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

Key Concepts

warm Engines Reverse warmth Engines (e.g.

You are watching: Is water condensing endothermic or exothermic

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

Introduction: warm Engines and Refrigeration

Refrigeration has enabled for an excellent advances in our ability to save food and also othersubstances safely for lengthy periods the time. In addition, the same technology that is usedto run refrigerators is also used in waiting conditioners, allowing people come live and also workcomfortably even in unbearably warm weather. Just how does this technology work to create coolair once the external conditions are really hot? as we candlestick see, refrigerators (and airconditioners) depend on the thermodynamic application well-known as the warm engine, also asthe molecule properties the the substance contained in the coils that the refrigerator.

One the the most important practical applications the the values of thermodynamics isthe warmth engine (Figure 1). In the warmth engine, warm is took in from a "workingsubstance" in ~ high temperature and also partially convert to work. (Heatengines are never ever 100% efficient, due to the fact that the remaining heat (i.e., the heat thatis not converted to work) is released to the surroundings, which space at a lowertemperature.) The steam engines offered to power at an early stage trains and also electric generators areheat engines in i m sorry water is the working substance.


Figure 1

In a warm engine, an input of warmth causes rise in the temperature that the functioning substance, enabling the functioning 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 come turn. This is the vital mechanism through which steam-powered trains operate.

In a reverse warm engine (Figure 2), the opposite result occurs. Job-related is converted toheat, i beg your pardon is released.


Figure 2

In a reverse warmth engine, a work-related input is converted to a heat output. In this case, the job-related (generated by electricity) condenses gas water (steam) and also pushes it right 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 very first U.S. Patent for arefrigeration machine, which uses a reverse heat engine (Figure 2) together the very first step inits operation. Gorrie, persuaded that the cure for malaria was cold (because outbreakswere terminated in the winter), search to develop a device that could make ice and cool apatient"s room in the warm Florida summer. In Dr. Gorrie"s refrigerator, air was compressedusing a pump, which led to the temperature that the waiting to increase (exchanging work forheat). Running this compressed air through pipes in a cold-water bathtub released the heatinto the water. The air to be then allowed to broaden again to atmospheric pressure, butbecause it had lost heat to the water, the temperature of the waiting was reduced than beforeand could be provided to cool the room.

Modern refrigerators run by the same reverse-heat-engine principle. Whereasa heat engine converts warmth (from a high-temperature area) come work, a refrigeratorconverts occupational to heat. Modern refrigerators usage substances various other than airas the coolant; the coolant substance transforms from gas come liquid together it go from higher tolower temperature. This readjust from gas to liquid is a phase transition, and also the energyreleased upon this transition is largely dependent top top the intermolecular interactions ofthe substance. Hence, to recognize the refrigeration cycle supplied in modernrefrigerators, it is crucial to very first discuss step transitions.

Questions on warmth Engines and also Refrigeration

In plenty of homes and businesses, warm pumps room replacing standard heaters come heat structures by using power to transfer heat to the within of the building. Is the heat pump an example of a warmth engine or a reverse warm engine? Briefly, describe your reasoning. Briefly, describe the procedure by which the heat pump transfers heat 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 form of issue that is uniform transparent in chemistry composition and physicalproperties, and that have the right to be differentiated from other phases with which it may be incontact by this definite properties and also composition. As presented in figure 3, a problem inthe heavy phase has actually a identify shape and also rigidity; a problem in the fluid phase has nodefinite shape, however has a identify volume, and a substance in the gas phase has nodefinite form or volume, yet has a shape and also volume established by the shape and also size ofthe container.


Figure 3

This schematic diagram shows the differences in physics 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 strictly configuration, offering the problem a definite shape and size. In a liquid, the particles space close together however may relocate with respect come one another, giving the problem a definite volume yet a fluid shape. In a gas, the particles may occupy the whole volume that the container, so the their shape and also volume room both identified by the container.

Molecular (Microscopic) View

One of the major differences in the three phases illustrated in number 3 is the numberof intermolecular interaction they contain. The corpuscle in a solid connect with all oftheir nearest next-door neighbors (recall the discussion of bonding in solids indigenous the tutorialentitled "Bands,Bonds, and Doping: just how Do LED"s Wrok?"), the corpuscle in a liquid interact withonly some of the surrounding particles, and the particles in a gas ideally have no interactionwith one another. By break or creating intermolecular interactions, a substance canchange from one step to another. Because that example, gas molecules condensation to form liquidsbecause of the existence of attractive intermolecular forces. The more powerful the attractiveforces, the better the stability of the liquid (which leads to a higher boiling pointtemperature). A transition between the phases of issue is called a phase transition. Thenames the the phase transitions in between solid, liquid, and also gas are displayed in number 4.


