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Ingenieurwesen

The Measure of Enthalpy and Entropy and the difference between the two concepts.

Mark AbkumFolgen

Florida State UniversityAnzeige

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- Mr. Bartelt Presents The greatest fight in the universe Enthalpy vs. Entropy
- EnthalpyEnthalpy We already know about enthalpy.We already know about enthalpy. Enthalpy is related to the amount ofEnthalpy is related to the amount of energy that is lost or gained by a systemenergy that is lost or gained by a system during a normal chemical process.during a normal chemical process. We have seen both endothermic andWe have seen both endothermic and exothermic processes in lab. Which doexothermic processes in lab. Which do you think is favored by nature?you think is favored by nature? Why?Why?
- ΔΔHHvapvap andand ΔΔHHfusfus Last unit we looked at the change inLast unit we looked at the change in enthalpy associated with a chemicalenthalpy associated with a chemical reaction and the energy flow as materialsreaction and the energy flow as materials (mostly water) are heated.(mostly water) are heated. Today we’re going to extend that lesson toToday we’re going to extend that lesson to include phase change.include phase change.
- ΔΔHHvapvap andand ΔΔHHfusfus If you put a thermometer in a glass of iceIf you put a thermometer in a glass of ice water, the temperature will read 273 Kwater, the temperature will read 273 K until all the ice melts. Only after all the iceuntil all the ice melts. Only after all the ice melts will the temperature begin its crawlmelts will the temperature begin its crawl up to room temperature.up to room temperature. For similar reasons, the temperature ofFor similar reasons, the temperature of boiling water will never exceed 373 K.boiling water will never exceed 373 K. Why is this?Why is this?
- ΔΔHHvapvap andand ΔΔHHfusfus Melting ice and boiling water are bothMelting ice and boiling water are both endothermic processes and as such theyendothermic processes and as such they both require energy.both require energy. If any of the water were to increase inIf any of the water were to increase in temperature that energy wouldtemperature that energy would immediately be transferred to the ice toimmediately be transferred to the ice to cause it to melt.cause it to melt. For waterFor water ΔΔHHvapvap = 43.9 kJ/mol= 43.9 kJ/mol ΔΔHHfusfus = 6.02 kJ/mol= 6.02 kJ/mol
- ΔΔHHvapvap andand ΔΔHHfusfus Examples, Given (for water)Examples, Given (for water) ΔΔHHvapvap = 43.9 kJ/mol= 43.9 kJ/mol ΔΔHHfusfus = 6.02 kJ/mol= 6.02 kJ/mol 1.1. How much energy is required to vaporizeHow much energy is required to vaporize 10.0 g of water? (easy)10.0 g of water? (easy) 2.2. How much energy is required to meltHow much energy is required to melt 10.0 g of water? (easy)10.0 g of water? (easy) 3.3. How much energy would it take to raiseHow much energy would it take to raise the temperature of ice at 268 K to 378the temperature of ice at 268 K to 378 K? (getting harder)K? (getting harder)
- Hard problemsHard problems How much boiling water would you needHow much boiling water would you need to add to a cup with 100.0 g of ice andto add to a cup with 100.0 g of ice and 100.0 g of water at equilibrium to get all100.0 g of water at equilibrium to get all the ice to melt? (hard)the ice to melt? (hard) If you doubled that amount of water whatIf you doubled that amount of water what would the final temperature be? (hard)would the final temperature be? (hard)
- A new lookA new look If 90.0 g of water at 300. K are added toIf 90.0 g of water at 300. K are added to 30.0 g of water at 350. K what will the30.0 g of water at 350. K what will the final temperature be?final temperature be? It sounds hard, but there’s a trick:It sounds hard, but there’s a trick: Just make a weighted average:Just make a weighted average: (90.0*300+30.0*350)/120 = 312.5(90.0*300+30.0*350)/120 = 312.5
- The baseline approachThe baseline approach Some problems will require a moreSome problems will require a more sophisticated approachsophisticated approach How many liters of steam at 1 atm andHow many liters of steam at 1 atm and 373 K will it take to melt exactly 75.0 g of373 K will it take to melt exactly 75.0 g of ice at STP?ice at STP? It sounds really hard, but it can be workIt sounds really hard, but it can be work quite simply with the baseline approachquite simply with the baseline approach
