Lecture 5 Spontaneity, Entropy and Gibbs Free Energy

Lecture 5 Spontaneity, Entropy and Gibbs Free Energy -...

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The total energy of the universe remains constant . E sys + E surr = E univ = 0 The total entropy of the universe increases . S univ = S sys + S surr > 0 THE LAWs of Thermodynamics A perfect crystal has zero entropy at absolute zero . S sys = 0 at 0 K
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Entropy: General Trends CHM139 F2011 Jockusch , Thermochemistry L5 2 Last lecture we talked about entropy and learned that it could be calculated from the number of microstates ( W ) a system possesses. S gas > S liquid > S solid Entropy increases with temperature More molecules have higher entropy than fewer molecules S = k ln( W ) Entropy is a measure of disorder
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For allotropes, S ° is higher in the form that allows the atoms more freedom of motion. Two forms of carbon: . Entropy and Structure S °= 5.69 J/mol K S °= 2.44 J/mol K graphite diamond
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NaCl AlCl 3 P 4 O 10 NO NO 2 N 2 O 4 S°( s ) 72.1 167 229 S °( g ) 211 240 304 For compounds, S ° increases with chemical complexity. Note: This trends only hold for substances in the same physical state . The effect of physical state dominates the effect of molecular complexity. Entropy and Structure Figure 20.11 Table: S °in J/(mol K) of selected chemical compounds
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Entropy and Structure CHM139 F2011 Jockusch , Thermochemistry L5 5 Which has a higher entropy? n hexane or cyclohexane
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Standard Molar Entropies 6 Standard Molar Entropy, S °: The entropy of 1 mole of the pure substance at 1 atm pressure and a specified temperature, usually 25°C. Standard molar entropies are
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Lecture 5 Spontaneity, Entropy and Gibbs Free Energy -...

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