Entropy and Free Energy

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Entropy and Free Energy

• 19.4

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After reading Section 19.4, you should know

• How to calculate the amount of free energy of a
  reaction
• How to use the free energy to determine whether a
  reaction will be spontaneous or not

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• Entropy a quantitative measure of the disorder
  of a system
• Symbol ?S0 (Joule / Kelvin mol)
• ?S0 ?S0 (products) - ?S0 (reactants)
• Table 19.2 on page 558

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Practice

• Using Table 19.2, determine the standard entropy
  change(?S0 ) for the following reaction.
• 2NO(g) O2(g) ? 2NO2(g)

?S0 ?S0 (products) - ?S0 (reactants)
?S0 -145.2 J/K
Remember to multiply the ?S0 by 2 since you
have 2 mol of NO and NO2
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• Gibbs Free Energy the energy available to do
  work
• Symbol ?G0 (kJ / mol)
• ?G0 ?G0 (products) - ?G0 (reactants)
• Table 19.4 on page 563

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Gibbs Free Energy

• If the ?H (enthalpy) and the ?S (entropy) are
  known, the (free energy) can be found by
• ?G0 ?H -T ?S0
• T temp in Kelvin
• Must convert units!!!
• ?G0 (kJ / mol)
• ?H0 (kJ / mol)
• ? S0 (Joule / Kelvinmol)

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Spontaneous or not??

• Spontaneous reactions release free energy
• ?G0 is negative
• The system loses free energy
• ?H0 negative
• ? S0 positive

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Spontaneous or not??

• Nonspontaneous reactions absorb free energy
• ?G0 is positive
• The system needs the free energy to make the
  reaction happen
• ?H0 positive
• ? S0 negative
• A system may be spontaneous under one set of
  conditions and nonspontaneous under another
  (example if the temperature changes)

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Values in the book

• Free Energy (?G0) values pg 563
• Enthalpy (?H0) values pg 316
• Entropy (? S0) values pg 558

10
After reading Section 19.4, you should know

• How to calculate the amount of free energy of a
  reaction
• How to use the free energy to determine whether a
  reaction will be spontaneous or not