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I. REVIEW OF THERMODYNAMICS, REDOX, ETC. I. Entropy: 1. Enthalpy = ΔH a. Both of these types of reactions can occur: Exothermic—heat out of the system—ΔH<0 Endothermic—heat into system—ΔH>0 2. Entropy = ΔS: measure of inherent disorder or randomness a. If ΔS>0, that means that there is increased entropy—increased disorder b. If ΔS<0, means entropy is negative—increased order c. Usually, can predict sign of ΔS by inspection. Ex: NaCl (s) Na + (aq) + Cl - (aq) ΔS>0 more ordered more disordered 2H 2(g) + O 2(g) 2H 2 O (g) ΔS<0 3 moles of gas 2 moles of gas d. Both of the examples above can happen, therefore, you can’t predict whether or not a reaction will occur just based on entropy. e. When comparing similar substances, the entropies of gases are much higher than those for liquids, and entropies of liquids are large than those for solids. f. Larger molecules have a larger entropy than smaller molecules g. Molecules with more complex structure have larger entropies than simpler molecules. h. ΔS o = ∑S o (products) - ∑S o (reactants) S o = entropy at standard conditions o = naut—standard conditions—25 o C, 1atm, 1.0M 3. Second Law of Thermodynamics: The entropy of the universe must always increase a. ΔS universe = ΔS system + ΔS surroundings b. For a spontaneous process, or a process that will occur, ΔS universe >0 c. It is possible for a process to occur that causes a more ordered system (like hydrogen gas and oxygen gas combining to form water vapor), but the entropy of the surroundings must be increased to compensate (for water vapor to form, heat must be given off). Ex: Penny Demo. 10 pennies in a stack Throw them up in the air—going from ordered to disordered, so this is spontaneous. However, to put them back into a stack, the system is becoming more ordered—therefore, surroundings have to become more disordered Students have to expend energy looking for pennies and putting them back into stack— surroundings becoming less disordered 4. Compute ΔS o rxn for this reaction: 2NO (g) + O 2(g) 2NO 2(g) 210.8J/K . mol 205.1 J/K . mol 240.0 J/K . mol Predict ΔS o rxn < 0 because becoming more ordered ΔS o rxn = [(2 mol NO 2 )( 240.0 J/K . mol)] – [(2 mol NO)( 210.8J/K . mol) + (1mol O 2 )( 205.1 J/K . mol)] ΔS o rxn = -146.7 J/K (see if it matches with what you predicted)
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5. Gibbs Free Energy: a. ΔG = ΔH o – TΔS o ΔG = Gibbs Free Energy—measure of spontaneity The term “free energy” represents the maximum energy available to do useful work. Therefore, in this context, the word “free” means “available.” If ΔG rxn < 0, the reaction is spontaneous If ΔG rxn = 0, the reaction is at equilibrium with K = 1 If ΔG rxn > 0, the reaction is not spontaneous b. If reaction is spontaneous, ΔG rxn < 0 ΔS universe > 0 always If ΔS system ↑, therefore, ΔS system > 0 (this is good for spontaneity), then, -T ΔS < 0 which helps
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This note was uploaded on 05/17/2008 for the course LBS 172 taught by Professor Laduca during the Spring '08 term at Michigan State University.

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