notes_Lecture_08_101810

notes_Lecture_08_101810 - Lecture 7: Thermo. Def. of...

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Lecture 7: Thermo. Def. of Entropy Reading: Zumdahl 10.5 and 10.6 Outline – Entropy and the second law of thermodynamics (10.5); How do we increase entropy? – Entropy changes in the surroundings and the effect of temperature on spontaneity (10.6) – Entropy changes in chemical reactions and the third law of thermodynamics (10.8) – APPLICATIONS: Spontaneity and temperature; entropy and phase changes
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Second Law of Thermodynamics Define: Δ S univ = Δ S sys + Δ S surr 1. Δ S univ > 0 … process is spontaneous 2. Δ S univ < 0 … process is spontaneous in opposite direction 3. Δ S univ = 0 … process is at equilibrium Note that we need to know Δ S for BOTH the system and surroundings to predict if a reaction will be spontaneous! The entropy of the universe is always increasing . In any spontaneous process , there is always an increase in the entropy of the universe .
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How do we increase entropy? Petrucci, Fig. 7.9 motion through space motion about the center of mass motion directed through chemical bonds 1. Increase the number of available KE modes. Increase V (more translational modes for gases) Cause phase change (more rotational and translational modes for liquids relative to solids, for gases relative to liquids) 2. Increase the energy in the available KE modes. Heat added to a system can be deposited in previously empty KE modes. ( translation and rotation are the KE modes most important to increased entropy)
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Expansion of an ideal gas In a thermally isolated system, an ideal gas expands into a vacuum spontaneously. More volume available to gas molecules… h more positions available to gas molecules h more microstates available to system h Dominant configuration: gas molecules evenly distributed in both bulbs h ENTROPY INCREASES
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Volume increases for both solvents, more translational modes available…more microstates available to solution than to either solvent by itself Solution Formation b ENTROPY INCREASES
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System Entropy in the Vaporization of Water More volume, more translational modes available in gas phase b more microstates available b dominant configuration: liquid-phase water molecules enter the gas phase and diffuse to fill bell jar b ENTROPY INCREASES
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notes_Lecture_08_101810 - Lecture 7: Thermo. Def. of...

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