Ch20 equilibrium.ppt

Philip DuttonUniversity of Windsor, Canada N9B 3P4

Prentice-Hall 2002

General ChemistryPrinciples and Modern ApplicationsPetrucci Harwood Herring 8th EditionChapter 20: Spontaneous Change: Entropy and Free Energy

Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 37Contents20-1Spontaneity: The Meaning of Spontaneous Change20-2The Concept of Entropy20-3Evaluating Entropy and Entropy Changes20-4Criteria for Spontaneous Change: The Second Law of Thermodynamics20-5Standard Fee Energy Change, G20-6Free Energy Change and Equilibrium20-7G and Keq as Functions of Temperature20-8Coupled ReactionsFocus On Coupled Reactions in Biological Systems

General Chemistry: Chapter 20

Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 3720-1 Spontaneity: The Meaning of Spontaneous Change


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 37Spontaneous ProcessA process that occurs in a system left to itself.Once started, no external actions is necessary to make the process continue.A non-spontaneous process will not occur without external action continuously applied.4 Fe(s) + 3 O2(g) 2 Fe2O3(s)H2O(s) H2O(l)


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 37Spontaneous ProcessPotential energy decreases.For chemical systems the internal energy U is equivalent to potential energy.

Berthelot and Thomsen 1870sSpontaneous change occurs in the direction in which the enthalpy of a system decreases.Mainly true but there are exceptions.


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 3720-2 The Concept of EntropyEntropy, S.The greater the number of configurations of the microscopic particles among the energy levels in a particular system, the greater the entropy of the system.

S > 0 spontaneous

U = H = 0


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 37The Boltzmann Equation for EntropyStates, S.The microscopic energy levels available in a system.Microstates, W.The particular way in which particles are distributed amongst the states. Number of microstates = W.The Boltzmann constant, k.Effectively the gas constant per molecule = R/NA.S = k lnW


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 37Boltzmann Distribution


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 37Entropy Change


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 3720-3 Evaluating Entropy and Entropy ChangesPhase transitions.Exchange of heat can be carried out reversibly.H2O(s, 1 atm) H2O(l, 1 atm) Hfus = 6.02 kJ at 273.15 K


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 37Troutons Rule


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 37Raoults LawPA = APA


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 37Absolute EntropiesThird law of thermodynamics.The entropy of a pure perfect crystal at 0 K is zero.Standard molar entropy.Tabulated in Appendix D.S = [ pS(products) - rS(reactants)]


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 37Entropy as a Function of Temperature


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 37Vibrational Energy and Entropy


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 3720-4 Criteria for Spontaneous Change:The Second Law of Thermodynamics.Stotal = Suniverse = Ssystem + SsurroundingsThe Second Law of Thermodynamics:Suniverse = Ssystem + Ssurroundings > 0All spontaneous processes produce an increase in the entropy of the universe.


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 37Free Energy and Free Energy ChangeHypothetical process:only pressure-volume work, at constant T and P.qsurroundings = -qp = -HsysMake the enthalpy change reversible.large surroundings, infinitesimal change in temperature.Under these conditions we can calculate entropy.


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 37Free Energy and Free Energy ChangeTSuniv. = TSsys Hsys = -(Hsys TSsys)-TSuniv. = Hsys TSsysG = H - TSG = H - TSFor the universe:For the system:Gsys = - TSuniverse


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 37Criteria for Spontaneous ChangeGsys < 0 (negative), the process is spontaneous.Gsys = 0 (zero), the process is at equilibrium.Gsys > 0 (positive), the process is non-spontaneous.


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 37Table 20.1 Criteria for Spontaneous Change


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 3720-5 Standard Free Energy Change, GThe standard free energy of formation, Gf.The free energy change for a reaction in which a substance in its standard state is formed from its elements in reference forms in their standard states.

The standard free energy of reaction, G.

