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Authors: Rachel Casiday and also Regina Frey department of inter-base.net, Washington university St. Louis, MO 63130
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Key Concepts

heat Engines Reverse warmth Engines (e.g.

You are watching: Do all exothermic phase changes have a negative value for the entropy change of the system?

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

Introduction: warm Engines and also Refrigeration

Refrigeration has permitted for good advances in our capability to save food and also othersubstances safely for lengthy periods the time. In addition, the same modern technology that is usedto operation refrigerators is likewise used in wait conditioners, enabling people to live and workcomfortably also in unbearably hot weather. Exactly how does this an innovation work to produce coolair when the external conditions are an extremely hot? as we chandelier see, refrigerators (and airconditioners) depend on the thermodynamic application known as the heat engine, also asthe molecule properties that the substance had in the coils the the refrigerator.

One that the most crucial practical applications of the principles of thermodynamics isthe warmth engine (Figure 1). In the warmth engine, warm is soaked up from a \"workingsubstance\" at high temperature and partially convert to work. (Heatengines are never ever 100% efficient, due to the fact that the remaining heat (i.e., the warmth thatis not converted to work) is released to the surroundings, which room at a lowertemperature.) The steam engines supplied to power early trains and electric generators areheat engines in i beg your pardon water is the working substance.


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Figure 1

In a warm engine, an entry of warm causes boost in the temperature of the working substance, permitting the working substance to carry out work. In this schematic diagram, the functioning substance is water. At high temperature, gas water (steam) pushes a piston, which reasons a wheel to turn. This is the important mechanism whereby steam-powered trains operate.


In a reverse warm engine (Figure 2), the opposite effect occurs. Job-related is convert toheat, i m sorry is released.


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Figure 2

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


In 1851, the Florida doctor John Gorrie was granted the an initial U.S. Patent because that arefrigeration machine, which offers a reverse warmth engine (Figure 2) together the first step inits operation. Gorrie, persuaded that the cure for malaria was cold (because outbreakswere terminated in the winter), sought to construct a maker that can 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 air to rise (exchanging job-related forheat). To run this compressed air v pipes in a cold-water bathtub released the heatinto the water. The air to be then allowed to increase again to atmospheric pressure, butbecause it had lost warmth to the water, the temperature the the air was lower than beforeand could be offered to cool the room.

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

Questions on warmth Engines and Refrigeration

In plenty of homes and businesses, warm pumps are replacing typical heaters come heat structures by using electrical energy to transfer warmth to the inside of the building. Is the heat pump an example of a heat engine or a reverse heat engine? Briefly, explain your reasoning. Briefly, describe the process by i m sorry the heat pump transfers heat into a building. What was the \"working substance\" in Dr. Gorrie\"s refrigerator?

Phases and Phase Transitions

Matter can exist in three different phases (physical states): solid, liquid, and gas. Aphase is a form of issue that is uniform throughout in chemistry composition and also physicalproperties, and that can be distinguished from various other phases v which it may be incontact by these definite properties and also composition. As shown in number 3, a problem inthe heavy phase has a definite shape and also rigidity; a substance in the fluid phase has actually nodefinite shape, yet has a identify volume, and a problem in the gas phase has actually nodefinite form or volume, however has a shape and volume identified by the shape and size ofthe container.


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Figure 3

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


Molecular (Microscopic) View

One the the major differences in the three phases illustrated in number 3 is the numberof intermolecular interactions they contain. The corpuscle in a solid connect with every oftheir nearest next-door neighbors (recall the discussion of bonding in solids native the tutorialentitled \"Bands,Bonds, and also Doping: just how Do LED\"s Wrok?\"), the corpuscle in a liquid connect withonly few of the adjacent particles, and the corpuscle in a gas ideally have actually no interactionwith one another. By break or developing intermolecular interactions, a substance canchange indigenous one step to another. Because that example, gas molecules condense to kind liquidsbecause the the visibility of attractive intermolecular forces. The more powerful the attractiveforces, the higher the security of the liquid (which leader to a greater boiling pointtemperature). A transition between the phases of issue is referred to as a phase transition. Thenames that the step transitions in between solid, liquid, and also gas are displayed in figure 4.


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Figure 4

This diagram mirrors the surname of the phase transitions in between solids, liquids, and gases. The arrowhead to the right of the chart demonstrates the these 3 phases have different enthalpies: gas has actually the highest enthalpy, liquid has an intermediate enthalpy, and also solid has the shortest enthalpy. Hence, every of the step transitions shown in this figure involves a adjust in the enthalpy the the substance.


