Chapter 17 Lecture Outline
17.1 Nature's Heat Tax: You Can't Win and You Can't Break Even
17.2 Spontaneous and Nonspontaneous Processes
- Spontaneous reactions (go forward)
- [delta]H is part of the picture
- [delta]H does not explain it all
(ie: evaporation of water)
17.3 Entropy and the Second Law of Thermodynamics
- Introduce entropy (S)
- Dispersion of Energy (
internet ©1997, Saunders)
- Dispersion of Matter (
internet ©1997, Saunders)
- Effect Entropy
- Phase of Matter (
internet ©1997, Saunders)
- Temperature (
internet ©1997, Saunders)
- Complexity of Molecule (
internet ©1997, Saunders)
- Ionic Solids (
internet ©1997, Saunders)
- Solubility(
internet ©1997, Saunders)
- Second law of thermodynamics "entropy of the universe increases for spontaneous processes"
- [delta]S = Sfinal - Sinitial
17.4 Heat Transfer and Changes in the Entropy of the Surroundings
17.5 Gibbs Free Energy
- Free Energy and Spontaneity
- Relate to Hiking
- climbing a mountain
- Takes work
- But reward?
- Reward must be > or = work for you to climb
- Going into a valley
- easy
- reward or penalty?
- tradeoff between easy and penalty
- free energy is [delta]G
- [delta]G = [delta]H - T*[delta]S
- Relate [delta]G to spontaneous reactions
17.6 Entropy Changes in Chemical Reactions: Calculating dSorxn
- Entropy at phase change
- Equlibrium between phases
- Difference in energy ([delta]H) offset by change in entropy (T*[delta]S)
- T*[delta]S = [delta]H (At equlibrium)
- Example with water
- Boils at 100 oC
- [delta]Hvap = 40.7 kJ/mole
- Calculate [delta]Svap
- Units
- [delta]H (kJ mole-1)
- [delta]S (J K-1 mole-1)
- Calculating [delta]Srxn from absolute S
- [delta]S for H2O(l) -> H2O (g)
- Compare to above
- (NOTE: S in table is at 298 K, changes some with temp.)
17.7 Free Energy Changes in Chemical Reactions: Calculating dGorxn
- Calculating [delta]G from [delta]Hrxn, [delta]Srxn, and [delta]Gf
- Note about [delta]H and [delta]S at different temperature
- Work an example with formation of water
- 2 H2 + O2 -> 2 H2O
- [delta]Hrxn = -285.9 kJ/mole
- [delta]Srxn = -265.7 J/mole K
- [delta]Grxn = -206.7 kJ/mole at 25°C
- Change temp, and vary [delta]G
- For reactions in table have class calculate [delta]H, [delta]S, and [delta]G at 25 oC
Reaction |
[delta]H, kJ/rxn |
[delta]S, J/(K rxn) |
[delta]Go, kJ/rxn (25oC) |
[delta]Go, kJ/rxn (300oC) |
2 O3 -> 3 O2 | -284.5 | 138.1 | -325 | -363.6 |
NO2 + O2 -> NO + O3 | 200.5 | 4.3 | 199.2 | 198.0 |
2 SO2 + O2 -> 2 SO3 | -198.3 | -189.0 | -142 | -103.8 |
NH4Cl -> NH3 + HCl | 176.1 | 284.3 | 91.4 | 13.2 |
- Look at trends in [delta]S (from reaction)
- Look at effect of T on [delta]G, Find at 300 °C
17.8 Free Energy Changes for Nonstandard States: The Relationship between dGorxn and dGrxn
- [delta]G = [delta]G° + RT lnQ (effect of concentration on [delta]G°)
Reaction | [delta]G°(kJ/mol) | [delta]G (kJ/mol) |
2 O3 (.001 bar) -> 3 O2 (0.2 bar) | -327 | -304 |
NO2 (10-6 bar) + O2 (0.2 bar) ->
NO (10-6 bar) + O3(10-6 bar) |
198 | 168 |
2SO2 (10-7bar) + O2 (0.2 bar) --> 2 SO3(10-7bar) | -142 | -138 |
NH4Cl -> NH3(10-8 bar) + HCl (10-8 bar) | 91.1 | -0.2 |
- Solutions
17.9 Free Energy and Equilibrium: Relating dGorxn and to the Equilibrium Constant (K)
- [delta]H = T [delta]S
- [delta]G = 0
- Q = K
- [delta]G° = -RT lnK
- Look at how Q effects [delta]G°
- For a spontaneous forward reaction [delta]G < 0
- For a non-spontaneous forward reaction [delta]G > 0
- Relate to figure 20.9
- Temperature effect on [delta]G and on K
- 2 NO2 --> N2O4 at low Temp ( Internet ©1997, Saunders)
- 2 NO2 --> N2O4 at high Temp ( Internet ©1997, Saunders)
- Varying Pressure for the reaction, calculate [delta]G:
H2 + Br2 --> 2 HBr
H2 (bar) | Br2 (bar) | HBr (bar) | [delta]G (kJ/mole) |
1 | 1 | 1 | -106.9 |
10-6 | 10-6 | 100 | -15.6 |
10-8 | 10-8 | 500 | 15.2 |
Solutions
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Last Updated Friday, May 25, 2001 1:59:43 PM