Chapter-19 - CHAPTER 19 ENTROPY and GIBBS FREE ENERGY...

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1 CHAPTER 19 ENTROPY and GIBBS FREE ENERGY spontaneous reactions entropy (S) and disorder 2 nd law of thermodynamics S(reaction) • Gibbs free energy (G, G) G(reaction) and equilibrium
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2 Spontaneous reactions 1 st law of thermodynamics (conservation of energy) endo or exothermic H(rxn) = f( H f °’s) H(rxn) = f(BE’s) complete rxn ? equilibrium ? spontaneous ?
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3 strongly exothermic rxns are generally spontaneous at RT P 4 (s) + 5 O 2 (g) P 4 O 10 (s) H -3000 kJ ! not all exothermic rxns are spontaneous at RT N 2 (g) + 3 H 2 (g) 2 NH 3 (g) H = -85 kJ diamond graphite H = -1.9 kJ due to very slow kinetics - how?
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4 endothermic rxns can be spontaneous - melting of ice at RT H = +6 kJ/mol - sublimation of CO 2 (s) (dry ice) at RT H = +25 kJ/mol - NH 4 NO 3 (s) + H 2 O NH 4 + (aq) + NO 3 - (aq) H = +26 kJ/mol - boiling of water at 100°C, 1 atm H = +44 kJ/mol
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5 spontaneity = irreversibility reactants products forw. rev. if the forward rxn is spontaneous and complete, then the reverse rxn cannot be (under the same conditions) 2 Na(s) + H 2 O 2 NaOH(aq) + H 2 (g) mixing vs. unmixing of aqueous solutions spontaneity depends on entropy change ( S)
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6 Entropy and disorder expansion of a gas into a vacuum spontaneous + irreversible ideal gas at constant T, U = 0, q = 0, w = 0 spontaneous increase in disorder of system (gas molecules dispersed over larger volume )
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7 spontaneous heat transfer from hot to cold T object and T surr. U object + U surr. = 0 < 0 > 0 spontaneous dispersion of energy among object and surroundings increased disorder T
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8 spontaneous disorder is due to higher statistical probability 2 “ordered” states (configurations) 6 “disordered” states a disordered state is 3 times more probable
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9 spontaneous expansion of a gas # molecules # states 2 2:0 1:1 0:2
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