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# Lecture 27 - For the reaction 2 Cs(g I2(g 2 CsI(g the...

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3/19/10 1 35 For the reaction 2 Cs (g) + I 2 (g) ! 2 CsI (g), the change in entropy of the system is A) Positive B) Negative C) Zero D) Cannot be determined 3 moles gas to 2 moles gas i. Determining ! S sys : What is the entropy of a compound? Need to compare all compounds to a reference system: The entropy of a pure perfect crystal at 0 K is ZERO. Tabulate values of absolute entropy (at a given T, 1 bar, 1 M): products Also denoted ! S o rxn 36 4.3.D. Calculating ! S sys and ! S sur to determine ! S univ ! S o = n p S o ( p ) " # n r S o ( r ) " reactants Third Law of Thermodynamics (no disorder in perfect system with no atomic motion) - Sº values on data sheet and in text Appendix B Change of entropy for a reaction, ! : 37 ii. Determining ! S sur : Given amount of heat will produce greater % change at lower T ! S sur ! 1/T - system can be a reaction, so ! H sys = ! H rxn Sign depends on direction of heat flow Endothermic rxn: ! H sys > 0 heat from sur. T sur " ! S sur < 0 Magnitude depends on T ! S sur > 0 Exothermic rxn: ! H sys < 0 heat to sur. T sur # ( | ! S sur | larger when T is small )

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