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Lecture 26

CHEM 1032 Lecture 26: Changes in Entropy and Free Energy

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CHEM 1032
Elizabeth Cerkez

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Changes in Entropy and Free Energy  ▯G = ▯H - T▯S o Gibbs Free Energy o Negative = spontaneous o Positive = nonspontaneous  ▯H – calculate: o standard enthalpies of formation (▯H ▯ f o Hess’s Law:  Think of reaction as steps  Summation of steps is overall ▯H  Calculate ▯S rxn o Using standard molar entropies o ▯S ▯ rxn S▯ productS▯ reactants  ▯H ▯ f= 0 for elements in their standard form o Gas: P = 1 atm o Liquids and solids: P = 1 atm T = 25▯C o Solutions: 1.0 M o ▯H ▯ fscale is arbitrary  Entropy has a set scale and a real definition of 0 rd  3 Law of Thermodynamics: o the entropy of a perfect crystal at absolute 0 (0▯K) is zero o S = klnW  = kln(1)  = 0  Factors that affect entropy: o State of substance o Molar mass of substance o Particular allotrope o Molecular complexity o Extent of dissolution  State of Substance:
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