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CHM1311 - Lecture(9) - ENTROPY AND FREE ENERGY ENTROPY...

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ENTROPY AND FREE ENERGY ENTROPY AND FREE ENERGY Petrucci, Chapter 19 Sections 19-1 to 19-5 1 (Skip sections 19-6 and 19-7 for now)
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When Do Reactions Occur? When Do Reactions Occur? An exothermic reaction is likely to be spontaneous, but will not necessarily be spontaneous: This reaction is so hot it is used in the welding of steel and iron! It is also spontaneous. (Thermite Reaction) Thermochemistry’s Goal: Predict whether or not a certain reaction can occur. 2 Fe 2 O 3 (s) + 2 Al(s) 2 Fe(s) + Al 2 O 3 (s) H = - 848 kJ - 848 kJ
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Spontaneity Spontaneity BUT many spontaneous reactions or processes are endothermic or even have H = 0 (neither absorb or release heat). NH 4 NO 3 (s) + heat NH 4 NO 3 (aq) Spontaneity is affected by something other than enthalpy. Spontaneity is also affected by ENTROPY 3
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Entropy, S Entropy, S Thus, the natural tendency in the universe is toward greater disorder or greater entropy. There are many ways to generate a disordered state (e.g. a shuffled deck of cards), but few ways to generate an ordered state (e.g. an unshuffled deck of cards). 4
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Probability Favors Disorder or Dispersion Probability Favors Disorder or Dispersion For 2 molecules, the probability of finding them both in the same flask is: ½ x ½ = ¼ For 6 x 10 23 molecules, the probability of finding them all in the same flask is: (1/2) 6 x 10^23 0 A disordered state has a higher probability of occurring. 5
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Entropy and Phase Entropy and Phase S (gases) >> S (liquids) > S (solids) S o (J/K•mol) H 2 O(liq) 69.91 H 2 O(gas) 188.8 Stronger forces between molecules prevent freedom of motion, thus causing lower entropy in solids. 6
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Entropy and Temperature Entropy and Temperature Entropy of a substance increases with temperature. more order in molecular conformation less order in molecular conformation 7
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Entropy and Molecular Complexity Entropy and Molecular Complexity Increase in molecular complexity leads to increase in S. S o (J/K•mol) CH 4 248.2 C 2 H 6 336.1 C 3 H 8 419.4 8
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Entropy and Solutions Entropy and Solutions Entropy usually increases when a pure liquid or solid dissolves in a solvent. Breaking of intermolecular solute-solute attractions: Entropy increases Breaking of intermolecular solvent-solvent attractions: Entropy increases Formation of solvent-solute attractions: Entropy decreases 9
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Which of the following processes would you expect to result in a greater positive change in entropy? o o 2 2 1. H O(l, 25 C) H O(l, 75 C) 2 2. CO (s, 101 kPa) CO(g, 2 kPa) 3 3 3. CH OH(l, 101 kPa) CH OH(g, 2 kPa) 4. Fe(s) Fe(l) 5. None of these processes involve positive entropy changes.
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The entropy change for the decomposition of ozone forming diatomic oxygen, -1 3 2 2O (g) 3O (g) H = 285 kJ mol - 1. is positive because two moles of gas are forming three moles of gas.
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