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Chemistry—Unit 3 Energy and Heating/Cooling · Chemistry—Unit 3 Energy and Heating/Cooling...
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©Modeling Instruction – AMTA 2013 1 U3 Energy and Heating/Cooling Chemistry—Unit 3 Energy and Heating/Cooling Energy is a substance-like quantity that is always involved whenever a system undergoes change (hotter-colder, faster-slower, higher-lower). A key to understanding energy is to recognize that energy is always and everywhere only energy. Energy is stored in a system in several different “accounts” and can be transferred between system and surroundings in different ways, but it does not come in different forms. When there is a change in the way the system stores energy or if energy is transferred between system and surroundings, something about the system changes, but the energy remains the same. One difficulty we have in understanding energy is that our everyday use of words can sometimes muddy the waters. For example, use of the word “heat” can leave the impression that it is somehow different from energy. It would be better if we viewed “heat” as one of the ways that energy is transferred from one object to another. While it is helpful to say that we “heat” an object (as a shortcut for “transfer energy to”), it is not useful to say that an object stores “heat”. It’s fine to describe an object that stores a lot of thermal energy as “hot”, but saying that it stores a lot of “heat” confuses energy with a way that it is moved from one object to another. Heating a system increases its thermal energy (E th ) through the collisions of more energetic particles with particles of lower energy; as a result, the particles in the system move more rapidly than before. Use of the –ing ending helps us view “heating” as a process of energy transfer through collisions of particles rather than as something different from energy. The quantity of energy transferred in this way is often referred to as “heat” (assigned the variable name Q), but it is important to remember that it is simply energy. Conversely, a system cools when its particles transfer thermal energy (through collisions) to particles in the surroundings. This process lowers the amount of thermal energy (E th ) stored by the system. Temperature is a useful tool because it allows us to assign a numerical value that helps us describe the thermal energy of a system (or surroundings). It is important to recognize that temperature and energy are not the same. Changes in temperature (ΔT) help us to determine the amount of thermal energy gained or lost by a system, as we shall discuss at a later time. We’re now ready to discuss the role of energy during phase change. We’ll first examine what happens when a solid melts. As energy is transferred into the system, the thermal energy (and motion) of the particles increases. At some temperature, the particles are vibrating to and fro so rapidly that they can no longer maintain the orderly arrangement of a solid. They break free of the attractions and begin to move around more freely – they become “liquid”. We use another account to
Transcript of Chemistry—Unit 3 Energy and Heating/Cooling · Chemistry—Unit 3 Energy and Heating/Cooling...
©Modeling Instruction – AMTA 2013 1 U3 Energy and Heating/Cooling
Chemistry—Unit 3 Energy and Heating/Cooling
Energy is a substance-like quantity that is always involved whenever a system undergoes change (hotter-colder, faster-slower, higher-lower). A key to understanding energy is to recognize that energy is always and everywhere only energy. Energy is stored in a system in several different “accounts” and can be transferred between system and surroundings in different ways, but it does not come in different forms. When there is a change in the way the system stores energy or if energy is transferred between system and surroundings, something about the system changes, but the energy remains the same. One difficulty we have in understanding energy is that our everyday use of words can sometimes muddy the waters. For example, use of the word “heat” can leave the impression that it is somehow different from energy. It would be better if we viewed “heat” as one of the ways that energy is transferred from one object to another. While it is helpful to say that we “heat” an object (as a shortcut for “transfer energy to”), it is not useful to say that an object stores “heat”. It’s fine to describe an object that stores a lot of thermal energy as “hot”, but saying that it stores a lot of “heat” confuses energy with a way that it is moved from one object to another. Heating a system increases its thermal energy (Eth) through the collisions of more energetic particles with particles of lower energy; as a result, the particles in the system move more rapidly than before. Use of the –ing ending helps us view “heating” as a process of energy transfer through collisions of particles rather than as something different from energy. The quantity of energy transferred in this way is often referred to as “heat” (assigned the variable name Q), but it is important to remember that it is simply energy. Conversely, a system cools when its particles transfer thermal energy (through collisions) to particles in the surroundings. This process lowers the amount of thermal energy (Eth) stored by the system. Temperature is a useful tool because it allows us to assign a numerical value that helps us describe the thermal energy of a system (or surroundings). It is important to recognize that temperature and energy are not the same. Changes in temperature (ΔT) help us to determine the amount of thermal energy gained or lost by a system, as we shall discuss at a later time. We’re now ready to discuss the role of energy during phase change. We’ll first examine what happens when a solid melts. As energy is transferred into the system, the thermal energy (and motion) of the particles increases. At some temperature, the particles are vibrating to and fro so rapidly that they can no longer maintain the orderly arrangement of a solid. They break free of the attractions and begin to move around more freely – they become “liquid”. We use another account to
