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Chapter 20 Lecture 3

# Chapter 20 Lecture 3 - Review Predicting Relative Entropy...

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Review: Predicting Relative Entropy Values Which of the following has higher S o a) Air at 25°C or air at 35°C S o increases with T b) CH 3 OH( l ) at 25°C or CH 3 OH( g ) at 25°C S o increases from liquid to gas ) N Cl( ) t 25°C N Cl( ) t 25°C c) NaCl( s ) at 25°C or NaCl( aq ) at 25°C S o typically increases with dissolution of solids d) N 2 ( g ) at 25°C or N 2 ( aq ) at 25°C S o decreases with dissolution of gases e) HCl( g ) at 25°C or HBr( g ) at 25°C S o increases with increasing the molar mass f) CO 2 ( g ) at 25°C or CH 3 OH( g ) at 25°C S o increases with increasing the molecular 20-29 complexity (# atoms)

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Entropy Changes in the System Standard Entropy of Reaction : S o rxn - the entropy change that occurs when all reactants and products are in their standard states. S o rxn = mS o products - nS o reactants Example : Calculate the standard entropy S o rxn of the reaction: N 2 O 4 ( g ) 2NO 2 ( g ) S o rxn = Σ mSº products - Σ nSº reactants = 2×Sº NO2( ) – 1×Sº N2O4( ) 2 S NO2(g) 1 S N2O4(g) From Appendix B: S o rxn = 2 mol×239.9 J/mol ڄ K – 1 mol×304.3 J/mol ڄ K = 175.5 J/K Note : for reactions involving gases: S o rxn > 0 if (# mol gaseous products) > (# mol gaseous reactants) 20-30 S o rxn < 0 if (# mol gaseous products) < (# mol gaseous reactants)