CHEM /21/11 IV. 2nd Law of THERMO

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Presentation transcript:

CHEM 433 - 10/21/11 IV. 2nd Law of THERMO • T dep. (heating/cooling) -> Measuring Sm° • Third Law Entropies - Standard Reaction Entropy Changes - T dependence of S READ: CHAPTER 3… HW #5 : Out when I get that far ….

How to Measure molar S: Measure Cp at many T’s Fit to equaiton and integrate: Cp/T dT) Measure trsS (as needed) Add up all the parts - this is eqn on board…

(s) vs. (l) vs. (g) ? Size ? (Svib) Why are most gas values lower than CO2? (this is subtle…)

Nernst Heat Theorem: For all processes: S —> 0 as T—> 0 (provided all substances are perfectly crystalline…) 3rd LAW of THERMO: The entropy of a perfect crystalline substance is zero at T=0. 3rd Law Entropies: Molar entropies obtained via assumption that S(0) = 0 (for all substances - crystalline or otherwise…)

For: H2 (g) + 1/2 O2 (g) —> H2O (l) rS° = ? Sm° = 130.7 205.0 69.9 (all values in J/K mol at 298K…) BTW: we calculated Ssurr for this last week - —> ~1000 J/K mol (or 1 kJ/K mol)