CH131_13A

CH131_13A - Entropy and the Direction of Spontaneous Change...

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Entropy and the Direction of Spontaneous Change (Chap. 13) Spontaneous change => the tendency of a reaction or process to follow a particular pathway or direction Consider what happens when we mix: ice + NaCl @ constant P T = ? Heat flow into or out of system? q P > 0 or q P < 0 ? q P = H > 0 an endothermic reaction - heat flows into system (heat absorbed by system) spontaneously! This system runs uphill spontaneously (with respect to H). H reactants products Endothermic reaction Thus, there must be more than energetics (1 st law considerations) important to determining the direction of spontaneous change.
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Additional concept in controlling the direction of spontaneous change is entropy, S . S => measure of disorder of the system (Na + - H 2 O, Cl - - H 2 O more disordered system than NaCl solid and pure liquid H 2 O) The direction of chemical or physical change that increases disorder has the greater tendency to be the direction of spontaneous change (informal statement of the Second Law of Thermodynamics ) Examples of physical changes that increase disorder (S) 1. Solid melting H 2 O(s) H 2 O(l) 2. Liquid vaporizing H 2 O(l) H 2 O(g) H 2 O molecules occupy a bigger volume
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Examples of a chemical change that increases disorder (S) N 2 O 4 (g) 2NO 2 (g) S > 0 (as written) Less ordered - 2 NO 2 molecules anywhere More ordered - 2 NO 2 molecules near each other Both energetics ( H) and entropy ( S) will play a role at a given set of conditions (T, P, etc.) in determining the direction of spontaneous change Ice & water example => S (disorder) increased on mixing: pure ice and NaCl crystal structures broken; ions mixed in more random arrangement (H 2 O molecules surround each ion!) S overcame H considerations to drive the “reaction” in the direction of spontaneous change in that case.
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Let’s consider compression/expansion of an (ideal) gas: V i = 1.00 L, T i = 25 o C, P i = 1.00 atm Compress isothermally ( T = 0) => V i = 0.50 L, T f = 25 o C, P i = 2.00 atm If compression carried our reversibly , (P ext = P gas ) infinitely slowly w rev
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This note was uploaded on 04/17/2008 for the course CAS CH131 taught by Professor Zigler during the Spring '08 term at BU.

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CH131_13A - Entropy and the Direction of Spontaneous Change...

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