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

  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)


17.3 Entropy and the Second Law of Thermodynamics

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

  2. 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)

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

  4. [delta]S = Sfinal - Sinitial


17.4 Heat Transfer and Changes in the Entropy of the Surroundings


17.5 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


17.6 Entropy Changes in Chemical Reactions: Calculating dSorxn

  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 oC
    2. [delta]Hvap = 40.7 kJ/mole
    3. Calculate [delta]Svap
    4. Units
      1. [delta]H (kJ mole-1)
      2. [delta]S (J K-1 mole-1)

  3. Calculating [delta]Srxn 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.)

17.7 Free Energy Changes in Chemical Reactions: Calculating dGorxn

  1. Calculating [delta]G from [delta]Hrxn, [delta]Srxn, and [delta]Gf
    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]Hrxn = -285.9 kJ/mole
      3. [delta]Srxn = -265.7 J/mole K
      4. [delta]Grxn = -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 oC

      Reaction [delta]H, kJ/rxn [delta]S, J/(K rxn) [delta]Go, kJ/rxn (25oC) [delta]Go, kJ/rxn (300oC)
      2 O3 -> 3 O2-284.5138.1-325 -363.6
      NO2 + O2 -> NO + O3200.54.3199.2198.0
      2 SO2 + O2 -> 2 SO3-198.3-189.0-142-103.8
      NH4Cl -> NH3 + HCl176.1284.391.413.2

      1. Look at trends in [delta]S (from reaction)
      2. 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

  1. [delta]G = [delta]G° + RT lnQ (effect of concentration on [delta]G°)
  2. 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) 198168
    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

  3. Solutions

17.9 Free Energy and Equilibrium: Relating dGorxn and to the Equilibrium Constant (K)

  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:
  8. H2 + Br2 --> 2 HBr

    H2 (bar) Br2 (bar) HBr (bar) [delta]G (kJ/mole)
    111-106.9
    10-610-6100-15.6
    10-810-850015.2

    Solutions


This page is maintained by
Scott Van Bramer
Department of Chemistry
Widener University
Chester, PA 19013

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

This page has been accessed 4815 times since 5/30/97.
Last Updated Friday, May 25, 2001 1:59:43 PM