- How to do itHow to do it When the system reaches equilibrium all speciesWhen the system reaches equilibrium all species will be liquid water at 273 K. That is our baseline.will be liquid water at 273 K. That is our baseline. Next we find how much energy will be required toNext we find how much energy will be required to melt all that ice. That’s easymelt all that ice. That’s easy kJ25.1 mol1 kJ6.02 18.0g mol1 75.0g =•• That’s how much energy it takes to get our ice to the baseline.That’s how much energy it takes to get our ice to the baseline. Since heat gained by ice is going to equal heat lost by steam we nowSince heat gained by ice is going to equal heat lost by steam we now need to find how much energy is released when one mole of steam isneed to find how much energy is released when one mole of steam is brought down to baseline (273 K as a liquid)brought down to baseline (273 K as a liquid)
- SteamSteam What happens when 1 mole of steam is broughtWhat happens when 1 mole of steam is brought down to 273 K and remains a liquid?down to 273 K and remains a liquid? 1.1. It condenses: 43.9 kJ will be released when thisIt condenses: 43.9 kJ will be released when this happenshappens 2.2. It cools from 373 K to 273 KIt cools from 373 K to 273 K ( ) ( ) kJ2.57 J/kJ1000 K100. gK J 4.1818.0g ΔH = • = This means that cooling a mole of water fromThis means that cooling a mole of water from boiling to freezing releases 75.2 kJ of energy.boiling to freezing releases 75.2 kJ of energy. Add that to the heat released from theAdd that to the heat released from the condensation process and you get 43.9+75.2 =condensation process and you get 43.9+75.2 = 119.1 kJ/mol119.1 kJ/mol
- Finishing upFinishing up If 119.1 kJ are released when one mole of steam is cooledIf 119.1 kJ are released when one mole of steam is cooled to 273 K (as water) then how many moles of steam will beto 273 K (as water) then how many moles of steam will be required to release the 25.1 kJ of energy required to meltrequired to release the 25.1 kJ of energy required to melt our 75.0 g of ice?our 75.0 g of ice? Easy ratio:Easy ratio: Finally use PV=nRT to solve for the volume in liters.Finally use PV=nRT to solve for the volume in liters. steammol0.211x kJ25.1 x kJ119.1 mol1 =⇒= K)(373 molK atmL 0.08206mol)(0.211atm)V(1 • • =
- EntropyEntropy If you thought that enthalpy was abstract,If you thought that enthalpy was abstract, welcome to Entropy.welcome to Entropy. Entropy can be thought of as the force inEntropy can be thought of as the force in the universe that pushes everythingthe universe that pushes everything toward disorder and chaos.toward disorder and chaos. The second law of thermodynamicsThe second law of thermodynamics states: In any spontaneous process therestates: In any spontaneous process there is always an increase in the entropy of theis always an increase in the entropy of the universe.universe.
- Enter ShivaEnter Shiva Shiva is the Hindo lord ofShiva is the Hindo lord of chaos. He/she (inchaos. He/she (in Hindoism he/she is aHindoism he/she is a hermaphrodite) is thehermaphrodite) is the “lord of chaos”.“lord of chaos”. Whenever a processWhenever a process produces an increase inproduces an increase in Entropy Shiva is happy.Entropy Shiva is happy.
- Enthalpy vs. Entropy (Shiva)Enthalpy vs. Entropy (Shiva) We know that the amount of energy in theWe know that the amount of energy in the universe is constant but the Entropy of theuniverse is constant but the Entropy of the Universe is always increasing.Universe is always increasing. Shiva is happy =Shiva is happy = Shiva’s lawShiva’s law ΔΔSSunivuniv == ΔΔSSsyssys ++ ΔΔSSsurrsurr For any reaction the change in entropy of the universe isFor any reaction the change in entropy of the universe is going to be equal to the change in entropy of our systemgoing to be equal to the change in entropy of our system plus the change in entropy of the surroundings. If theplus the change in entropy of the surroundings. If the change in entropy of the universe is positivechange in entropy of the universe is positive and theand the reaction will happen spontaneously. If it’s negativereaction will happen spontaneously. If it’s negative and it will not happen, end of story.and it will not happen, end of story.
