Lecture6 - Take Home Lessons Thermodynamics Simple Elegant...

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1 Chapter 20 Thermodynamics: Entropy, Free Energy and the Direction of Chemical Reactions “Take Home Lessons” Thermodynamics: Simple Elegant Relations that Teach Us the Directions of Reactions and Maximum Extractable Work Entropy makes the world go round Entropy is simply the number of different possible permutations/probability to have certain arrangement of atoms AND energy Entropy also referred to as disorder Entropy 0 (always increases iff change)…2 nd Law Entropy = 0 at T=0; S = k lnW (W =1)….3 rd Law Enthalpy can be related to Entropy via q transfer from or to surroundings…gives relationship to propensity of system to increase bond strength or go down hill in energy Thermodynamics: Entropy, Free Energy, and the Direction of Chemical Reactions 20.1 The Second Law of Thermodynamics: Predicting Spontaneous Change energy/heat needed …. Probability of gaining net energy to drive reaction related to entropy (Second Law of Thermodynamics Δ S>0) 20.2 Calculating the Change in Entropy of a Reaction (Third Law of Thermodynamics S=0 at T=0 absolute determination of entropy) 20.3 Entropy, Free Energy, and Work 20.4 Free Energy, Equilibrium, and Reaction Direction { RECALL S = k ln W where S is entropy, W is the number of ways of arranging the components of a system, and k is a constant (the Boltzman constant), R/N A (R = universal gas constant, N A = Avogadro’s number. •A system with relatively few equivalent ways to arrange its components (smaller W) has relatively less disorder and low entropy. •A system with many equivalent ways to arrange its components (larger W) has relatively more disorder and high entropy. Δ S universe = Δ S system + Δ S surroundings > 0 This is the second law of thermodynamics.

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2 Entropy Changes in the System Δ S 0 rxn - the entropy change that occurs when all reactants and products are in their standard states. Δ S 0 rxn = Σ Δ S 0 products - Σ Δ S 0 reactants The change in entropy of the surroundings is directly related to an opposite change in the heat of the system and inversely related to the temperature at which the heat is transferred. Δ S surroundings = - Δ H system T Figure 20.10 Components of Δ S 0 universe for spontaneous reactions exothermic system becomes more disordered exothermic system becomes more ordered endothermic system becomes more disordered Sample Problem 20.3: SOLUTION: Determining Reaction Spontaneity PROBLEM:
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Lecture6 - Take Home Lessons Thermodynamics Simple Elegant...

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