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Entropy and Free Energy

by: BarrettTMason

Entropy and Free Energy CHEM 1040

Marketplace > Auburn University > Chemistry > CHEM 1040 > Entropy and Free Energy
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About this Document

These notes are from 2/10/2016 to 2/12/2016. We had a test that Monday, so no notes from that date.
Fundamentals of Chemistry II
Dr. Michael Squillacote
Class Notes




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This 4 page Class Notes was uploaded by BarrettTMason on Friday February 19, 2016. The Class Notes belongs to CHEM 1040 at Auburn University taught by Dr. Michael Squillacote in Winter 2016. Since its upload, it has received 12 views. For similar materials see Fundamentals of Chemistry II in Chemistry at Auburn University.


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Date Created: 02/19/16
2/10/2016 Entropy and Free Energy - Entropy (a.k.a. randomness) and heat energy combine into concept of free energy to explain processes that occur Spontaneous Processes - Any process that occurs w/o input of additional energy (i.e. w/o outside intervention) Ex: A ball rolling down a hill. - It doesn’t matter how fast the process occurs. Ex: The slowing of Earth’s rotation over millions of years. Ex: ◦ CH 4 2O → 2O + 2H 2 ∆H 2 rxn= -892kJ (exothermic) o Very slow reaction H + HO → H O ∆H ◦ = -56.2kJ (exothermic) 2 rxn - Almost all exothermic reactions (when ∆H is negative) are spontaneous. o But endothermic can be spontaneous as well. Ex: H 2(s) → H O2aq) ∆H ◦rxn= 601kJ (endothermic) NH NO (s) + H O → NH (aq) + NO (aq) ∆H- ◦ = 25kJ 4 3 2 4 3 rxn ◦ 2HgO(s) → 2Hg(aq) + O (g) ∆2 rxn 90.7kJ - Why are these endothermic reactions spontaneous? o Because randomness increases o Solids are turning into liquids and gases which are more random than solids Conclusion: Randomness (entropy) and heat energy (enthalpy) combined determines whether a process is spontaneous or not. Measuring Entropy Equation: S = (k) ln(W) o S = entropy o K = Boltzman constant o W = # of possible microstates Note: Microstates - The number of microstates of any substance is the number of possible ways the particles can be arranged in a space. o In the picture above, the particles in the box on the left have fewer possible microstates because they have less room to arrange themselves. If that box grew, the number of possible microstates will increase. - The formula for calculating the entropy of the system in this case is: o ∆S sys= nR ln (W final initial o ∆S sys= S finalS initial o If ∆S > 0, randomness increases Standard Entropies S is in J * K *mol- H 2(aq) = 69.9 H O(g) = 188.8 2 I2(s) = 116.2 I2(g) = 260.7 - S solid < S liquid << S gasO Second Law of Thermodynamics - When a spontaneous event occurs in our universe it is accompanied by an overall increase in the entropy of our universe. ∆S = ∆S sys+ ∆S surrounding sys - ∆S univ 0 Generic Reaction: aA + bB → cC + dD O O O O - ∆S rxn = [cS (C) + dS (D)] – [aS (A) + bS (B)] - (or) ∆S sys = ∑n[S (products)] - ∑n[S (reactants)] 2/12/2016 Goal is to predict if a process is spontaneous. - Ex: 2HgO(s) → 2Hg(l) + O (g2 o S HgO(s) = 72 J/k*mol o S Hg(l) = 77.4 J/k*mol o o S O (g2 = 205 o ∆S oRXN= 2(77.4) + 205 – 2(72) o o ∆S RXN= 215.8 J/k*mol - Makes sense that entropy is high because randomness is increasing due to a solid yielding a liquid and gas which are more random than a solid. Try to find ∆S of surroundings. - ∆S > 0 = ∆S + ∆S univ sys surr - Suppose reaction is exothermic; a.k.a. ∆H RXN< 0 - If heat is released, surroundings warm up and S surrill increase. o ∆S is proportional to -∆H RXN - Suppose you do an exothermic reaction at a low temp and then again at a high temp. o ∆S surrow) > ∆S surrigh) o ∆S surrs proportional to 1/T ∆S surr= -∆H RXN/T o Ex: N 2 3H → 2NH 3 ∆H RXN = -92.6 kJ - ∆SoRXN = 2(S NH )3– (S N 2 – 3(S H )2 = 2(193) – (192) – 3(131) = -199 J/k*mol - At 25 C, is reaction spontaneous? - ∆Suniv ∆S RXN- ∆H RXNT 3 = -199J/k – 92.6*10 J/298k = 112 J/k (positive so reaction is spontaneous) Where do standard entropies come from? Third Law of Thermodynamics - Entropy of a perfect crystalline substance is zero at absolute zero (0k) of temperature. o A.k.a. substance is no longer random. 298k 0k - ∆S of anything = S anything S anything Free Energy Function G - Gibbs free energy formed to couple together entropy and enthalpy. - ∆Suniv= ∆S RXN - ∆HRXN/T - -T∆Suniv= ∆H RXN - T∆S RXN o -T∆S univs Gibbs free energy (G) - G = H – TS - ∆G RXN= ∆H - T∆S RXN o If ∆G RXN < 0, then reaction is spontaneous. o If ∆G RXN > 0, then reaction is not spontaneous. o If ∆G RXN = 0, then reaction is at equilibrium.


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