Phase transitions space a type of chemistry reaction. Many of the chemistry reactionsstudied in Chem 151 and also 152 show off the break or creating of bonds in ~ molecules;phase transitions involve the breaking or developing of intermolecular forces (attractiveinteractions in between molecules). Hence, similar to other chemical reactions, the is necessaryto discuss the power that is soaked up or provided off during the break or creating ofintermolecular interaction in a step transition.

Phase transitions involving the break of intermolecular attractions (i.e.,fusion (melting), vaporization, and also sublimation) call for an entry of energy to overcomethe attractive forces in between the corpuscle of the substance. Phase transitions involvingthe development 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 therefore the amount ofenergy required to conquer these attractive pressures (as well together the lot of energyreleased once the attractions are formed) depends on the molecular properties that thesubstance. Generally, the more polar a molecule is, the more powerful the attractiveforces between molecules are. Hence, more polar molecules typically require moreenergy to overcome the intermolecular attractions in one endothermic phase transition, andrelease more energy by developing intermolecular attractions during an exothermic phasetransition.

Thermodynamic (Macroscopic) View

In addition to the microscopic, molecular watch presented above, we can define phasetransitions in terms of macroscopic, thermodynamic properties. It is crucial to bear inmind the the microscopic and macroscopic views space interdependent; i.e., thethermodynamic properties, such together enthalpy and temperature, the a substance are dependenton the molecular behavior of the substance.

Phase transitions are accompanied by transforms in enthalpy and also entropy. In this tutorial,we will concern ourselves mostly with alters in enthaply. The energy change involved inbreaking or creating intermolecular attractions is generally supplied or released in theform that heat. Adding heat causes intermolecular attractions to it is in broken.How does this occur? warm is a deliver of power to molecules, leading to the molecules toincrease their motion as defined by the kinetic theory of gases (discussed in thetutorial entitled, \"GasLaws save Lives: The inter-base.net Behind Airbags\"), and also thereby weakening theintermolecular pressures holding the molecules in place. Likewise, molecule loseheat to form intermolecular attractions; when warm is lost, the molecules moveslower and therefore have the right to interact an ext with other nearby molecules.

Because phase changes generally take place at consistent pressure (i.e., in areaction vessel open to the atmosphere), the heat deserve to be described by a adjust in enthalpy(ΔH=qp=nCp ΔT, whereby n is thenumber of moles of the substance and Cp is the molar heat capacity at constantpressure). For step transitions entailing the breaking of intermolecularattractions, heat is added and ΔH is positive, because thesystem is going indigenous a lower-enthalpy step to a higher-enthalpy phase, together shownby the direction that the vertical arrowhead to the right of figure 4. Hence, fusion,vaporization, and also sublimation space all endothermic phase transitions. For phasetransitions entailing the creating of intermolecular attractions, warm is released and ΔH is negative, because the device is going from 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 readjust for every of the phase-transition processes named in figure 4 is presented inTable 1, below.


Phase change Direction that ΔH

Fusion (Melting) (solid come liquid)

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

Table 1

This table shows the authorize of the enthalpy change for each of the step transitions explained above. Recall the endothermic processes have actually a positive 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 for phase transitions have the right to be included or subtracted follow toHess\"s law. (Recall native Chem 112 and the advent to the experiment that,according to Hess\"s law, as soon as chemical reaction 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 change of step transitions can likewise be used to explain differences inmelting points and boiling points of substances. A provided substance has actually a characteristicrange of temperature at which that undergoes every of the step transitions (at a givenpressure). This temperatures are named for the phase transition that occurs at thetemperature (e.g., melt point). In general, the better the enthalpychange for a phase transition is (the much more heat required for an endothermic transition, orreleased because that an exothermic transition), the better the temperature is in ~ which thesubstance undergoes the step transition. For example, liquids through strongintermolecular attractions require more heat to vaporize 보다 liquids through weakintermolecular attractions; therefore, the boiling point (vaporization point) for theseliquids will certainly be greater than because that the liquids v weaker intermolecular attractions.

Questions top top Phases and Phase Transitions

A student actions the melting points that two typical household crystalline solids: sodium chloride (NaCl) and sucrose (C12H22O11). She finds the the melting suggest of salt chloride is much greater than the melting suggest of sucrose. Briefly, explain why the melting suggest for NaCl is higher than for C12H22O11, in terms of the kind of attractive forces in the solids and your molecular knowledge of phase transitions. as soon as you ar your finger right into a glass of water instantly after adding an ice cream cube, and again five minutes later, you find that the water feeling cooler after some of the ice has begun to melt. Briefly, explain this phenomenon in regards to your thermodynamic expertise of phase changes.

