110126 - 1 CHM3400 - Lecture 9 Jan 26 3 rd Law of TD: Gibbs...

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Unformatted text preview: 1 CHM3400 - Lecture 9 Jan 26 3 rd Law of TD: Gibbs free energy (Chapter 4 p. 95-110) entropy as an absolute value 3 rd Law of TD meaning of entropy Gibbs free energy G Dependence of G on temperature and pressure How does a refrigerator work? 2 Entropy S = k B ln(W) Entropy is an absolute value W stands for the number of microstates for a system in a given macrostate . How many microstates exist for a perfect crystal at 0 K? = S Lim K T = = T P T P T T d C dT T C S ln 3 3 rd Law of TD Every substance has a finite positive entropy, but at the absolute zero of temperature the entropy may become zero, and it does in the case of a pure, perfect crystalline substance. S=0 at 0 K Phase transitions 4 Approaching zero Kelvin In reality, it is actually impossible to get down to 0 K. Current world record, see Science 301 , 2003, 1513. Because number of microstates becomes so low that cooling rate becomes very low 2500 Na atoms in a optical trap 5 Entropy change of chemical reactions aA + bB cC + dD Hypothetical reaction: = ) tan ( ) ( ts reac S v products S v S o o o r Molar entropies at a particular temperature Stoichiometric coefficient 6 Example 4.5Example 4....
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110126 - 1 CHM3400 - Lecture 9 Jan 26 3 rd Law of TD: Gibbs...

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