GEN CHEM & QUAL ANALY
GEN CHEM & QUAL ANALY CHM 2046
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This 11 page Class Notes was uploaded by Brigitte Wyman on Friday September 18, 2015. The Class Notes belongs to CHM 2046 at University of Florida taught by George Christou in Fall. Since its upload, it has received 9 views. For similar materials see /class/207005/chm-2046-university-of-florida in Chemistry at University of Florida.
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Date Created: 09/18/15
Chapter 18 Thermodynamics Entropy and Free Enemy Section 182 Spontaneous Change and Eguilibrium We must now address My reactions occur and what controls the equilibrium position of a reaction To do this we must introduce the idea of a spontaneous process spontaneous means occurs by itself a spontaneous reaction or change occurs by itself until it reaches equilibrium whereas a non spontaneousquot reaction is in a direction away from equilibrium and can thus occur only if we force it to happen by inputting energy Let39s consider reactions that lie 100 o to one side or the other to help get the points across and then we will be more general later Examples 1 Sodium metal Nas will spontaneously react with Clz gas to give NaCl but NaCl will M spontaneously give Na metal and Clz gas unless we input energy 2 Iron metal will spontaneously rust when exposed to water and oxygen gas but rust will not spontaneously give Fe metal These examples are obvious but what about 9 Pb s 2 Agquot aq quot sz Ag 5 Which is the spontaneous direction in which the reaction will go by itself and which is the direction we must force it to go Similarly consider something falling eg a ball or chalkboard eraser Note spontaneousness or spontaneityquot does M tell us anything about rates of reaction only about whether it will occur by itself What factors determine which direction of a reaction is Spontaneous We Shall See in Chapter 18 and involves the 2nd Law of Thermodynamics First let39s refresh our memories of the 1ST Law First Law of Thermodynamics Conservation of Energy AE q w E internal energy of system sum of potential and kinetic energies q heat w work As chemists our system is usually a reaction in a beaker or39 flask everything else is surroundings Euniverse Esystem Esurroundings The universe consists of everything its energy is constant AEumV 0 AEuniv AEsys AEsurr 0 Ila AESys and AEsurr cancel each other out energy E cannot be created or destroyed This is the First Law of Thermodynamics NOTE First Law does not by itself help us to understand and predict the spontaneous direction however How is E the internal energy of the system related to H the enthalpy we learned about in Chapter 6 H E PV P pressure V volume AH AE PAV at constant pressure For most reactions in solution AV is insignificant AH H AE AH enthalpy change the heat gained or lost at constant pressure Can we predict spontaneity from the sign of AH Ila are all exothermic rxns AH lt 0 spontaneous are all endothermic rxns AH gt 0 nonspontaneous 19Th century scientists used to think so for a long time but the answer is NO CH4 g 2 02 g gt CO2 g 2 H20 g AH rxn 802 kJ Na 5 Cl2 g gt NaCl 5 AH rxn 411 kJ These spontaneous as written left to right their AH lt 0 exothermic But in general AH lt 0 does M define spontaneity There are reactions with AH gt 0 endothermic that are also spontaneous Mspontaneous reactions are usually exothermic but not always eg melting of ice H20 5 gt H20 l AH rxn 602 kJ evaporation of H20 H20 l gt H20 g AH rxn 440 kT AH gt 0 for both yet both occur spontaneously AH by itself does not determine spontaneity We must keep looking to find the law that determines spontaneity Sections 1834 Entropy and the 2quotd Law of Thermodynamics Spontaneity is determined by a thermodynamic function called ENTROPY symbol 5 units JoulesKelvin JK This is the basis of the 2nd law of Thermodynamics and allows us to predict when a process is spontaneous First let39s think about entropy What is an increase in entropy Increasing the dispersal of energy or matter over more energy states corresponds to an increase in entropy eg dispersing spreading energy over more