- What makesWhat makes Disorder!!! A gas his more scattered andDisorder!!! A gas his more scattered and spread outspread out Look on page 787Look on page 787 Entropy is increased if there are moreEntropy is increased if there are more possible arrangements at the end of thepossible arrangements at the end of the reaction than beforereaction than before The mores spread out the better. Shiva isThe mores spread out the better. Shiva is eagerly awaiting theeagerly awaiting the heat death of the universeheat death of the universe, when, when maximum entropy is achieved. This willmaximum entropy is achieved. This will take a while, fear not.take a while, fear not.
- TemperatureTemperature Entropy is always increasing but we canEntropy is always increasing but we can manipulate entropy by changing themanipulate entropy by changing the temperature.temperature. What has more disorder, ice or liquidWhat has more disorder, ice or liquid water?water? Then why does ice ever form?Then why does ice ever form? HH22OO(l)(l) HH22OO(s)(s) ΔΔS = -#S = -# So how can this happen????????????So how can this happen????????????
- Endo or exothermic?Endo or exothermic? Is the freezing of water endothermic orIs the freezing of water endothermic or exothermic?exothermic? This release of energy from the systemThis release of energy from the system makes the surroundings more disordered.makes the surroundings more disordered. Hence,Hence, ΔΔSSsurrsurr = +#= +# ΔΔSSunivuniv == ΔΔSSsurrsurr ++ ΔΔSSsyssys As long asAs long as ΔΔSSunivuniv is +is + and it mayand it may proceed.proceed.
- ΔΔSSunivuniv == ΔΔSSsurrsurr ++ ΔΔSSsyssys The sign ofThe sign of ΔΔSSsyssys is easy to predict. If your systemis easy to predict. If your system gets more orderly, thegets more orderly, the ΔΔSSsyssys is negativeis negative , but if it, but if it gets more chaotic thengets more chaotic then ΔΔSSsyssys is positiveis positive .. But the system is only half the picture, theBut the system is only half the picture, the surroundings are important as well.surroundings are important as well. When ice melts, it throws energy into theWhen ice melts, it throws energy into the surroundings and makes them more chaoticsurroundings and makes them more chaotic .. So the important question is:So the important question is: What causes freezing to take place at certainWhat causes freezing to take place at certain temperatures and not at other temperatures?temperatures and not at other temperatures?
- ΔΔSSsurrsurr is T dependentis T dependent The above equations are important.The above equations are important. When water freezesWhen water freezes ΔΔSSsyssys becomes more orderedbecomes more ordered But the process is exothermic, which makes theBut the process is exothermic, which makes the surroundings more chaoticsurroundings more chaotic One term (One term (ΔΔSSsyssys oror ΔΔSSsurrsurr) needs to dominate) needs to dominate ΔΔSSsyssys is temperature dependentis temperature dependent At high temperturesAt high tempertures ΔΔSSsyssys becomes small and weakbecomes small and weak T ΔH ΔSsurr −= syssurruniv ΔSΔSΔS +=
- ΔΔSSsurrsurr is T dependentis T dependent syssurruniv ΔSΔSΔS += T ΔH ΔSsurr −= ( ) ( ) 0ΔSTΔH Thus ΔSTΔH-ST ΔS T ΔH -ΔS ΔSΔSΔS sys sysuniv sysuniv syssurruniv <− −=∆ += += The reaction is spontaneous
- Gibbs free energyGibbs free energy Gibbs free energy is defined asGibbs free energy is defined as G=H-TS (does this look familiar?)G=H-TS (does this look familiar?) Where H = enthalpyWhere H = enthalpy T = temp (K)T = temp (K) S = entropyS = entropy The free energy equation can be rewritten:The free energy equation can be rewritten: ΔΔG =G = ΔΔH - TH - T ΔΔSS Note: that ifNote: that if ΔΔG is negative, the reaction willG is negative, the reaction will occur spontaneously.occur spontaneously. -(-(ΔΔH - TH - T ΔΔS) = T(S) = T(ΔΔSSunivuniv) = -) = -ΔΔGG
- Let’s look at a few examplesLet’s look at a few examples ΔΔG =G = ΔΔH - TH - T ΔΔSS ΔΔHH ΔΔSS ΔΔGG Spontaneous ?Spontaneous ? -- ++ -- YES!!!!!!!YES!!!!!!! ++ -- ++ NO!!!!!!!!NO!!!!!!!! -- -- ?? Depends on TDepends on T ++ ++ ?? Depends on TDepends on T
- CondensationCondensation When water condenses your systemWhen water condenses your system becomes more organizedbecomes more organized However, condensation is exothermic andHowever, condensation is exothermic and releases energy which makes thereleases energy which makes the surroundings more disorderedsurroundings more disordered This means that temperature determines ifThis means that temperature determines if water will freezewater will freeze But how do we calculateBut how do we calculate ΔΔS?S?
