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Outline: 1/24/07

Outline: 1/24/07. CAPA TA’s – Wed & Sunday evening Today: Chapter 14 Thermodynamics: Enthalpy (Heat) Entropy Free Energy. Thermo = heat dynamics = movement. Want to test your knowledge?. (and those keypads once again?). What’s the functional group here?. O.

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Outline: 1/24/07

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  1. Outline: 1/24/07 • CAPA TA’s – Wed & Sunday evening • Today: Chapter 14 Thermodynamics: Enthalpy (Heat) Entropy Free Energy • Thermo = heat • dynamics = movement

  2. Want to test your knowledge? (and those keypads once again?)

  3. What’s the functional group here? O • Aldehyde • Alcohol • Ketone • Acid • Ester OH

  4. What provides the polymeric backbone to DNA’s double helix? • Ribose sugars • Phosphates • Amino acids • 1 & 2 • 1 & 3

  5. Why are we so interested in heat? • Chemical reactions that produce heat • Chemical reactions that absorb heat • “Exothermic” • “Endothermic” Heat is related to whether the reaction will happen!

  6. The first law…. • DE = q + w and w = - PDV • DE = q- PDV • q = DE + PDV • For constant pressure: • PDV = D(PV) • q = D(E + PV) • Make a new state variable: E + PV = H • “Constant pressure heat” = Enthalpy (H)

  7. Another definition: • Enthalpy of formation (DHfo): • The constant-pressure heat required to form a chemical compound from the elements (in their most stable form). • DefineDHfo = 0 for elements • Look up tables ofDHfo (Appendix D)

  8. Examples: 0 kJ/mol 0 kJ/mol - 285 kJ/mol • H2(g) + O2 (g) H20 (l) • DH for formation reaction: DHfo • DHrxn = SDHproducts - SDHreactants

  9. Example of DHfo usefulness: • CH4(g) + O2(g) H2O (g) + CO2(g) • How much heat is generated if you burn 1.0 mole of methane gas?

  10. Example of DHfo usefulness: • CH4(g) + 2O2(g) 2 H2O (g) + CO2(g) 1. Balance equation... 2. Info from Appendix D: DHfo (CH4(g)) = -74.6 kJ/mol DHfo (O2(g)) = 0.0 kJ/mol DHfo (H2O(g)) = -241.8 kJ/mol DHfo (CO2(g)) = -393.5 kJ/mol 1 2 2 1

  11. Example of DHfo usefulness: • CH4(g) + 2O2(g) 2 H2O (g) + CO2(g) 1. Balance equation... 2. Info from Appendix D… 3. Products – Reactants… -802.5 kJ/mol • Practice !!!

  12. Remember?

  13. A new topic! Entropy • Start with definition: • Spontaneity: Every chemical process has a spontaneous direction. • e. g. 2 H2 + O2 2 H2O (fast) • apple  brown apple (slow)

  14. What governs spontaneity? • DH (heat of reaction) ? • DE (energy of reaction) ? Both exothermic and endothermic reactions can be spontaneous… • T (temperature) ? • ….. Something else ? Answer: Entropy

  15. Definition: • Entropy = “randomness” • = “disorder” • = S Second Law of thermodynamics: Entropy always increases in spontaneous reactions (entropy of the universe that is...) DS = q/T > 0 (spontaneous)

  16. (s)  ()  (g) Argon example • At a molecular level, entropy (order/disorder) is easy to visualize:

  17. Entropy is a “state function”: • DS = Change in entropy (randomness) • = related to the flow of heat • (at constant temperature) • TDS =qTwhere qT is the heat of • reaction at constant T • Don’t forget (T in Kelvin) • Examples: Phase changes ()  (g) • constant temp bath

  18. Macroscopic H2O example: (1) 36 g water freezes into ice cube in a freezer at -10°C spontaneously (2) 36 g ice cube melts to water at 5°C spontaneously How can both spontaneous reactions of ice/water have increasing entropy?

  19. Ice cube example (cont’d): • 2 mols H2O(s) 2 mols H2O() • (@ 5oC) • What is DS for this reaction? • q(H2O) = n DHfus = 2 mol (6.0 kJ/mol) • = (heat absorbed by water) = +12 kJ • DS(H2O) = q(H2O) / T = +12 kJ/273K • = +44 J/K • (more entropy / more disordered)

  20. Consider the universe? • q(H2O)= -q(freezer) = -12 kJ • DS(freezer) = q(freezer) / T = -12kJ/278K • = -43.2 J/K (less entropy / less disordered) • Overall entropy (of universe): • DS(universe) = DS(H2O) + DS(freezer) • = +44.0 + -43.2 = +0.8 J/K (more entropy / more disordered)

  21. What about freezing water? • 2 mols H2O(l) 2 mols H2O(s) • (freezer @ -10oC) • What is DS for this reaction? • q(H2O) = n DHfus = 2 mol (-6.0 kJ/mol) • = (heat lost by water) = -12 kJ • DS(H2O) = q(H2O) / T = -12 kJ/273K • = -44 J/K • (less entropy / less disordered)

  22. But why is it spontaneous? • q(H2O)= -q(freezer) = +12 kJ • DS(freezer) = q(freezer) / T = +12kJ/263K • = +45.6 J/K (more entropy / more disordered) • Overall entropy (of universe): • DS(universe) = DS(H2O) + DS(freezer) • = -44.0 + 45.6 = +1.6 J/K (more entropy / more disordered)

  23. On what does entropy depend? • obvious • Temperature • Phase (s)  ()  (g) • Molar mass • Concentration • less obvious • Generally, as molar mass , intermolecular disorder  • Table 14-2: H2 = 130.7 J/K mol (page 580) F2 = 202.8 J/K mol • Cl2 = 223.1 J/K mol

  24. On what does entropy depend? • obvious • Temperature • Phase (s)  ()  (g) • Molar mass • Concentration • less obvious p.126

  25. Let’s test those minds… Quiz #2 Please put away all books, papers, etc.

  26. Quiz #2 You may leave when you are done… hand them into me on your way out.

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