroscopic, thermodynamic properties. It is necessary to bear inmind that the microscopic and macroscopic views space interdependent; i.e., thethermodynamic properties, such as enthalpy and also temperature, the a substance space dependenton the molecular behavior of the substance.

Phase transitions are accompanied by alters in enthalpy and also entropy. In this tutorial,we will concern ourselves largely with changes in enthaply. The energy readjust involved inbreaking or creating intermolecular attractions is primarily supplied or released in theform the heat. Adding heat causes intermolecular attractions to it is in broken.How walk this occur? heat is a move of power 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 moment-g.com Behind Airbags"), and thereby weakening theintermolecular forces holding the molecules in place. Likewise, molecules loseheat to form intermolecular attractions; when warm is lost, the molecules moveslower and also therefore can interact more with other surrounding molecules.

Because phase transforms generally take place at consistent pressure (i.e., in areaction vessel open to the atmosphere), the heat have the right to be explained by a adjust in enthalpy(ΔH=qp=nCp ΔT, wherein n is thenumber of mole of the substance and Cp is the molar warm capacity in ~ constantpressure). For phase transitions entailing the break of intermolecularattractions, heat is added and ΔH is positive, since thesystem is going native a lower-enthalpy step to a higher-enthalpy phase, together shownby the direction that the vertical arrowhead to the right of number 4. Hence, fusion,vaporization, and also sublimation space all endothermic step transitions. For phasetransitions including the creating of intermolecular attractions, heat is released and also ΔH is negative, because the device is going indigenous a higher-enthalpyphase to a lower-enthalpy phase, as displayed in figure 4. Hence, freezing,condensation, and deposition space all exothermic step transitions. The direction that theenthalpy readjust for every of the phase-transition processes called in number 4 is displayed inTable 1, below.

Phase shift Direction the ΔH

Fusion (Melting) (solid come liquid)

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

Table 1

This table reflects the authorize of the enthalpy change for each of the step transitions described above. Recall that endothermic processes have a optimistic enthalpy change, and exothermic processes have a an adverse enthalpy change.

As with other chemical reactions, due to the fact that enthalpy is a state function, ΔH because that phase transitions can be added or subtracted follow toHess"s law. (Recall native Chem 112 and also the introduction to the experiment that,according come Hess"s law, once chemical reaction are included or subtracted to accomplish a netreaction, the matching ΔH"s are added or subtracted toobtain the ΔH for the net reaction.)

The enthalpy adjust of phase transitions can likewise be used to describe differences inmelting points and boiling clues of substances. A offered substance has actually a characteristicrange of temperature at which that undergoes each of the phase transitions (at a givenpressure). This temperatures are called for the phase shift that occurs at thetemperature (e.g., melt point). In general, the higher the enthalpychange for a phase transition is (the an ext heat compelled for an endothermic transition, orreleased because that an exothermic transition), the greater the temperature is at which thesubstance undergoes the step transition. For example, liquids with strongintermolecular attractions require more heat come vaporize 보다 liquids v weakintermolecular attractions; therefore, the boiling point (vaporization point) because that theseliquids will be higher than for the liquids through weaker intermolecular attractions.

Questions ~ above Phases and also Phase Transitions

A student actions the melting points the two common household crystalline solids: salt chloride (NaCl) and sucrose (C12H22O11). She finds that the melting suggest of salt chloride is much higher than the melting suggest of sucrose. Briefly, define why the melting allude for NaCl is higher than because that C12H22O11, in regards to the type of attractive pressures in the solids and also your molecular knowledge of step transitions. once you place your finger into a glass that water immediately after including an ice cube, and also again five minutes later, you find that the water feeling cooler after few of the ice has begun to melt. Briefly, explain this phenomenon in regards to your thermodynamic understanding of phase changes.


Now, we shall usage our expertise of warm engines and also phase transitions to explainhow refrigerators work. The enthalpy changes linked with step transitions may be usedby a warm engine (Figure 1) to perform work and to transfer heat between (1) the substanceundergoing a phase change and (2) its surrounding environment. In a warm engine, a"working substance" absorbs warm at a high temperature and also converts part of thisheat to work. In a secondary process, the remainder of the warm is exit to the surroundingsat a lower temperature, since the warmth engine is not 100% efficient.

