Authors: Rachel Casiday and also Regina Frey Department of y2kcenter.org, Washington University St. Louis, MO 63130

Key Concepts

Heat Engines Reverse Heat Engines (e.g.

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, Refrigerators) Phases of Matter Solid Liquid Gas Phase Transitions Fusion/ Freezing Vaporization/ Condensation Sublimation/ Deposition Breaking or Formation of Intermolecular Attractions in Phase Transitions Change in Enthalpy (ΔH) of Phase Transitions Refrigeration Cycle (Note: This section consists of an computer animation.)

Introduction: Heat Engines and also Refrigeration

Refrigeration has actually enabled for good developments in our capability to store food and also othersubstances safely for long periods of time. In addition, the very same technology that is usedto run refrigerators is additionally used in air conditioners, enabling people to live and also workcomfortably also in unbearably hot weather. How does this innovation occupational to produce coolair once the external conditions are extremely hot? As we shall see, refrigerators (and airconditioners) rely on the thermodynamic application known as the warmth engine, as well asthe molecular properties of the substance consisted of in the coils of the refrigerator.

One of the many crucial valuable applications of the principles of thermodynamics isthe heat engine (Figure 1). In the heat engine, heat is took in from a "workingsubstance" at high temperature and partially converted to job-related. (Heatengines are never 100% efficient, because the staying warmth (i.e., the warmth thatis not converted to work) is released to the surroundings, which are at a lowertemperature.) The heavy steam engines provided to power at an early stage trains and electric generators arewarmth engines in which water is the functioning substance.


Figure 1

In a warm engine, an input of heat causes a rise in the temperature of the working substance, enabling the functioning substance to perdevelop work-related. In this schematic diagram, the functioning substance is water. At high temperature, gaseous water (steam) pushes a piston, which causes a wheel to revolve. This is the necessary mechanism through which steam-powered trains operate.

In a reverse heat engine (Figure 2), the oppowebsite effect occurs. Work is converted towarm, which is released.


Figure 2

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

In 1851, the Florida medical professional John Gorrie was granted the initially U.S. Patent for arefrigeration machine, which provides a reverse heat engine (Figure 2) as the first action inits procedure. Gorrie, convinced that the cure for malaria was cold (because outbreakswere terminated in the winter), sought to build a machine that can make ice and also cool apatient"s room in the warm Florida summer. In Dr. Gorrie"s refrigerator, air was compressedmaking use of a pump, which caused the temperature of the air to increase (exaltering work-related forheat). Running this compressed air with pipes in a cold-water bath released the heatinto the water. The air was then allowed to expand also aobtain to atmospheric pressure, butbecause it had actually shed heat to the water, the temperature of the air was lower than beforeand can be provided to cool the room.

Modern refrigerators operate by the exact same reverse-heat-engine principle. Whereasa heat engine converts warm (from a high-temperature area) to work, a refrigeratorconverts work to warmth. Modern refrigerators use substances various other than airas the coolant; the coolant substance alters from gregarding liquid as it goes from higher toreduced temperature. This adjust from gregarding liquid is a phase change, and the energyreleased upon this shift is mostly dependent on the intermolecular interactions ofthe substance. Hence, to understand the refrigeration cycle supplied in modernrefrigerators, it is vital to first comment on phase transitions.

Questions on Heat Engines and also Refrigeration

In many dwellings and also businesses, warmth pumps are replacing standard heaters to warmth structures by making use of electrical power to transport warmth to the inside of the structure. Is the warmth pump an instance of a warm engine or a reverse heat engine? Briefly, define your reasoning. Briefly, explain the process by which the warm pump transfers heat right into a structure. What was the "functioning substance" in Dr. Gorrie"s refrigerator?

Phases and Phase Transitions

Matter deserve to exist in 3 different phases (physical states): solid, liquid, and gas. Aphase is a type of matter that is uniform throughout in chemical composition and also physicalproperties, and that deserve to be distinguiburned from other phases through which it may be incontact by these definite properties and composition. As displayed in Figure 3, a substance inthe solid phase has actually a definite shape and also rigidity; a substance in the liquid phase has actually nodefinite shape, however has a definite volume, and a substance in the gas phase has actually nodefinite form or volume, but has a shape and also volume established by the form and size ofthe container.


