Lecture 5 (Statistics and Entropy)

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Department
Chemistry & Chemical Biology
Course Code
CHEM 1214
Professor
Thomas Gilbert

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Lecture 5 (1/16/2013) • Statistics and Entropy (S): o Boltzmann’s equation:  S = k ln W • where:  k   =1.38 ´ 10  J/K        = RAN   W  = # of accessible energy “microstates” (how dispersed  the energy in a system is) • Thermodynamic Entropy: o Isothermal Process:  A process that takes place at constant T. o Reversible Process:  A process that can be run in the reverse, restoring the initial  system, with no net energy flow into or out of the system. o For an isothermal process:    ΔS sys /rev /T sys • qrev flow of heat into or out from a system during a  reversible process. • Entropy Change & Spontaneity: o Consider an ice cube at 0.0°C (273 K) placed on a counter top at room  temperature (293 K). o Will it melt spontaneously?  ΔS univ= ΔSsys ΔS surr  Heat flows into ice cube: ΔS  = q /273 sys rev  Heat flows out of counter top: ΔS surr= −qrev93 • Negative because ice cube causes table top to lose heat  ΔS = (q /273) + (−q /293) > 0   univ rev rev  Yes, ice cubes melt spontaneously at room temperature. Who  knew! • Entropy and Temperature: o Entropy increases as temperature increases o Decrease in temperature leads to decrease in entropy o … • Third Law of Thermodynamics: o Third Law of Thermodynamics:  The entropy of a perfect crystal is zero at absolute zero. (It has  only 1 microstate.) o Absolute Entropy:  The entropy of a substance at some temperature  above 0 Kelvin.  Calculated from measurement of molar heat capacities  as a function of
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