As shown in number 2, a refrigerator can be believed of together a warm engine in reverse. Thecooling result in a frozen fridge is achieved by a bicycle of condensation and also vaporizationof the nontoxic compound CCl2F2 (Freon-12). As displayed inFigure 5, the refrigerator consists of (1) one electrically-powered compressor the does workon Freon gas, and (2) a collection of coils that permit heat to it is in released external (on theback of) the frozen refrigerator or took in from within the refrigerator together Freon passes throughthese coils.


Figure 5

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

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


Figure 6

This diagram reflects the significant steps in the refrigeration cycle. Because that a summary of each action (indicated by the environment-friendly numbers), view the numbered steps below. In this figure, blue dots stand for Freon gas, and also solid blue areas represent liquid Freon. Small arrows show the direction the heat flow 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 presented in the figure over and defined below. Click the blue button below to download QuickTime 4.0 to see the movie.


external of the refrigerator, the electrically-run compressor does job-related on the Freon gas, raising the push of the gas. Together the push 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 exterior of the refrigerator. Heat (q) flows from the high-temperature gas come the lower-temperature wait of the room bordering the coil. This warmth loss causes the high-pressure gas come condense to liquid, as motion of the Freon molecules decreases and intermolecular attractions room formed. Hence, the occupational done on the gas through the compressor (causing one exothermic phase shift in the gas) is convert to heat offered off in the waiting in the room behind the refrigerator. If friend have ever felt the coils on the ago of the refrigerator, you have experienced the heat given off during the condensation the Freon. Next, the liquid Freon in the outside coil passes with an expansion valve right into a coil within the insulated compartment the the refrigerator. Now, the liquid is in ~ a low push (as a an outcome of the expansion) and also is lower in temperature (cooler) than the surrounding air (i.e., the air inside the refrigerator). Since heat is transferred from locations of greater temperature to locations of lower temperature, warm is soaked up (from inside the refrigerator) through the fluid Freon, leading to the temperature inside the frozen refrigerator to it is in reduced. The took in heat starts to rest the intermolecular attractions that the liquid Freon, enabling the endothermic vaporization process to occur. When every one of the Freon transforms to gas, the cycle have the right to start over. The bike described over does not run continuously, but rather is managed by athermostat. When the temperature within the frozen fridge rises over the collection temperature,the thermostat start the compressor. When the refrigerator has actually been cooled listed below the settemperature, the compressor is rotate off. This regulate mechanism enables the refrigeratorto conserve electrical energy by just running as lot as is essential to save the refrigeratorat the desired temperature.

Questions on Refrigeration

just how would the effectiveness of a refrigerator be impacted if the food inside the refrigerator is packed very tightly and very close come the interior coils, so that there is no air flow to the internal coils? Briefly, define your reasoning. Ammonia (NH3) was one of the early refrigerants used prior to Freon. It is no longer used in family members refrigerators, since of the toxicity of ammonia should there it is in a leak. The boiling point of NH3 is similar to that of Freon. based upon molecular structure only, i m sorry substance, ammonia or Freon, would certainly you mean to have a larger enthalpy adjust of vaporization (ΔHvap)? Briefly, define your answer. based upon your prize to component (a), i m sorry substance, ammonia or Freon, would certainly you mean to it is in a much better refrigerant? Briefly, explain your answer.


Refrigerators are basically heat engines working in reverse. Vice versa, a warm engineconverts warmth to work, reverse heat engines convert work to heat. In the refrigerator, theheat the is produced is moved 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 expanding the Freon,combined through phase transitions between the gaseous and liquid phases of Freon. Occupational isdone ~ above the Freon by a compressor, and also the Freon then releases warmth to the air exterior ofthe frozen fridge (as it experience the exothermic condensation native a gas come a liquid). Toregenerate the gaseous Freon because that compression, the Freon passes with an inner coil,where it experience the endothermic vaporization indigenous the fluid phase to the gaseousphase. This endothermic process causes the Freon come absorb warm from the air within therefrigerator, cooling the refrigerator.

Additional Links:


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

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


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

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

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Revised January 2001.