II-A-2.3a-Heating+Cooling Reading
©Modeling Instruction – AMTA 2013 2 U3 Energy and Heating/Cooling
describe the way the system stores energy when the particles exist as a liquid rather than as a solid; we call this phase energy, Eph. Particles in the liquid phase store more phase energy than do particles in the solid phase. As you recall from the experiment, during the melting of the solid, the temperature remained more or less constant, despite the fact that energy was being continually transferred to the system. To explain this, consider the fact that energy is required to overcome the attractions that bind the particles in an orderly array. Apparently, at the melting point, energy entering the system can no longer be stored in the motion of particles in the solid phase - the particles are moving too rapidly to remain as solid. Instead, the particles trade thermal energy for phase energy as they break free from their neighbors and are able to move around more freely. This decrease in thermal energy is temporary, however, as energy is still being supplied via collisions to the particles in the system. A closer examination of the plateau region of the heating curve would reveal tiny zigzags in the temperature, like the teeth of a hacksaw blade. Energy enters the thermal account (raising the temperature) and then is immediately transferred to the phase account (lowering the temperature) as the particles break free from their mutual attractions. This internal energy transfer keeps the temperature more or less constant during the phase change. This process of energy shuttling between accounts continues until the solid is completely melted. It may be helpful to consider an analogy for this process. Let's substitute money for energy and substitute a checking account for the thermal energy account and a savings account for the phase energy account. Let’s also say that the checking account is set up to have a maximum balance of $1000. So long as the checking balance is lower than this amount, money can be deposited into this account. Once the balance reaches $1000, however, any money entering the checking account is quickly transferred to the savings account. If $50 is deposited into the checking account, the balance becomes too high so the excess is transferred to savings. This transfer increases the amount in the savings account by $50 and the checking account balance returns to $1000. Once all the particles in the system are in the liquid phase, energy transfers to the system are once again stored in the thermal account and the temperature increases. This process continues until the temperature reaches the boiling point. At this temperature, the particles are moving too rapidly to remain in the liquid phase. Thermal energy is again exchanged for phase energy as the particles break free from one another and enter the gas phase. Now, let’s examine a situation where energy is transferred out of a system during a phase change. An example of such a situation is the condensation of water vapor. In order for a collection of gaseous water particles to condense (become bound to one another in the liquid phase) they must transfer phase energy to the thermal account. The particles of liquid water are now hotter than they once were. When
©Modeling Instruction – AMTA 2013 3 U3 Energy and Heating/Cooling
these higher temperature particles in the liquid phase come into contact with lower temperature particles in the surroundings, a transfer of thermal energy from system to surroundings via heating occurs. The system cools and the immediate surroundings get warmer. The bottom line is that any time energy enters or leaves a system via heating (collisions of particles), the motion of the particles changes first. This means that energy entering or leaving a system does so via the thermal energy account. When the temperature of a single phase changes, the only account that changes is the thermal energy. During a phase change, the thermal account experiences small but temporary changes as it serves as the conduit for energy moving from the phase account to the surroundings (during freezing or condensing) or from the surroundings to the phase account (during melting or vaporization).
Does water boil faster if you put salt in thewater?
Yes and no. If you look at how fast water boils when you add a small amount of salt to it, such aswhen cooking your noodles, the change is insignificant between pure water and the salted water.However, if you take two identical pots and add one gallon of pure water to one pot and one gallon of20 percent salt water to the other and heat the two pots on identical stoves, the pot containing the saltwater will come to a boil first. Surprised?
To truly answer the question, one must look at what it takes to boil a container of water.
