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Chem 300 - Ch 21/#2 Today’s To Do List

Chem 300 - Ch 21/#2 Today’s To Do List. S m + Molecular Mass + Molecular Structure = ? When Entropies don’t agree… Calculating D r S of Chem Reactions. The Impossibility of Reaching 0 K in a finite # of steps. Current world record low temp: ~20 nK (20x10 -9 K) T = 4 K (bp He)

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Chem 300 - Ch 21/#2 Today’s To Do List

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  1. Chem 300 - Ch 21/#2 Today’s To Do List • Sm + Molecular Mass + Molecular Structure = ? • When Entropies don’t agree… • Calculating DrS of Chem Reactions

  2. The Impossibility of Reaching 0 K in a finite # of steps • Current world record low temp: • ~20 nK (20x10-9 K) • T = 4 K (bp He) • T ~ 1 K (boiling He under vac) • T < 1 K (adiabatic demagnetization)

  3. Adiabatic Demagnetization

  4. Sm & Molecular Structure • S = f(T, Cp) • Cp = f(mass, degrees of freedom) • Mass = f(no. & kind of atoms)

  5. Some Smº of Substances • He(g) 126 Ar(g) 155 • H2(g) 131 N2(g) 192 • O2(g) 205 CO2(g) 214 • Hg(l) 76 H2O(l) 70 • C6H6(l) 173 C6H12(l) 204 • C(s)[diam] 2.4 C(s)[graph] 5.7 • C6H22O11(s) 360

  6. Smº as a function of molar mass

  7. Discrepancies • Calc. Exper. • CO2(s) 213.8 213.7 • CH4(s) 186.3 186.3 • CO(s) 160.3 155.6 • S(calc) – S(exp) = 4.7 J/mol-K

  8. Discrepancies (contin.) • DS = 4.7 due to imperfect arrangement of CO’s in crystal • 2 orientations possible: •  •  • # of crystal config = W = 2N (N = Avog #) • From stat mech: • S = k ln W = k ln 2N = R ln 2 = 5.7 J/mol-K

  9. Cyclic Path to calc. Smº of [Br2(g)] @ 298 K

  10. DrSº Calculation • 2 CO(g) + O2(g)  2 CO2(g) • DrSº = 2 Sº(CO2) – 2 Sº(CO) – Sº(O2) • = (2) (214) - (2) (198) - (205) • = - 173 J/mol-K

  11. Next Time • Start of Chapter 22: • Gibbs & Helmholtz Functions • Maxwell Relations

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