Lecture26 - Chem 162, Lect 26, Spring 2011 First Law of...

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Chem 162, Lect 26, Spring 2011 First Law of Thermodynamics Conservation of energy: U Univ = U sys + U surr = constant U Univ = U sys + U surr = 0 Energy is conserved U sys = q + w q = heat added to system w = work done on system This means that any change in energy of a system must come from interactions with the surroundings (either through q or w, or both). Second Law of Thermodynamics The driving force for a spontaneous process is an increase in the entropy of the Universe. S Univ = S sys + S surr > 0 spontaneous process < 0 non-spontaneous process = 0 at equilibrium The second law gives the direction of a spontaneous process. Although energy is conserved in either direction, the actual (spontaneous) direction is such that the entropy of the universe increases. 1
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T S Univ is an energy quantity, so this means that energy is degraded in spontaneous processes — converted from a more usable to a less usable form after the spontaneous process. Third Law of Thermodynamics The entropy of a perfect crystal at 0 Kelvin is zero. There is only one arrangement (microstate). To evaluate whether a reaction is spontaneous, we must determine both S sys and S surr . We now know that S sys = q rev /T What about S Surr ? We need to look at what determines S Surr , because it is S Universe overall that matters in terms of spontaneity. We sort of have a handle on the sign and magnitude of S sys , what about S Surr ? Entropy changes in the surroundings are primarily determined by
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Lecture26 - Chem 162, Lect 26, Spring 2011 First Law of...

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