Figure 4

This diagram shows the surname of the phase transitions in between solids, liquids, and also gases. The arrowhead to the right of the chart demonstrates the these three phases have different enthalpies: gas has the highest enthalpy, liquid has actually an intermediate enthalpy, and solid has actually the lowest enthalpy. Hence, every of the step transitions displayed in this figure entails a adjust in the enthalpy that the substance.

Phase transitions space a kind of chemical reaction. Many of the chemistry reactionsstudied in Chem 151 and 152 indicate the break or creating of bonds in ~ molecules;phase transitions indicate the break or developing of intermolecular forces (attractiveinteractions in between molecules). Hence, similar to other chemistry reactions, the is necessaryto talk about the power that is absorbed or offered off throughout the breaking or forming ofintermolecular interactions in a phase transition.

Phase transitions involving the break of intermolecular attractions (i.e.,fusion (melting), vaporization, and also sublimation) need an entry of power to overcomethe attractive forces between the particles of the substance. Phase transitions involvingthe formation of intermolecular attractions (i.e., freezing, condensation, anddeposition) release power as the particles embrace a lower-energy conformation. Thestrength of the intermolecular attractions in between molecules, and therefore the quantity ofenergy compelled to conquer these attractive forces (as well together the lot of energyreleased once the attractions room formed) relies on the molecule properties of thesubstance. Generally, the more polar a molecule is, the stronger the attractiveforces in between molecules are. Hence, an ext polar molecules generally require moreenergy to get rid of the intermolecular attractions in an endothermic phase transition, andrelease an ext energy by developing intermolecular attractions during an exothermic phasetransition.

Thermodynamic (Macroscopic) View

In enhancement to the microscopic, molecular see presented above, us can define phasetransitions in regards to macroscopic, thermodynamic properties. The is crucial to bear inmind that the microscopic and macroscopic views are interdependent; i.e., thethermodynamic properties, such together enthalpy and temperature, of a substance room dependenton the molecular behavior of the substance.

Phase transitions space accompanied by alters in enthalpy and also entropy. In this tutorial,we will concern ourselves greatly with changes in enthaply. The energy readjust involved inbreaking or developing intermolecular attractions is mostly supplied or exit in theform that heat. Adding heat reasons intermolecular attractions to it is in broken.How go this occur? warmth is a move of energy to molecules, causing the molecule toincrease their movement as defined by the kinetic concept of gases (discussed in thetutorial entitled, "GasLaws save Lives: The aramuseum.org Behind Airbags"), and also thereby weakening theintermolecular forces holding the molecule in place. Likewise, molecules loseheat to kind intermolecular attractions; when warm is lost, the molecules moveslower and therefore have the right to interact much more with other surrounding molecules.

Because phase transforms generally take place at continuous pressure (i.e., in areaction vessel open up to the atmosphere), the heat can be described by a change in enthalpy(ΔH=qp=nCp ΔT, wherein n is thenumber of mole of the substance and also Cp is the molar warm capacity in ~ constantpressure). For phase transitions including the break of intermolecularattractions, warmth is added and ΔH is positive, since thesystem is going native a lower-enthalpy phase to a higher-enthalpy phase, together shownby the direction the the vertical arrow to the appropriate 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, since the system is going indigenous a higher-enthalpyphase to a lower-enthalpy phase, as presented in number 4. Hence, freezing,condensation, and deposition are all exothermic phase transitions. The direction that theenthalpy change for each of the phase-transition processes named in figure 4 is presented inTable 1, below.

Phase transition Direction that ΔH

Fusion (Melting) (solid come liquid)

ΔH>0; enthalpy increases (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 to liquid) ΔH0; enthalpy reduce (exothermic process)
Deposition (gas to solid) ΔH0; enthalpy to reduce (exothermic process)

Table 1

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

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

The enthalpy change of phase transitions can additionally be supplied to describe differences inmelting points and also boiling point out of substances. A provided substance has actually a characteristicrange of temperatures at which the undergoes every 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 greater the enthalpychange because that a phase change is (the an ext heat required for one endothermic transition, orreleased for an exothermic transition), the higher the temperature is at which thesubstance experience the step transition. Because that example, liquids with strongintermolecular attractions require much more heat come vaporize than liquids with weakintermolecular attractions; therefore, the boiling suggest (vaporization point) for theseliquids will certainly be greater than for the liquids with weaker intermolecular attractions.

Questions ~ above Phases and Phase Transitions

A student steps the melting points of two common household crystalline solids: sodium chloride (NaCl) and also sucrose (C12H22O11). She finds the the melting suggest of sodium chloride is much higher than the melting allude of sucrose. Briefly, describe why the melting suggest for NaCl is greater than for C12H22O11, in terms of the form of attractive forces in the solids and also your molecular expertise of phase transitions. when you location your finger right into a glass of water automatically after adding an ice cream cube, and also again five minutes later, you discover that the water feels cooler after few of the ice cream has begun to melt. Briefly, describe this phenomenon in terms of your thermodynamic knowledge of step changes.


