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Lecture 12

# Chem 402 Lecture 12: L12 2:13:17 Premium

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Department
University College - Chemistry
Course
University College - Chemistry Chem 402
Professor
Barnes Alexander
Semester
Spring

Description
13 February 2017 L12: The Joule-Thompson Coefficient and Heat Capacities I. Heat Capacities A. Joule Thompson Coefficient 1. Meaning ππ» ππ» a. ππ» = ( )ππππ+ ( ) πππ=ππΆ ππ + ππΆ π ππ π π½π b. If ππ½π= 0, an ideal gas or a real gas invT β’ For an ideal gas, βH = nCpβT ππ βπ c. π π½π= ( ππ=π»( ) βπxπ»erimentally) d. For an expansion, π > 0 π½π β’ βT/βP = (-)/(-) β’ Need βT to be negative (cool on expansion) for π to be positive π½π e. For an expansion, ππ½π < 0 β’ βT/βP = (+)/(-) β’ Need βT to be positive (heat on expansion) forπ½π to be negative 2. For a van der Waals gas: π a. π π½πβ ππβ π π π βπ b. Ifππ < π, then T > ππ = π πππ£ thus (βπ)π»< 0) β’ If βP < 0, then βT > 0 and gas heats on expansion β’ It takes energy to expand when π½π > 0, such energy comes from the translational and rotational degrees of freedom (decreases energy level occupations to expand gas) β not from vibrational energy (ground state around room temp) c. If π > π, then T< π = π , thus (( ) > 0) ππ ππ πππ£ βπ π» β’ If βP < 0, then βT < 0 and gas cools on expansion d. Most real gases have Tinv>> 300K 3. JT Experiment + Barnes Lab a. NMR polarization with even populations for 300 MHz separation b. EPR polarization with even populations for 198 GHz separation c. At thermal eqβb, higher population in lower energy state (look 6 sig figs out for differences) d. For βspectral density junctionβ, a given point on the stretch tells us the frequency of changing flip spin state β’ Graphs frequency (x axis) vs. degeneracy*energy (y-axis) β’ Will get increased sensitivity by bringing down the temperature (from 298K
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