3/24/10 1 57 What is the boiling point of ethanol, CH 3 CH 2 OH(l), under standard conditions ? CH 3 CH 2 OH(l) ! CH 3 CH 2 OH(g) ! H f o-238.6 kJ/mol-201.2 kJ/mol S o 127 J/mol K 238 J/mol K At equilibrium, ! G º = 0 Use: ! G º = ! H º- T ! S º ! S o = 238 " 127 = 111 J/K A) 298 K B) 373 K C) 273 K D) 337 K E) 0.3 K ! H o = -201.2 – (-238.6) = 37.4 kJ T = ! H o ! S o = 37.4 " 10 3 J 111 J/K = 337 K T < 337 K < T Spontaneous condensation Spontaneous boiling T B or b.p Experimental b.p. 352 K Assume T-independent 4.4.D. Non-standard free energy (Section 20.4) relative amounts of reactants or products could also drive a reaction i.e., when you no longer have 1bar/1M/T (298 K) conditions Now use: ! G = ! Gº + RT ln Q Q = Reaction Quotient rxn at equilibrium At equilibrium : ! Gº = " RT ln K 58 ! G is driving force of a reaction done at standard P or [ conc.] conditions Q < K ! G < 0 rxn proceeds to right; forward reaction Q > K ! G > 0 rxn proceeds to left; reverse reaction Q = K
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