The time it takes a bucket of liquid to boil is controlled by essentially three things. The first is howmuch heat or energy you put into the bucket. The second is how fast the temperature rises inresponse to the heat input (the liquid's heat capacity), and the third is the boiling point of the liquid.Assuming that we can control our stoves and add the same amount of energy to each pot, thisvariable becomes insignificant.
The boiling point of water does rise if you add salt to it, but only by about 2°C (4°F) to 102°C (216°F).Remember, water boils at 100°C (212°F). This is an insignificant change for adding such a largeamount of salt. For you science nerds out there, the boiling point increase is calculated using the"ebullioscopic" constant of water. This leads us to the important variable, how fast water or salt waterheats up, or the solution's heat capacity.
The heat capacity of water is very high. What this means is that it takes a lot of energy to raise thetemperature of water 1°C; in fact, the calorie is defined as the amount of energy that it takes to heatone gram of water to 1°C. Not to digress, but the high heat capacity of water is good, especially if youlive on a planet where two-thirds of the surface is covered by water - it helps regulate the globaltemperature.
Now back to the question. If you look at the heat capacity of salt water, you will find that it is less thanpure water. In other words, it takes less energy to raise the temperature of the salt water 1°C thanpure water. This means that the salt water heats up faster and eventually gets to its boiling point first.
Why does salt water have a lower heat capacity? If you look at 100 grams of pure water, it contains100 grams of water, but 100 grams of 20 percent salt water only contains 80 grams of water. Theother 20 grams is the dissolved salt. The heat capacity of dissolved salt is almost zero whencompared to the high heat capacity of water. This means that the heat capacity of a 20-percent saltsolution is 80 percent that of pure water. Twenty percent salt water will heat up almost 25 percentfaster than pure water and will win the speed race to the boiling point.
Please note that this will not hold true if you take two identical pots containing one gallon of watereach and add the salt to one pot because then the volume of liquid in the salted pot will be greaterthan the one gallon starting point.
This month's Whizard is Mike Dammann, manager of the Inorganics Section in the Chemistry andChemical Engineering Division.
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Boiling point elevation occurswhen the boiling point of asolution becomes higher thanthe boiling point of a puresolvent. The temperature atwhich the solvent boils isincreased by adding any non-volatile solute. A commonexample of boiling pointelevation can be observed byadding salt to water. The boilingpoint of the water is increased.Boiling point elevation, likefreezing point depression, is acolligative property of matter.
This means it depends on the numberof particles present in a solution and noton the type of particles or their mass.
The amount of boiling point elevationcan be calculated using the Clausius-Clapeyron equation and Raoult's law.For an ideal dilute solution:
Boiling Point = Boiling Point +ΔT
where ΔT = molality * K * i
with K = ebullioscopic constant(0.52°C kg/mol for water) and i = Van't
Hoff factor
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Adding salt to water increases its boiling point. Markus Schweiss
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Department of PhysicsUniversity of Illinois at Urbana-Champaign
Q & A: Boiling and Freezing points of pure and salty water
Most recent answer: 02/24/2014
Q: What is the boiling and freezing point of both fresh and saltwater?- maria (age 13)congress middle, lake worth , F.L., Florida
A: Hi Maria,
For pure water, the boiling point is 100 degrees Celsius (212 Fahrenheit) at one atmosphere of pressure, and the melting point is 0degrees Celsius (32 degrees Fahrenheit) at one atmosphere of pressure. At lower pressures (at high altitudes, for example, inDenver, Colorado), the boiling point will be perhaps a couple of degrees lower.
For saltwater, the boiling point is raised, and the melting point is lowered. By how much depends on how much salt there is. I'llassume the salt is sodium chloride, NaCl (table salt). The melting point is lowered by 1.85 degrees Celsius if 29.2 grams of salt aredissolved in each Kg of water (called a "0.5 molal solution" of salt. The Na and Cl dissociate right away when dissolved, and so for a0.5 molal solution of salt, there is a 1.0 molal concentration of ions). The boiling point is raised by 0.5 degrees Celsius for water with29.2 grams of salt dissolved in each kg of water.
If your concentrations of salt are different, then you can scale the boiling point elevation and melting point depression predictionsdirectly with the concentration.