Now, we shall usage our knowledge of heat engines and also phase transitions come explainhow refrigerators work. The enthalpy changes linked with phase transitions might be usedby a warmth engine (Figure 1) to carry out work and to transfer heat between (1) the substanceundergoing a phase transition 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 secondary process, the remainder of the warm is released to the surroundingsat a lower temperature, due to the fact that the heat engine is no 100% efficient.

As presented in figure 2, a refrigerator deserve to be believed of as a warm engine in reverse. Thecooling effect in a frozen refrigerator is achieved by a cycle of condensation and vaporizationof the nontoxic compound CCl2F2 (Freon-12). As presented inFigure 5, the refrigerator contains (1) one electrically-powered compressor that does workon Freon gas, and (2) a series of coils that allow heat to it is in released external (on theback of) the refrigerator or took in from within the refrigerator as Freon passes throughthese coils.


Figure 5

This is a schematic chart of the major functional components of a refrigerator. The major features incorporate a compressor include Freon (CCl2F2) gas, an external heat-exchange coil (on the outside back of the refrigerator) in i m sorry the Freon passes and also condenses, an development valve, and a heat-exchange coil inside the insulated compartment the the refrigerator (blue) in which the Freon is vaporized, soaking up heat from within the refrigerator (and thus lowering the temperature).

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


Figure 6

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

Please click on the pink button below to see a QuickTime movie mirroring an computer animation of the refrigeration cycle presented in the figure over and defined 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 job-related on the Freon gas, enhancing the push of the gas. As the push of the gas increases, therefore does that temperature (as suspect 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 to the lower-temperature wait of the room neighboring the coil. This heat loss reasons the high-pressure gas to condense to liquid, as motion of the Freon molecules decreases and intermolecular attractions room formed. Hence, the work-related done ~ above the gas through the compressor (causing one exothermic phase transition in the gas) is convert to heat given off in the air in the room behind the refrigerator. If you have ever before felt the coils ~ above the earlier of the refrigerator, you have experienced the heat given off during the condensation the Freon. Next, the fluid Freon in the exterior coil passes through an development valve into a coil within the insulated compartment that the refrigerator. Now, the liquid is in ~ a low pressure (as a result of the expansion) and is lower in temperature (cooler) 보다 the neighboring air (i.e., the air inside the refrigerator). Since heat is moved from areas of higher temperature to areas of reduced temperature, warm is soaked up (from within the refrigerator) through the fluid Freon, resulting in the temperature within the frozen refrigerator to it is in reduced. The took in heat starts to rest the intermolecular attractions that the liquid Freon, permitting the endothermic vaporization procedure to occur. When all of the Freon changes to gas, the cycle can start over. The cycle described above does not run continuously, however rather is regulated by athermostat. As soon as the temperature inside the frozen fridge rises above the collection temperature,the thermostat starts the compressor. When the refrigerator has been cooled below the settemperature, the compressor is turn off. This manage mechanism enables the refrigeratorto conserve electricity by just running as much as is necessary to keep the refrigeratorat the wanted temperature.

Questions on Refrigeration

exactly how would the effectiveness of a frozen fridge be impacted if the food inside the refrigerator is packed an extremely tightly and an extremely close to the interior coils, so the there is no air flow to the interior coils? Briefly, explain your reasoning. Ammonia (NH3) was among the early refrigerants used before Freon. It is no longer used in household refrigerators, due to the fact that of the toxicity of ammonia should there it is in a leak. The boiling allude of NH3 is comparable to that of Freon. based on molecular structure only, which substance, ammonia or Freon, would certainly you suppose to have actually a larger enthalpy adjust of vaporization (ΔHvap)? Briefly, explain your answer. based on your prize to component (a), which substance, ammonia or Freon, would you intend to be a better refrigerant? Briefly, describe your answer.


Refrigerators are basically heat engines functioning in reverse. Vice versa, a warmth engineconverts warmth to work, reverse warmth engines convert work come heat. In the refrigerator, theheat the is generated is transferred to the external of the refrigerator. Come cool therefrigerator, a "working substance", or "coolant", such as Freon isrequired.The refrigerator works by a bicycle of compressing and expanding the Freon,combined through phase transitions in between the gaseous and liquid phases of Freon. Occupational isdone top top the Freon by a compressor, and the Freon then releases heat to the air outside ofthe frozen fridge (as it experience the exothermic condensation indigenous a gas to a liquid). Toregenerate the gas Freon for compression, the Freon passes v an internal coil,where it experience the endothermic vaporization indigenous the liquid 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 Bursten. aramuseum.org: The central Science, 7th ed., p. 395-98.

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


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

The development of this accuse was supported by a provide from the Howard HughesMedical Institute, with the Undergraduate organic Sciences education program, GrantHHMI# 71199-502008 to Washington University.

See more: What Is The Electron Configuration For Arsenic ? Arsenic » Properties Of Free Atoms

Revised January 2001.