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Outline: 1/26/07

Outline: 1/26/07. Chemistry Seminar – today @ 4pm Pick up Quiz #2 – from me Today: Chapter 14 (cont’d) Entropy Free Energy Example calculations. Quiz #2 Average: 6.1 (20.5% got 9 or 10….). On what does entropy depend?. obvious. Temperature Phase (s)  (  )  (g).

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Outline: 1/26/07

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  1. Outline: 1/26/07 • Chemistry Seminar – today @ 4pm • Pick up Quiz #2 – from me • Today: Chapter 14 (cont’d) Entropy Free Energy Example calculations

  2. Quiz #2 Average: 6.1 (20.5% got 9 or 10….)

  3. 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

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

  5. Given thermodynamic tables, DSrxn can be calculated: • CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g) • 186.3 2(205.1) 213.7 2(188.8) • J/K mol J/ K mol J/K mol J/K mol • DSrxn = S Soproducts - S Soreactants • = -5.2 J/K • But isn’t it spontaneous ? • Need to check with the universe…

  6. How useful is entropy? • If you can calculate the DS of the universe you can tell if a reaction is spontaneous... • DSuniverse not always easy to calculate...

  7. The last thermo variable: DG • Gibb’s Free Energy: • DGsys = DHsys- TDSsys • If DG < 0 the system spontaneously reacts…. • For DG you only need to consider the system • (not the system + surroundings !)

  8. Prove it! • Since DHsys = qsys (at constant P) • = - qsurr • Then DHsys = - T DSsurr • Plug into: DGsys = DHsys- TDSsys • DGsys = - T DSsurr- TDSsys • DGsys = - T (DSsurr+ DSsys ) • or DGsys = - T (DSuniverse )

  9. Given thermodynamic tables, DGrxn can be calculated: • CH4(g) + 2 O2(g)=> CO2(g) + 2 H2O(g) • -50.7 0 -394.4 2(-228.6) • kJ/mol kJ/mol kJ/mol kJ/mol • DGrxn = SDGoproducts - SDGoreactants • = -801 kJ/mol • very spontaneous at 298 K !

  10. Pb2+(aq) + SO42-(aq)  PbSO4(s) • Ca2+(aq) + SO42-(aq)  CaSO4(s) Reality check: • Predict which is spontaneous…. • Qualitative Chemistry rules revisited!

  11. -24.4 -774.5 -813.4 • kJ/mol kJ/mol kJ/mol DGrxn = -14.5 kJ/mol spontaneous! • -553.6 -774.5 -1321.8 • kJ/mol kJ/mol kJ/mol DGrxn = 6.3 kJ/mol non-spontaneous! Which is spontaneous? • Pb2+(aq) + SO42-(aq)  PbSO4(s) • Ca2+(aq) + SO42-(aq)  CaSO4(s)

  12. Better yet, DGrxn at different temperatures can be calculated: • DHorxn and DSorxn are relatively independent of temperature • DGTrxn DHorxn- TDSorxn

  13. DGTrxn DHorxn- TDSorxn p.126

  14. The beauty of DGTrxnis that you can now determine whether a reaction is spontaneous at any reasonable temp! Example:4 CuO(s) 2Cu2O(s) + O2 (g)At what temperature will this reaction become spontaneous? • Or conversely, at what temperature a reaction will become spontaneous!

  15. Example: • 4 CuO(s) 2Cu2O(s) + O2(g) • DGTrxn 0 when first spontaneous…. Solve: 0 = DHorxn- TDSorxn • For this reaction: • DHorxn= + 227 kJ • DSorxn = + 221 J/K • (from appendix D) • Or: + 227 kJ= + T (0.221 kJ/K) • T =1027 K

  16. Summary to date: • DE, DH, DS and DG are defined • First law calculations: DE = q + w • DHrxn, DHphasechange problems • DS calculations: T DS = q • DGorxn= DHorxn- TDSorxn problems • Since DHorxn and DSorxn are relatively independent of temperature: • DGTrxn DHorxn- TDSorxn

  17. Practice Problems: Chapter 14 • 14.11, 14.15, 14.17, 14.19, 14.23, 14.25, 14.27, 14.31, 14.35, 14.37, 14.38, 14.41, 14.43, 14.49, 14.51, 14.53, 14.55, 14.57, 14.61, 14.65, 14.67, 14.71, 14.75, 14.77, 14.79, 14.81, 14.91, 14.101, 14.103

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