These numbers come from the CRC Handbook of Chemistry and Physics.
Tom
(published on 10/22/2007)
Follow-Up #1: pressure and freezing
Q: Increasing pressure on water raises its boiling point does it raise its freezing point- Garrymuswellbrook nsw australia
A: Raising pressure actually lowers water’s freezing point a little. That’s because ice occupies more volume than liquid water, sosqueezing tends to drive it to become liquid.
That’s rather unusual for freezing points, since the solid occupies less space than the liquid for most substances.
Mike W.
(published on 10/22/2007)
Follow-Up #2: efficiently salting water
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Q: So, following the conclusion that salt water has a higher boiling point than fresh water, comes the question; From a perspective ofenergy consumption, when to add salt to the water when boling spagetti ? I.e.; if I add the salt to the water while its still cold, it takesmore energy to make it boil ? If I add it when the water boils at 100 C, the water stops boiling, and needs to consume more energy tocome up to teh new boiling point ? At the end, same energy is required to make a portion of spagetti I'd assume ? :-)- jacob hansen (age 39)Naestved, denmark
A: You're basically right. Either way you start out with plain water and salt at room temperature. The noodles go in when you have saltywater at its boiling point. The energy difference between that and the starting point doesn't depend on when you add the salt.
A little energy is lost to evaporation on the way, however. If you wait until the water is boiling, then add salt, then boil again, the wateris spending a little more time near boiling than it would if you add the salt first. So you lose a little more energy. It's marginally moreefficient to add the salt first.
Mike W.
(published on 06/18/2009)
Follow-Up #3: salt water boiling
Q: approx how much minnimum salt (in grams) needs to be added to one litre of kitchen tap water to make the boiling temperatureincrease above 100 degree Celcius? ******* i'm desperate for an answer, cuz it's for an assignment if u cant give an answer, do uhave any idea of some useful websites that can help me? sincerely anonymous- anonymousaustralia
A: I've marked your question as a follow-up to one that already had an answer. Any amount of salt will raise the boiling point above100°C. The question is how much above that you wish to get. The answer above should help with that.
Mike W.
(published on 04/21/2011)
Follow-Up #4: salt nucleating steam
Q: I am confused now. Whenever I boil water for pasta I add the salt when I get impatiant waiting for it to boil. When I add the salt whenit is near boiling it instantly will boil. This is the exact opposite of then physics being described here. Is this due to anotherphunomina? Does the salt provide a point for the steam to nuecleate on?- Luke (age 30)Boston
A: Exactly, you guessed it.
Mike W.
(published on 01/17/2012)
Follow-Up #5: Using salt in humidifiers
Q: I'm sick and I'm trying to get my humidifier to put as much water in the air as possible. This is one of the cheap humidifiers that boilsthe water and makes steam. I think I read in the directions that if you add a pinch of salt it put more water in the air. How would thatwork? I mean, that would raise the boiling point, so maybe the water gets hotter, like 103 degrees celsius - but seems like it wouldjust take longer to boil. Once it hits 103 why would more steam come out than at 100?- John (age 45)Overland Park KS, USA
A: I use that sort of humidifier. Adding salt does work. The heating comes from the electrical current flowing through the water. Tapwater has a pretty low conductivity, so not much current flows. Adding salt raises the conductivity, since the ions are electricallycharged. You actually have to be a bit careful not to add too much salt, since you don't want to blow a fuse.
The effects on the boiling point are very minor compared to the effects on the conductivity.
BTW, although these humidifiers are cheap they do have the nice advantage that since they output water vapor, not drops, you don'shave to worry about bacteria etc. getting sprayed into the air.
Mike W.
(published on 02/24/2014)
Follow-up on this answer.
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Question: Does Adding SaltLower the Boiling Point ofWater?
Does adding salt lower theboiling point of water? You mayhave heard this and wondered ifit was true. Here is a look at thescience behind salt and boilingwater.
Answer: No, adding salt doesnot lower the boiling point ofwater. Actually, the opposite istrue. Addin salt to water resultsin a phenomenon called boilingpoint elevation. The boiling pointof water is increased slightly, butnot enough that you would
notice the temperature difference.
