Chapter 19 Lecture Outline

Introduction to Entropy

1. Spontaneous reactions (go forward)

2. [delta]H is part of the picture

3. [delta]H does not explain it all (ie: evaporation of water)

4. Introduce entropy (S)
   1. Dispersion of Energy ( internet ©1997, Saunders)
   2. Dispersion of Matter ( internet ©1997, Saunders)

5. Effect Entropy
   1. Phase of Matter ( internet ©1997, Saunders)
   2. Temperature ( internet ©1997, Saunders)
   3. Complexity of Molecule ( internet ©1997, Saunders)
   4. Ionic Solids ( internet ©1997, Saunders)
   5. Solubility( internet ©1997, Saunders)

6. Second law of thermodynamics "entropy of the universe increases for spontaneous processes"

7. [delta]S = S_final - S_initial

Calculating Entropy

1. Entropy at phase change
   1. Equlibrium between phases
   2. Difference in energy ([delta]H) offset by change in entropy (T*[delta]S)
   3. T*[delta]S = [delta]H (At equlibrium)

2. Example with water
   1. Boils at 100°C
   2. [delta]H_vap = 40.7 kJ/mole
   3. Calculate [delta]S_vap
   4. Units
      1. [delta]H (kJ mole^-1)
      2. [delta]S (J K^-1 mole^-1)

3. Calculating [delta]S_rxn from absolute S
   1. [delta]S for H₂O(l) -> H₂O (g)
   2. Compare to above
   3. (NOTE: S in table is at 298 K, changes some with temp.)

Gibbs Free Energy

1. Free Energy and Spontaneity
   1. Relate to Hiking
      1. climbing a mountain
         1. Takes work
         2. But reward?
         3. Reward must be > or = work for you to climb
      2. Going into a valley
         1. easy
         2. reward or penalty?
         3. tradeoff between easy and penalty

   2. free energy is [delta]G

   3. [delta]G = [delta]H - T*[delta]S

   4. Relate [delta]G to spontaneous reactions

2. Calculating [delta]G from [delta]H_rxn, [delta]S_rxn, and [delta]G_f
   1. Note about [delta]H and [delta]S at different temperature

   2. Work an example with formation of water
      1. 2 H₂ + O₂ -> 2 H₂O
      2. [delta]H_rxn = -285.9 kJ/mole
      3. [delta]S_rxn = -265.7 J/mole K
      4. [delta]G_rxn = -206.7 kJ/mole at 25°C
      5. Change temp, and vary [delta]G

   3. For reactions in table have class calculate [delta]H, [delta]S, and [delta]G at 25°C

      Reaction	[delta]H, kJ/rxn	[delta]S, J/(K rxn)	[delta]G°, kJ/rxn (25°C)	[delta]G°, kJ/rxn (300°C)
      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

      1. Look at trends in [delta]S (from reaction)
      2. Look at effect of T on [delta]G, Find at 300 °C

[delta]G and Concentration

1. [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

2. Solutions

[delta] G at equlibrium

1. [delta]H = T [delta]S
2. [delta]G = 0
3. Q = K
4. [delta]G° = -RT lnK
5. Look at how Q effects [delta]G°
1. For a spontaneous forward reaction [delta]G < 0
2. For a non-spontaneous forward reaction [delta]G > 0
3. Relate to figure 20.9
6. Temperature effect on [delta]G and on K
   1. 2 NO₂ --> N₂O₄ at low Temp ( Internet ©1997, Saunders)
   2. 2 NO₂ --> N₂O₄ at high Temp ( Internet ©1997, Saunders)

7. 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

8. Work on Homework Problems

Please send any comments, corrections, or suggestions to svanbram@science.widener.edu.

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