atoms represents an increase in entropy think of a hot metal in contact with a cold one the heat will be spread out spontaneously eg dispersing spreading matter over a bigger volume represents an increase in entropy think of a gas in one container distributing between it and an empty container when connected Entropy is often related to the idea of DISORDER Increasing disorder ie decreasing order is increasing entropy Spreading dispersing heat energy over a bigger amount of metal or spreading dispersing the gas molecules over a greater volume are both examples of going to a situation with greater disorder smaller order Other examples Order Disorder new deck of cards shuffled deck crystal gas molecules atoms protein amino acids house pile of bricks Nature has an inherent tendency towards greater dispersal spreading out of energy and matter ie to greater disorder eg gas vs crystal A gas is much more disordered than a crystal A measure of dispersal or disorder in a system is ENTROPY 5 Increasing entropy S increasing dispersal disorder le sdisordered system gt sordered system eg 5gas gtgt 5crystal S is a state function like enthalpy H or energy E it depends on the present state of the system not how it got there The Boltzmann Equation for Entropy Entropy S is related to disorder number of equivalent ways of distributing energy In 1877 Boltzmann defined 5 S k lnW W No of ways system can be arranged k k3 Boltzmann39s Constant Interesting but rarely useful in practice because on a molecular level we rarely know or can measure W Thus entropy 5 measures the extent of disorder resulting from dispersal of energy and matter We can now state the rule for a spontaneous process Section 184 Second Law of Thermodynamics quot2quotd Law of Thermodynamics in a spontaneous process the change in the entropy of the universe is positive i e ASunimse gt 0 ie spontaneous processes occur in the direction that increases the entropy of the universe M but doesn39t help yet with figuring out which is the spontaneous direction of a reaction we39ll get to that later Now Asuniver se Assystem Assurroundings 39 ASSS ASSW gt 0 for a spontaneous process Note ASSYS may or may not be gt 0 positive Assurr may or may not be gt 0 positive but Asunme ALWAYS gt 0 for a spontaneous process First let39s look at ASSYS or ASH in detail we39ll return to ASSurr later Standard Molar Entropies 5 Remember from Chapter 6 that enthalpy H cannot be determined absolutely only changes AH However absolute entropies m be determined because of The 3quotd Law of Thermod namics A perfect crystal has zero entropy at absolute zero ie SSys 0 at 0 K As with AH we normally quote 5 at standard conditions for molar amounts ie 5 standard molar entropy JmolK or Jmol391K391 Standard conditions 1 atm pressure for gases 1 M molarities for solution 5 Predicting Relative Values See Appendix 3 for tables of standard molar entropies 5 at 298 K 25 C See also the table Table 191 Some Standard Molar Entropy Values at 298 K Entropy 5 Entropy 5 Element JK mol Compound JK rnol Cgraphite 56 CH4g 1863 Cdianiund 2377 C7Hg 2292 Cvapar 1581 C3H3g 2703 Cas 4159 CH30H 1272 Arg 1549 C0g 1977 Hg 1307 cog 2137 Hg 2051 H0g 18834 N2g 1916 H0I 6995 F2g 2028 HClg 1862 125 2231 NaCls 7211 Bra 1522 Mg0s 2685 125 1161 CaC03s 917 Let39s use ideas of dispersaldisorder and W ways of distributing energy 1 2 3 for di As temperature increases 5 increases ie AS gt 0 When more ordered phase gt more disordered phase AS gt 0 eg Na 5 gt Na 9 AS 1022 Jmol391K391 5 514 5 1536 Jmol391K391 Jmol391K391 Dissolution of solid or liguid increases its entropy M we must be careful to also consider the entropy of the solvent 5 NaCl 5 721 but AlClg s 109 NaCl aq 1151 AlClg aq 148 ssolution of NaCl AS gt 0 but for dissolution of AlClg AS lt 0 Why Although AlClg breaks up into more disordered Al3 and 3 Cl39 the water molecules become highly ordered around small highly charged Al3 50 usually A5 for dissolution gt 0 positive occasionally A5 lt 0 if it involves a small highly charged ion such as AI3 For nonionic substances A5 much smaller due to no separation into ions For mixing H20 and MeOH or EtOH there is little change to