Figure 3

This schematic diagram shows the distinctions in physical properties and pshort article arrangement between a substance in the solid, liquid, and gas phases. In a solid, the particles are densely packed in a rigid configuration, giving the substance a definite shape and also size. In a liquid, the pshort articles are cshed together yet may move with respect to one one more, giving the substance a definite volume but a fluid form. In a gas, the pposts may occupy the whole volume of the container, so that their form and also volume are both characterized by the container.

Molecular (Microscopic) View

One of the major distinctions in the 3 phases portrayed in Figure 3 is the numberof intermolecular interactions they contain. The pposts in a solid interact through all oftheir nearemainder next-door neighbors (recontact the discussion of bonding in solids from the tutorialentitled "Bands,Bonds, and Doping: How Do LED"s Wrok?"), the pshort articles in a liquid communicate withonly some of the surrounding pposts, and the particles in a gas ideally have no interactionvia one an additional. By breaking or creating intermolecular interactions, a substance canadjust from one phase to one more. For instance, gas molecules conthick to create liquidsbereason of the visibility of attractive intermolecular forces. The more powerful the attractivepressures, the better the stcapacity of the liquid (which leads to a higher boiling pointtemperature). A change between the phases of issue is dubbed a phase change. Thenames of the phase transitions in between solid, liquid, and gas are presented in Figure 4.


Figure 4

This diagram reflects the names of the phase transitions in between solids, liquids, and gases. The arrow to the ideal of the diagram demonstprices that these three phases have actually various enthalpies: gas has actually the highest possible enthalpy, liquid has an intermediate enthalpy, and solid has actually the lowest enthalpy. Hence, each of the phase transitions presented in this figure entails a adjust in the enthalpy of the substance.

Phase transitions are a type of chemical reactivity. Most of the chemical reactionsstudied in Chem 151 and 152 involve the breaking or developing of bonds within molecules;phase transitions involve the breaking or forming of intermolecular forces (attractiveinteractions between molecules). Hence, as via other chemical reactions, it is necessaryto discuss the power that is absorbed or provided off during the breaking or developing ofintermolecular interactions in a phase transition.

Phase transitions involving the breaking of intermolecular attractions (i.e.,fusion (melting), vaporization, and also sublimation) need an input of power to overcomethe attractive pressures between the pshort articles of the substance. Phase transitions involvingthe formation of intermolecular attractions (i.e., freezing, condensation, anddeposition) release energy as the pwrite-ups embrace a lower-energy condevelopment. Thetoughness of the intermolecular attractions between molecules, and also therefore the amount ofpower compelled to get rid of these attractive pressures (and also the amount of energyreleased when the attractions are formed) depends on the molecular properties of thesubstance. Usually, the even more polar a molecule is, the more powerful the attractiveforces in between molecules are. Hence, even more polar molecules generally call for morepower to overcome the intermolecular attractions in an endothermic phase change, andrelease more energy by forming intermolecular attractions in the time of an exothermic phaseshift.

Thermodynamic (Macroscopic) View

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

Phase transitions are accompanied by alters in enthalpy and also entropy. In this tutorial,we will certainly problem ourselves mainly via changes in enthaply. The energy readjust involved inbreaking or creating intermolecular attractions is mainly provided or released in theform of heat. Adding heat reasons intermolecular attractions to be broken.How does this occur? Heat is a transport of power to molecules, resulting in the molecules torise their motion as explained by the kinetic concept of gases (debated in thetutorial entitled, "GasLaws Save Lives: The y2kcenter.org Behind Airbags"), and also thereby weakening theintermolecular pressures holding the molecules in area. Likewise, molecules losewarmth to form intermolecular attractions; as soon as heat is lost, the molecules moveslower and also therefore deserve to interact even more with other surrounding molecules.

Because phase alters primarily take place at constant press (i.e., in areactivity vessel open to the atmosphere), the warmth have the right to be defined by a adjust in enthalpy(ΔH=qp=nCp ΔT, wright here n is thenumber of moles of the substance and Cp is the molar warmth capacity at constantpressure). For phase transitions including the breaking of intermolecularattractions, warm is included and also ΔH is positive, bereason themechanism is going from a lower-enthalpy phase to a higher-enthalpy phase, as shownby the direction of the vertical arrow to the appropriate of Figure 4. Hence, fusion,vaporization, and also sublimation are all endothermic phase transitions. For phasetransitions including the forming of intermolecular attractions, warm is released and ΔH is negative, bereason the mechanism is going from a higher-enthalpyphase to a lower-enthalpy phase, as presented in Figure 4. Hence, freezing,condensation, and deplace are all exothermic phase transitions. The direction of theenthalpy readjust for each of the phase-change processes named in Figure 4 is presented 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 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)
Deplace (gas to solid) ΔH0; enthalpy decreases (exothermic process)

Table 1

This table mirrors the authorize of the enthalpy readjust for each of the phase transitions explained above. Recontact that endothermic processes have a positive enthalpy adjust, and exothermic procedures have actually an unfavorable enthalpy readjust.