You would have to add 58 grams of saltjust to raise the boiling point of a liter ofwater by one half of a degre Celsius.
If you add salt to water, be sure to addit before boiling the water. Adding saltto water that is already boiling maycause the water to splash up and boilmore vigorously for a few seconds.
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Salt does not lower the boiling point of water. In fact, it causesthe water to boil at a slightly higher temperature. Markus Schweiss
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Q & A: Salt and the boiling point of water
Most recent answer: 02/23/2013
Q: Why does adding salt make the boiling temperature of water rise???- soeun leeAuckland Girls Grammar, Auckland, New Zealand
A: Souen- That's a good question. It turns out to be easy to give an answer to someone who's studied a little Statistical Mechanics, butI'll try to give an answer that doesn't assume that sort of background.
Salt (or other solutes, like sugar) can easily dissolve in liquid water. However, taking the solute out of the water and putting it in thegas phase (air) requires a lot of energy. At temperatures around the water boiling point, these solutes stay in the liquid.
Now the total pressure in the liquid and the air at the boundary are the same- otherwise one would push the other into a smallerspace. Part of the pressure in the liquid comes from the solutes, not the water. So the pressure due to the water alone is reducedcompared to that of pure water at the same temperature. The vapor pressure, meaning the pressure of water vapor that would stayin equilibrium with the liquid, is reduced by the same amount because of the solutes. (I've simplified and approximated a little here,since the pressure doesn't quite break up into separate parts due to the salt and the water.)
Water boils when the vapor pressure of the water gets to be as big as the pressure of the atmosphere. At that point, vapor bubbles inthe water can grow. You have to heat the liquid with solutes up more to get the vapor pressure in it to equal the atmosphericpressure, so it has a higher boiling point.
A very similar argument explains why solutes also lower the freezing point. Since the solutes are almost completely excluded fromthe solid (like from the gas) they stabilize the liquid. A search of this site will turn up some answers about freezing salt water.
Mike W. (and Tom J.)
(published on 10/22/2007)
Follow-Up #1: Why does salt water have a higher boiling point than distilled water?
Q: Why does salt water have a higher boiling point than distiller water? Can you please explain in terms of the positive and negativecharges of the particles and the rubbing off of electrons please? Thank you. My teacher taught it to us but I missed the lesson.- Ash (age 14)Australia
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A: Ash- I've marked your question as a follow-up to a similar question, which has a version of the answer.
Your teacher may have given an answer "in terms of the positive and negative charges of the particles and the rubbing off ofelectrons", but that sounds fundamentally wrong. Any solute in water raises the boiling point, so long as the solute stays in the liquidwater. It's true that part of why salt dissolves well in water is that it falls apart into charged particles, but some uncharged moleculesalso dissolve in water and also raise the boiling point.
Another way of putting the answer is to say that if the solute stays in the liquid, there is less room for it to find various different statesas the liquid boils away. When not so many states are available, we say that the "entropy" is reduced. The basic rule that tells uswhat will happen is that nature always heads toward an increase in entropy. So having solutes in there goes against boiling, whichdoesn't then occur until the temperature is higher. At the higher temperature it turns out that boiling still increases net entropy, thanksto the water molecules getting more space to run around.
By the way, something about this topic seems to bring out goofy answers. You might enjoy this:
http://van.physics.illinois.edu/qa/listing.php?id=16388
Mike W.
(published on 02/23/2013)
Follow-up on this answer.
© 1994 - 2015 The Board of Trustees at the University of Illinois :: Department of Physics :: College of Engineering :: University of Illinois at Urbana-ChampaignDepartment of Physics 1110 West Green Street Urbana, IL 61801-3080Questions? Contact Us
Phase changesTransitions between solid, liquid, and gaseous phases typically involve largeamounts of energy compared to the specific heat. If heat were added at aconstant rate to a mass of ice to take it through its phase changes to liquid waterand then to steam, the energies required to accomplish the phase changes(called the latent heat of fusion and latent heat of vaporization ) would lead toplateaus in the temperature vs time graph. The graph below presumes that thepressure is one standard atmosphere.
Temperature scales Water phase changes Boiling point
Water
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Energy Involved in the PhaseChanges of Water
The data for the vaporization phase change presumes that the pressure is onestandard atmosphere.