Refrigeration

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

As shown in number 2, a refrigerator have the right to be thought of together a heat engine in reverse. Thecooling impact in a refrigerator is completed by a bike of condensation and also vaporizationof the nontoxic compound CCl2F2 (Freon-12). As presented inFigure 5, the refrigerator consists of (1) one electrically-powered compressor the does workon Freon gas, and also (2) a collection of coils that allow heat to it is in released external (on theback of) the frozen refrigerator or took in from within the refrigerator as Freon overcome throughthese coils.


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Figure 5

This is a schematic chart of the major functional components 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 i beg your pardon the Freon passes and condenses, an expansion valve, and a heat-exchange coil within the insulated compartment the the frozen fridge (blue) in i beg your pardon the Freon is vaporized, taking in heat from within the frozen refrigerator (and for this reason lowering that temperature).


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


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Figure 6

This diagram mirrors the major steps in the refrigeration cycle. For a description of each action (indicated through the environment-friendly numbers), check out the numbered procedures below. In this figure, blue dots stand for Freon gas, and solid blue areas represent liquid Freon. Tiny arrows suggest the direction that heat flow into or out of the refrigerator coils.

Please click the pink button listed below to check out a QuickTime movie mirroring an computer animation of the refrigeration cycle presented in the figure above and defined below. Click the blue button listed below to download QuickTime 4.0 to view the movie.

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external of 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 increases, so does its temperature (as guess by the ideal-gas law). Next, this high-pressure, high-temperature gas beginning the coil ~ above the exterior 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 reasons the high-pressure gas to condense to liquid, as movement of the Freon molecule decreases and also intermolecular attractions are formed. Hence, the job-related done ~ above the gas through the compressor (causing an exothermic phase transition in the gas) is convert to heat offered off in the wait in the room behind the refrigerator. If girlfriend have ever felt the coils top top the back of the refrigerator, you have actually experienced the heat provided off throughout the condensation that Freon. Next, the fluid Freon in the external coil passes through an expansion valve into a coil inside the insulated compartment that the refrigerator. Now, the liquid is at a low press (as a an outcome of the expansion) and also is lower in temperature (cooler) than the neighboring air (i.e., the air inside the refrigerator). Since heat is moved from areas of greater temperature to locations of reduced temperature, warmth is absorbed (from inside the refrigerator) by the fluid Freon, resulting in the temperature within the frozen refrigerator to be reduced. The soaked up heat starts to break the intermolecular attractions that the fluid Freon, allowing the endothermic vaporization process to occur. When all of the Freon alters to gas, the cycle deserve to start over. The bicycle described above does no run continuously, but rather is controlled by athermostat. When the temperature within the refrigerator rises over the set temperature,the thermostat beginning the compressor. When the refrigerator has actually been cooled listed below the settemperature, the compressor is turn off. This regulate mechanism allows the refrigeratorto conserve electrical energy by only running as much as is crucial to save the refrigeratorat the wanted temperature.

Questions ~ above Refrigeration

exactly how would the effectiveness of a frozen fridge be impacted if the food within the frozen fridge is packed really tightly and very close to the internal coils, so that there is no air circulation to the inner coils? Briefly, describe your reasoning. Ammonia (NH3) was among the early on refrigerants used prior to Freon. That is no much longer used in family members refrigerators, since of the toxicity the ammonia must there be a leak. The boiling point of NH3 is similar to that of Freon. based on molecular framework only, i m sorry substance, ammonia or Freon, would certainly you intend to have a larger enthalpy change of vaporization (ΔHvap)? Briefly, explain your answer. based on your price to part (a), i beg your pardon substance, ammonia or Freon, would you intend to be a much better refrigerant? Briefly, define your answer.

Summary

Refrigerators are basically heat engines functioning in reverse. Whereas a heat engineconverts heat to work, reverse heat engines convert work to heat. In the refrigerator, theheat that is created is transferred to the outside of the refrigerator. To cool therefrigerator, a \"working substance\", or \"coolant\", such together Freon isrequired.The refrigerator functions by a bike of compressing and expanding the Freon,combined with phase transitions in between the gaseous and also liquid phases of Freon. Work-related isdone top top the Freon by a compressor, and also the Freon then releases warm to the air exterior ofthe frozen refrigerator (as it undergoes the exothermic condensation native a gas to a liquid). Toregenerate the gas Freon for compression, the Freon passes with an interior coil,where it undergoes the endothermic vaporization from 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:

References:

Brown, Lemay, and also Bursten. inter-base.net: The main Science, 7th ed., p. 395-98.

Petrucci and Harwood. General inter-base.net, 7th ed., p. 435, 699-701, 714-15.

Acknowledgements:

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

The advancement of this indict was sustained by a provide 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.