化学原理Chemistry课件-post+9+Thermodynamics.ppt
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1、Chapter 18 Thermodynamics In this Chapter,we will address several aspects as follows:I)How to predict whether a reaction under a given conditions can occur or not?II)How to make the reaction happen,if it does not proceed under primary conditions?p Spontaneity and Entropy(S).p The Second and Third La
2、ws of thermodynamics.p Free Energy(G)and Equilibrium(K).Spontaneous process:spontaneousnon-spontaneous18.2Go ahead by itself,without help from surroundings.spontaneousNon-spontaneousWhat are the factors controlling the spontaneity?CH4(g)+2O2(g)CO2(g)+2H2O(l)DH0=-890.4 kJH+(aq)+OH-(aq)H2O(l)DH0=-56.2
3、 kJH2O(s)H2O(l)DH0=6.01 kJNH4NO3(s)NH4+(aq)+NO3-(aq)DH0=25 kJH2O Energy Freedom Entropy(S):a measure of system disorder.orderSdisorderSDS=Sf-SiDS 0For a given substance:Ssolid Sliquid 018.2Heating hydrogen gas from 600C to 800CCrystallization from a supersaturated solutionDS 0For a process:Entropy i
4、s also a State function.E1H1S1E2H2S2n1 P1V1T1n2 P2V2T2State 1State 2DE=E2 E1DH=H2 H1DS=S2 S1The value of S is only determined by the state,regardless of how the state is reached.How to quantify the Entropy?MicrostatesW=1W=4W=6SIn 1868,Boltzmann proposedS=k ln WDS=Sf-SiFor a process:DS=k lnWfWiBoltzm
5、ann constant k=1.3810-23 J/KFor CH3OH and C2H5OH:Smethanol 0(1)Spontaneous process:DSuniv=DSsys+DSsurr=0(2)Equilibrium process:In order to predict the direction of a chemical reaction,however,both DSsys and DSsurr need to be calculated,which is a question remaining to be solved.How to calculate DSsu
6、rr,the entropy change in the surroundings?Endothermic DHsys 0DSsurr DHsysD DSsurr 0For a constant-pressure process,it is known that Qp=DH.Exothermic DHsys 0D DSsurr 0for a spontaneous process:DSuniv=DSsys-0surrsysTHDTsurr DSuniv=Tsurr DSsys-DHsys 0-Tsurr DSuniv=DHsys-Tsurr DSsys 0DG 0 non-spontaneou
7、s DG=0 at equilibrium.Definition:G=H-T S G is called Gibbs free energydG=dH T dS S dTat constant P and T:DHsys-Tsys DSsys 0For a process at constant P and T DGsys=DHsys Tsys DSsysNote that S has a true and absolute value,which can be determined by experiment,or by statistics.This is quite different
8、from E and H,the values of which are not available by experiments.For any perfect crystalline substance at 0 K,the entropy S=0.The Third Law of Thermodynamicsp the particles(atom,molecule or ion)arranged in space in a mostly optimized way;p No defect sites both in bulk and on the surface;p Free of i
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