CHEM 6B WK8 Session Slide

Page 5: Brainstorming: Work and Energy

• Concepts of Thermodynamics:

  • Heat:

  • System vs. Surrounding

  • Types of Systems: Open, Closed, Isolated

  • Energy Symbols:

    • U (internal energy)

    • +, − for various processes

  • Pressure considerations

Page 6: Energy Equation

• Key Equations:

  • q = C ∙ ∆T

  • Where:

    • C = heat capacity

    • q = heat transfer

  • Variations:

    • q = m ∙ Cs ∙ ∆T

    • q = n ∙ Cm ∙ ∆T

    • q cal = C cal ∙ ∆T

    • q surroundings = -q system

  • Thermodynamic properties:

    • H = U + PV

    • ∆H = ∆U + P∆V

    • ∆U = q + w

    • W = -P∆V

    • ∆H = q at constant pressure

Page 7: Heat at Constant Conditions

• At Constant Volume:

  • ∆U = q (work = 0)

  • Cv = ∆U/ ∆T

  • Cv,s = ∆U / m ∙ ∆T

  • Cv,m = ∆U / n ∙ ∆T

• At Constant Pressure:

  • ∆H = q

  • Cp = ∆H / ∆T

  • Cp,s = ∆H / m ∙ ∆T

  • Cp,m = ∆H / n ∙ ∆T

Page 8: Specific Heat Capacities

• Atoms and Molecules:

  • Linear Molecules: Cv,m ? R

  • Nonlinear Molecules:

    • Cv,m 5/2 R, Cp,m ? R

    • Relationship: Cv,m < Cp,m

Page 9: Phase Change Energies

• Enthalpy Changes:

  • ∆H vaporization, ∆H fusion, ∆H sublimation

  • Heat Capacity Relationships:

    • q = m ∙ Cs ∙ ∆T

    • q = n ∙ Cm ∙ ∆T

    • q = mol/g ∙ ∆H (phase change)

Page 10: Reaction Enthalpies

• Key Concepts:

  • ∆H rxn: Product vs. Reactant side relationship

  • Reverse Reaction:

  • Standard State and Hess’s Law

  • Formula:

    • ∆H̊ rxn = Ʃ n ∆H̊ f (products) - Ʃ n ∆H̊ f (reactants)

  • Bond Energy:

    • ∆HB Bond breaking vs. Bond forming

    • ∆H̊ rxn = Ʃ HB (reactants) - Ʃ HB (products)

Page 11: Entropy Concepts

• Entropy (S):

  • Formulas:

    • ∆S = q rev / T

    • ∆S = n ∙ R ∙ ln (V2 / V1)

    • ∆S = n ∙ R ∙ ln (P1 / P2)

    • ∆S = C ∙ ln (T2 / T1)

Page 12: Phase Change and Entropy

• Transition Temperatures:

  • Tb: Transition temperature, Vapor pressure = ? atm

  • Tf: Transition temperature, Melting pressure = ? atm

• Entropy Changes:

  • ∆S = q rev / T

    • ∆S vaporization = ∆H vap / T

    • ∆S fusion = ∆H fus / T

Page 13: Boltzmann's Formula and Reaction Entropy

• Entropy Calculation:

  • S = k ∙ ln (W)

  • Comparison:

    • Large & Complex vs. Small & Simple: Liquid vs. Solid vs. Gas

    • ∆S ̊rxn = Ʃ n ∆Sm ̊ (products) - Ʃ n ∆Sm ̊(reactants)

  • Sign of ∆S:

    • ∆n (g) > 0 vs. ∆n (g) < 0

Page 14: Total Entropy

• Total Entropy Calculation:

  • ∆S total = ∆S system + ∆S surroundings

  • Surroundings Calculations:

    • ∆S surroundings = q surroundings, rev / T

    • ∆H system at constant pressure

  • Entropy signs indicating spontaneous processes:

    • ∆S > 0 → (spontaneous, equilibrium)

    • ∆S = 0 → (equilibrium)

    • ∆S < 0 → (nonspontaneous)

Page 15: Gibbs Free Energy

• Relation: G = H - TS

• Changes in Gibbs Energy:

  • ∆G = ∆H - T∆S

  • Implications of signs of ∆G:

    • ∆G > 0 → (nonspontaneous)

    • ∆G = 0 → (equilibrium)

    • ∆G < 0 → (spontaneous)

Page 16: Standard Gibbs Free Energy of Formation

• Standard Gibbs Free Energy Change:

  • ∆G ̊ f = Standard Gibbs free energy of formation

  • Example: A (g) + B (g) → AB (g)

    • ∆G ̊ f < 0: AB (g) is thermodynamically stable

    • ∆G ̊ f > 0: AB (g) is thermodynamically unstable

  • Formula:

    • ∆G ̊ rxn = Ʃ n ∆G ̊ f (products) - Ʃ n ∆G ̊ f (reactants)

Page 17: Vapor Pressure Concepts

• Influences on Vapor Pressure:

  • Partial pressure of components above liquids

  • Effect of intermolecular forces:

    • IMF increases → vapor pressure ?

    • IMF decreases → vapor pressure ?

    • Temperature increases → vapor pressure ?

• Boiling Points:

  • Boiling vapor pressure = ? pressure

    • IMF increases → boiling point ?

    • IMF decreases → boiling point ?

Page 18: Phase Diagram

• Components of Phase Diagram:

  • Locations of three physical states

  • Phase boundaries, Triple point, Critical point

  • Solubility Concept:

    • Like dissolves like

Page 19: Mock Practice Exam

• Preparation:

  • Cheat sheet, Calculator, Periodic table required

Page 20: Exam QR

• References:

  • WI 25 Stallings

Page 21: Answers to Practice Exam

1. D

2. B

3. D

4. E

5. E

6. C

7. B

8. A

9. B

10. D

11. E

12. A

13. D

14. C

15. B

Page 22: Conclusion

• Closing Remarks:

  • CHEM 6B Closer WI 25 Stallings