Hbonding interactions A5 small Pure MeOH l 5 127 MeOH aq 5 132 A5 5 Jmol391K391 4 Dissolution of a gas Gases are very disordered so dissolution gives a more ordered solution entropy always decreases on dissolution A5 lt 0 02g 2050 JmolK but 02aq 1109 JmolK However mixing one gas into another M increase entropy 5 Complexity of the substance For a given phase 5 depends on atomic size and molecular complexity eg Li gt Na gt K gt Rb gt Cs down a group 5 s 28 51 64 69 83 Jmol391K391 Also CF4 g 2615 lt CCI4 g 3097 Jmol391K391 For different forms of an element 5 is less for the form with stronger more directional bonds eg C diamond vs C graphite 5 diamond lt 5 graphite For compounds ionic or molecular 5 greater for greater complexity number of atomsions involved E phase is the same 69 Clg lt dz 9 09 lt 029 lt 039 1651 2230 161 205 238 CH4 9 lt CzHe 9 C3H8 9 C4H10 9 1861 2295 2699 310 Careful CH4 9 lt CHgOH g MLer compare 186 238 gas with gas bl CH4 9 gt CHSOH I otherwise complexity 186 127 rule doesn39t hold Calculating Entropy Changes A Let39s also learn to predict the sign of A5 erx useful Example 1 N2 9 3 H2 9 2 2 NH3 9 Change in entropy AS 5 products 5 reactants Prediction 4 moles of gas gt 2 moles of gas greater disorder gt less disorder less order gt more order 39 we predict change in entropy AS will be lt 0 negative Calculation A50rxn or A50 stopr od 39 Znsoreacts m and n are coefficients in eqn AS 2mol5 NH3 1mol N2 3mol5 H2 AS 2 193 1 915 31306 JK A5 197 JK note units this is AS for the reaction as written above AS lt 0 as predicted Example 2 Na 5 gt Na g prediction solid gt gas we predict AS rxn gt 0 calculation Asor xn soprod 39 soreact 15361 5145 JK AS 10216 JK gt 0 as predicted Example 3 C3H8 g 5 02 g gt 3 COZ g 4 H20 l prediction 6 moles gas gt 3 moles gas Decrease in disorder we predict AS rxn lt 0 M in rxns with gases liquids or solids gases are so much more disordered than liquidssolids that they dominate what happens to Asorxn calculation AS rxn soprods soreacts AS an 32137 4699 12699 52050 JK A5 3742 JK lt 0 as predicted Now let39s look at ASSW for the last reaction above This is actually a spontaneous reaction ASumV gt 0 since Asumv gt 0 and ASrxn lt 0 39 it m be that ASSurr gtgt 0 Remember Asuniv ASrgtltn Assurr if reaction is spontaneous Asumv gt 0 ASrgtltn Assurr gt 0 So if Asmw 0 negative then ASSurr must be gt 0 ositive to make the sum of the two ASrxn ASSurr also be gt 0 LeT39s Think abouT ASEW more closely IT Turns ouT ThaT ASSW is relaTed To AHM Why Think abouT iT39 AHrxn gives heaT q T0 or Takes heaT from The surroundings The enTropy ASSurr of The surroundings will be changed 1 If AHrxn lt 0 exoThermic qSys lt 0 qsurr gt 0 ASSurr gt 0 2 If AHrxn gt 0 endoThermic qSys gt 0 qsurr lt 0 ASSurr lt 0 Thus heaTenThalpy changes in The sysTem The reacTion cause SSUPP To change and Therefore ASSurr st 0 39 ASSurr depends on qSys ie AHW and The TemperaTure T of surroundings The relaTionship is Assw So ASSurr is given by Aern AHSYS and The T aT which The reacTion Takes place So consider again N2 g 3 H2 g gt 2 NH3 g AS rxn 197 JK AHor xn 2mol459 kJmol1mol0 kJmol 3mol0 kJmol 918 kJ 91800 J o AHrxn 91800J As surr T W A5 umV AS sys AS Surr 197 JK 308 JK 111 JK A5 umV 111 JK A5 umV gt 0 reacTion is sponTaneous as wriTTen lefT TorighT Entropy Changes at Equilibrium What is the situation for a reaction that has reached equilibrium At equilibrium no net change is occurring Asum 0 Assys ASSN because ASumV Assys Assurr 0 Example H20 I 2 H20 9 AHvaPorimion 407 kJmol this is an equilibrium at 100 C 373 K AS SYS AS PmdA5 mc1959868JK 109 JK A50 AHosys AHovap T T 373K 109 JK Asuniv Assys Assurr 0 This is true for both forward and reverse directions at equilibrium no movement away from equilibrium in either direction is spontaneous Ila no change Summa A reaction is spontaneous if Asumv gt 0 le Asuniv Assys Assurr gt 0 For a spontaneous reaction if AH lt 0 exothermic ASSurr gt 0 and Assys can be gt 0 g lt 0 if AH gt 0 endothermic ASSurr lt 0 and Assys must be gt 0 A ASSUY Y A T At equilibrium no movement from equilibrium position is spontaneous Assys Assurr
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