Why negative potentialenergy?
Heat offusion
Division ofenergy
Heat ofvaporization
Water
Index
Phasechange
concepts
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Why is the Potential EnergyNegative?
In discussing the energy of the phase changes in water, we found that thepotential energy is treated as a negative quantity. An analogy with a mechanicalsystem with gravitational potential energy and kinetic energy might be helpfulin understanding the logic of a negative energy quantity. You are always free tochoose the zero of potential energy, and it seems logical to choose the zero ofpotential energy such that a free molecule at rest has zero energy. A boundparticle at rest then has negative potential energy.
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Water phase changes
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Details of Heating WaterIt is known that 100 calories of energy must be added to raise thetemperature of one gram of water from 0° to 100°C. Part of that energyincreases the kinetic energy of the molecules, and some adds to the potentialenergy.
The sizes of the blockswhich represent thekinetic energy of themolecules at 0°C and100°C provide a visualillustration of themeaning of temperatureand the nature of theabsolute or Kelvintemperature scale. Fromthe definition of kinetic
temperature, the size ofthe block is seen to beproportional totemperature, and theratios of the heights ofthe KE blocks is theratio of the temperatures.But the kinetictemperature is inherentlythe absolutetemperature, so that theratio of the heights ofthe blocks is 373K/273K. So the absolutetemperature is actuallyproportional to thetranslational kineticenergy of the molecules,while the Celsiustemperatures are justchosen for convenience.
Water phase changes More detail about energy changes
Water
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Some energy details related toheating water
In the process of heating water from from 0 to 100 C, 100 calories of energymust be added. Part of that energy increases the kinetic energy of themolecules, and some adds to the potential energy. To assess the amountadded to kinetic energy, the molecular speeds at the two temperatures may beevaluated with the Boltzmann speed distribution.
The net gain in kinetic energy is then 16.7 calories/gram when the water isheated from 0 to 100 C. The remainder of the energy goes into weakeningthe attractive forces between the water molecules. This weakening of theintermolecular forces manifests itself in the reduction of the surface tensionof water as it is heated.
In the process of vaporization of water, a large amount of energy must beadded to overcome the remaining cohesive forces between the molecules andan additional amount of energy goes into PdV work to expand the gas fromits very small liquid volume to the volume occupied by the resulting vapor.
If the heat of vaporization of water at 100°C is 539 calories, then subtractingthe 41 calorie work component suggests that the actual binding energy of thewater molecules at 100°C is 539-41=498 calories.
Why is the heat of vaporization more at bodytemperature?
An interesting feature of the process of cooling the human body byevaporation is that the heat extracted by the evaporation of a gram ofperspiration from the human skin at body temperature (37°C) is quoted inphysiology books as 580 calories/gm rather than the nominal 540calories/gm at the normal boiling point. The question is, why is it larger atbody temperature?
The main part of the answer is that the binding energy of the water moleculesis greater at that lower temperature, and it therefore takes more energy tobreak them apart into the gaseous state. The change in the heat ofvaporization can be roughly calculated using what we know from the specificheat of water, 1 calorie/gm °C. It takes 37 calories to heat a gram of waterfrom 0°C to 37°C, but the change in the kinetic energy is much less than that:
Index
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concepts
It was shown above that the kinetic energy of the water molecules onlyincreases by 61.7 - 45 = 16.7 calories/gm when the water is heated from zeroto 100°C but we know it takes 100 calories to do that heating. Therefore thecontribution to weakening the water bonds is 83.3 calories/gm. Using theresult for water at 37°C it is evident that 52.4 calories of additional energymust be supplied at 37°C to vaporize the water.
There is one additional element in modeling the heat of vaporization at bodytemperature - the PdV work required to expand the water into its gaseousform is slightly less at 37°C. By analogy with the work calculation above,that work is found to be 34.2 calories/gm, 6.8 calories/gm less than at 100°C.
This model then suggests a heat of vaporization at 37°C:
Body temperature heat of vaporization = 539 cal/gm + 52.4 cal/gm - 6.8cal/gm = 585 cal/gm.
So this simple model agrees fairly well with the quoted 580 cal/gm.
Water phase changes
Water
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