Chapter 19: Entropy and Free Energy

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Chapter 19 Entropy and Free Energy
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Thermodynamics

• will a reaction be spontaneous?
• H2O (s) -----gt H2O(l) spontaneous?
• H2O (l) -----gt H2O(s) spontaneous?
• 4Fe(s) 3O2(g) -----gt 2Fe2O3(s) spontaneous?
  

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Change in Enthalpy (Review)

• ?Ho ?Hf(prod) - ?Hf(react)
• ?H negative exothermic ?H positive
  endothermic
• a) CaCO3(s) -----gt CaO(s) CO2(g)
• b) 4Fe(s) 3O2(g) -----gt 2Fe2O3(s)
• c) NaCl(s) H2O(l) -----gt NaOH(aq) HCl(aq)

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Enthalpy as Predictor?

• in 1860s it was reported that enthalpy change
  was a way to predict spontaneity.
• similar to ?o values (?o gt 0 spontaneous)
• hypothesize ?H lt 0 spontaneous
• is this reasonable? N2 N2 I2
  I2
• N2 I2 I2
• N2
• Spontaneous? Yes ?H 0
• N2 I2 I2 N2
• I2 I2 N2
• N2
• Also H2O (s) -----gt H2O(l) spontaneous? Yes
  exothermic? No
• Also, CaCO3(s) -----gt CaO(s) CO2(g)
• although not spontaneous at room temperature,
  above 1200K becomes spontaneous

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Change in Entropy, ?S

• entropy is a measure of disorder
• high entropy high disorder
• low entropy high order
• ?S ?So(products) - ?So(reactants)
• entropy of atom combination Appendix L Table 20
• Trends Ssolids lt Sliquids lt Sgases
• lighter molecules lt heavier molecules
• fewer molecules lt more molecules e.g. A
  ----gt 2B
• NOTE UNITS! J/K not kJ, like enthalpies.

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Using Entropy Values

• a) CaCO3(s) -----gt CaO(s) CO2(g)
• b) 4Fe(s) 3O2(g) -----gt 2Fe2O3(s)
• c) NaCl(s) H2O(l) -----gt NaOH(aq) HCl(aq)

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Gibbs Free Energy, ?G

• for a reaction carried out at constant T P
• ? ?G ?H - T?S
• ?G lt 0 reaction is spontaneous
• ?G gt 0 reaction is not spontaneous
• ?G 0 reaction is at equilibrium
• Two factors tend to make ?G negative
• 1. negative ?H values
• 2. positive ?S values

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Effect of Temperature on Reaction Spontaneity

• ?H ?S Value of ?G notes
• _ Always _ spontaneous at all T
• _ Always never spontaneous
• low T high T _ spontaneous only
  at high T
• _ _ low T - high T spontaneous only at low T
• ? ?G ?H - T?S

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Using ?G to Predict Reaction Spontaneity

• a) CaCO3(s) -----gt CaO(s) CO2(g)
• ??? 178.0kJ ?S 0.1605kJ/K
• b) 4Fe(s) 3O2(g) -----gt 2Fe2O3(s)
• ?H -1648.4kJ ?S -0.5497kJ/K
• c) NaCl(s) H2O(l) -----gt NaOH(aq) HCl(aq)
• ?????60.5kJ ?S -0.0375kJ/K

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Equilibrium

• at what temperature will a system be at
  equilibrium?
• ?G ?H - T?S 0
• ?H T?S
• ?H /?S T