Just like other chemical reactions, bereason enthalpy is a state attribute, ΔH for phase transitions deserve to be added or subtracted according toHess"s law. (Recontact from Chem 112 and also the Review to the experiment that,according to Hess"s law, as soon as chemical reactions are added or subtracted to accomplish a netreactivity, the matching ΔH"s are included or subtracted toattain the ΔH for that net reaction.)

The enthalpy adjust of phase transitions can also be used to define differences inmelting points and also boiling points of substances. A provided substance has a characteristicarray of temperatures at which it undergoes each of the phase transitions (at a givenpressure). These temperatures are called for the phase shift that occurs at thetemperature (e.g., melting point). In general, the higher the enthalpychange for a phase transition is (the even more warmth required for an endothermic change, orreleased for an exothermic transition), the higher the temperature is at which thesubstance undergoes the phase transition. For instance, liquids through strongintermolecular attractions call for even more warmth to vaporize than liquids through weakintermolecular attractions; therefore, the boiling point (vaporization point) for theseliquids will certainly be better than for the liquids with weaker intermolecular attractions.

Questions on Phases and also Phase Transitions

A student procedures the melting points of two widespread family members crystalline solids: sodium chloride (NaCl) and sucrose (C12H22O11). She finds that the melting allude of sodium chloride is much higher than the melting suggest of sucrose. Briefly, describe why the melting suggest for NaCl is greater than for C12H22O11, in regards to the type of attrenergetic forces in the solids and your molecular expertise of phase transitions. When you area your finger into a glass of water automatically after including an ice cube, and also aacquire 5 minutes later on, you discover that the water feels cooler after some of the ice has started to melt. Briefly, define this phenomenon in regards to your thermodynamic understanding of phase transforms.


Now, we shall use our understanding of heat engines and also phase transitions to explainexactly how refrigerators occupational. The enthalpy alters linked through phase transitions may be usedby a heat engine (Figure 1) to execute work-related and to deliver heat between (1) the substanceundergoing a phase transition and also (2) its neighboring setting. In a heat engine, a"working substance" absorbs warm at a high temperature and also converts component of thisheat to job-related. In a secondary procedure, the remainder of the warm is released to the surroundingsat a reduced temperature, because the warmth engine is not 100% efficient.

As displayed in Figure 2, a refrigerator have the right to be assumed of as a warm engine in reverse. Thecooling result in a refrigerator is completed by a cycle of condensation and vaporizationof the nontoxic compound CCl2F2 (Freon-12). As displayed inFigure 5, the refrigerator has (1) an electrically-powered compressor that does workon Freon gas, and also (2) a collection of coils that enable warmth to be released outside (on theearlier of) the refrigerator or absorbed from inside the refrigerator as Freon passes throughthese coils.


Figure 5

This is a schematic diagram of the major functional components of a refrigerator. The significant features include a compressor containing Freon (CCl2F2) gas, an external heat-exadjust coil (on the external earlier of the refrigerator) in which the Freon passes and condenses, an development valve, and also a heat-exreadjust coil inside the insulated compartment of the refrigerator (blue) in which the Freon is vaporized, absorbing warmth from inside the refrigerator (and for this reason lowering its temperature).

Figure 6 (below) traces the phase transitions of Freon and also their associatedheat-exreadjust events that take place throughout the refrigeration cycle. The procedures of therefrigeration cycle are described listed below the figure. (The numbers in the number correspondto the numbered procedures listed below.)


Figure 6

This diagram reflects the major measures in the refrigeration cycle. For a summary of each step (shown by the green numbers), check out the numbered procedures listed below. In this figure, blue dots represent Freon gas, and also solid blue locations reexisting liquid Freon. Small arrows indicate the direction of warm flow right into or out of the refrigerator coils.