- CalculatingCalculating ΔΔSS ΔΔS is calculated in a similar fashion toS is calculated in a similar fashion to ΔΔH.H. ΔΔS is looked up in the back of the bookS is looked up in the back of the book Back to the example of the freezing waterBack to the example of the freezing water HH22OO(g)(g) HH22OO(l)(l) ΔΔSSreactionreaction == ΔΔSSproductsproducts –– ΔΔSSreactantsreactants (Just like(Just like ΔΔH)H) ΔΔSSreactionreaction = 69.96 – 188.7 = -118.7= 69.96 – 188.7 = -118.7 JJ/mol*K/mol*K NOTE:NOTE: ΔΔS is in Joules NOT kJS is in Joules NOT kJ Kmol J 188.7S Kmol J 69.96S (g)2 (l)2 OH OH • = • =
- CalculatingCalculating ΔΔGG ΔΔSSreactionreaction = 188.7 – 69.96 = -118.7= 188.7 – 69.96 = -118.7 Now that we haveNow that we have ΔΔS all we need it T andS all we need it T and ΔΔHH and we can findand we can find ΔΔG usingG using HH22OO(g)(g) HH22OO(l)(l) ΔΔHHreactionreaction == ΔΔHHproductsproducts –– ΔΔHHreactantsreactants ΔΔHHreactionreaction = -286 –(-242) = - 44 kJ/mol= -286 –(-242) = - 44 kJ/mol Kmol kJ 2.422H Kmol kJ 286-H (g)2 (l)2 OH OH • −= • =
- CalculatingCalculating ΔΔG (cont…)G (cont…) ΔΔSSreactionreaction = - 118.7 J/mol= - 118.7 J/mol -0.1187 kJ/mol-0.1187 kJ/mol ΔΔHHreactionreaction = - 44 kJ/mol= - 44 kJ/mol Now we plug and chug. If T = 300 K we wouldNow we plug and chug. If T = 300 K we would expect condensation to take place spontaneously.expect condensation to take place spontaneously. ΔΔG =G = ΔΔH - TH - T ΔΔSS ΔΔG = -44 –(300)(-0.1187)G = -44 –(300)(-0.1187) ΔΔG = -8.39 kJ/molG = -8.39 kJ/mol But what about @ 444 K???But what about @ 444 K??? ΔΔG = -44 –(444)(-0.1187)G = -44 –(444)(-0.1187) ΔΔG = +8.7 kJ/molG = +8.7 kJ/mol
- ΔΔG and Boiling point TempG and Boiling point Temp ΔΔG can be used to predict boiling point.G can be used to predict boiling point. At the boiling point HAt the boiling point H22OO(g)(g) and Hand H22OO(l)(l) are inare in equilibrium, hence the reaction is spontanious inequilibrium, hence the reaction is spontanious in both directions or in neither (take your pick)both directions or in neither (take your pick) RegardlessRegardless ΔΔG = 0G = 0 ΔΔSSreactionreaction = - 0.1187 kJ/mol= - 0.1187 kJ/mol ΔΔHHreactionreaction = - 44 kJ/mol= - 44 kJ/mol ΔΔG =G = ΔΔH - TH - T ΔΔSS 0 = -44 – (T)(-0.1187)0 = -44 – (T)(-0.1187) T=371 KT=371 K 98ºC (pretty close to water’s BP)98ºC (pretty close to water’s BP)

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