G = [ p Gf(products) - r Gf(reactants)]


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 3720-6 Free Energy Change and Equilibrium


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 37Free Energy Change and Equilibrium


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 37Relationship of G to G for Nonstandard Conditions2 N2(g) + 3 H2(g) 2 NH3(g)G = H - TSG = H - TS For ideal gases H = H G = H - TS


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 37Relationship Between S and Sqrev = -w = RT lnVfVi


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 372 N2(g) + 3 H2(g) 2 NH3(g)SNH3 = SNH3 Rln PNH3SN2 = SN2 Rln PN2SH2 = SH2 Rln PH2Srxn = 2(SNH3 Rln PNH3) 2(SN2 Rln PN2) 3(SH2 Rln PH2)


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 37G Under Non-standard Conditions G = H - TS G = G + RT ln Q


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 37G and the Equilibrium Constant KeqG = G + RT ln QG = -RT ln Keq


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 37Criteria for Spontaneous ChangeEvery chemical reaction consists of both a forward and a reverse reaction.The direction of spontaneous change is the direction in which the free energy decreases.


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 37Significance of the Magnitude of G


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 37The Thermodynamic Equilibrium Constant: ActivitiesFor ideal gases at 1.0 bar:Therefore, in solution:PV=nRT or P=(n/V)RT, pressure is an effective concentration The effective concentration in the standard state for an ideal solution is c = 1 M.


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 37ActivitiesFor pure solids and liquids:a = 1For ideal gases: a = P(in bars, 1 bar = 0.987 atm)For ideal solutes in aqueous solution:a = c(in mol L-1)


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 37The Thermodynamic Equilibrium Constant, KeqA dimensionless equilibrium constant expressed in terms of activities.Often Keq = KcMust be used to determine G.


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 3720-7 G and Keq as Functions of TemperatureG = H -TSG = -RT ln Keq


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 37Temperature Dependence of Keq-H= R slope= -8.3145 J mol-1 K-1 2.2104 K= -1.8102 kJ mol-1


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 3720-8 Coupled ReactionsIn order to drive a non-spontaneous reactions we changed the conditions (i.e. temperature or electrolysis)Another method is to couple two reactions.One with a positive G and one with a negative G.Overall spontaneous process.


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 37Smelting Copper OreCu2O(s) + C(s) 2 Cu(s) + CO(g)Spontaneous reaction!-50 kJ


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 37Focus On Coupled Reactions in Biological SystemsADP3- + HPO42- + H+ ATP4- + H2OG = -9.2 kJ mol-1But [H3O+] = 10-7 M not 1.0 M.G = -9.2 kJ mol-1 + 41.6 kJ mol-1 = +32.4 kJ mol-1 = G'The biological standard state:


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 37Focus On Coupled Reactions in Biological Systems


Prentice-Hall 2002General Chemistry: Chapter 20Slide * of 37Chapter 20 Questions Develop problem solving skills and base your strategy not on solutions to specific problems but on understanding.

Choose a variety of problems from the text as examples.

Practice good techniques and get coaching from people who have been here before.


Chemistry 140 Fall 2002Chemistry 140 Fall 2002*Chemistry 140 Fall 2002Chemistry 140 Fall 2002*Chemistry 140 Fall 2002Chemistry 140 Fall 2002*Ludwig Boltzmann.Associated the number of energy levels in a system with the number of ways of arranging the particles in these levels.

Chemistry 140 Fall 2002Chemistry 140 Fall 2002*Failures of this rule are understandable:Water and hydrogen bonding. Sliquid is lower than expected so S is larger than 87.

Chemistry 140 Fall 2002Chemistry 140 Fall 2002* are the stoichiometric coefficients.Chemistry 140 Fall 2002Chemistry 140 Fall 2002*Want to evaluate the system only, not the surroundings.Chemistry 140 Fall 2002Chemistry 140 Fall 2002*Activity is the effective concentration divided by the effective concentration in the standard state.

Ch20 equilibrium.ppt

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