Please click on the pink button listed below to view a QuickTime movie showing an animation of the refrigeration cycle displayed in the number above and also described listed below. Click the blue switch below to download QuickTime 4.0 to view the movie.


Beyond the refrigerator, the electrically-run compressor does work-related on the Freon gas, enhancing the pressure of the gas. As the push of the gas boosts, so does its temperature (as predicted by the ideal-gas law). Next off, this high-press, high-temperature gas enters the coil on the external of the refrigerator. Heat (q) flows from the high-temperature gregarding the lower-temperature air of the room surrounding the coil. This warmth loss reasons the high-push gregarding condense to liquid, as activity of the Freon molecules decreases and also intermolecular attractions are developed. Hence, the occupational done on the gas by the compressor (causing an exothermic phase shift in the gas) is converted to heat given off in the air in the room behind the refrigerator. If you have actually ever before felt the coils on the ago of the refrigerator, you have actually skilled the warm provided off throughout the condensation of Freon. Next off, the liquid Freon in the outside coil passes through an expansion valve right into a coil inside the insulated compartment of the refrigerator. Now, the liquid is at a low press (as a result of the expansion) and is reduced in temperature (cooler) than the bordering air (i.e., the air inside the refrigerator). Due to the fact that heat is moved from areas of better temperature to locations of reduced temperature, warmth is took in (from inside the refrigerator) by the liquid Freon, causing the temperature inside the refrigerator to be lessened. The took in heat starts to break the intermolecular attractions of the liquid Freon, permitting the endothermic vaporization process to occur. When every one of the Freon changes to gas, the cycle have the right to start over. The cycle described over does not run consistently, yet quite is controlled by athermostat. When the temperature inside the refrigerator rises over the set temperature,the thermostat starts the compressor. Once the refrigerator has actually been cooled listed below the settemperature, the compressor is turned off. This control system enables the refrigeratorto conserve electricity by just running as much as is crucial to store the refrigeratorat the preferred temperature.

Questions on Refrigeration

How would the efficiency of a refrigerator be influenced if the food inside the refrigerator is packed exceptionally tightly and also extremely close to the internal coils, so that there is no air flow to the inner coils? Briefly, explain your reasoning. Ammonia (NH3) was one of the at an early stage refrigerants used prior to Freon. It is no much longer supplied in family refrigerators, because of the toxicity of ammonia should tright here be a leak. The boiling suggest of NH3 is similar to that of Freon. Based on molecular framework just, which substance, ammonia or Freon, would certainly you mean to have actually a bigger enthalpy readjust of vaporization (ΔHvap)? Briefly, describe your answer. Based on your answer to component (a), which substance, ammonia or Freon, would certainly you expect to be a better refrigerant? Briefly, define your answer.


Refrigerators are essentially heat engines functioning in reverse. Whereas a heat engineconverts warmth to occupational, reverse heat engines transform work to warmth. In the refrigerator, theheat that is generated is transferred to the outside of the refrigerator. To cool therefrigerator, a "functioning substance", or "coolant", such as Freon isrequired.The refrigerator functions by a cycle of compushing and also expanding the Freon,combined via phase transitions between the gaseous and also liquid phases of Freon. Work isdone on the Freon by a compressor, and the Freon then releases warm to the air outside ofthe refrigerator (as it undergoes the exothermic condensation from a gregarding a liquid). Toregeneprice the gaseous Freon for compression, the Freon passes with an inner coil,wbelow it undergoes the endothermic vaporization from the liquid phase to the gaseousphase. This endothermic procedure causes the Freon to absorb heat from the air inside therefrigerator, cooling the refrigerator.

Additional Links:


Brown, Lemight, and also Bursten. y2kcenter.org: The Central Science, 7th ed., p. 395-98.

Petrucci and Harlumber. General y2kcenter.org, 7th ed., p. 435, 699-701, 714-15.


The authors thank Dewey Holten, Michelle Gilbertboy, Jody Proctor and also CarolynHerman for many type of helpfulsuggestions in the creating of this tutorial.

The breakthrough of this tutorial was supported by a provide from the Howard HughesMedical Institute, via the Undergraduate Biological Sciences Education program, GrantHHMI# 71199-502008 to Washington College.

See more: Which Line Is A Line Of Symmetry For This Regular Polygon? Lines Of Symmetry Of Plane Shapes

Revised January 2001.