Lec-13-Chap18-1-b

Lec-13-Chap18-1-b - Lec-13-Chap18-1-b 9/30/2009 Lec-13:...

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Lec-13-Chap18-1-b 9/30/2009 Directionality of Chemical Reactions Entropy Changes. 3 rd Principle. Standard Molar Entropies. 2 nd Principle 1 Lec-13: Thermodynamics: Directionality of Chemical Reactions. Entropy Chapter 18 1 Directionality of Chemical Processes It would be very useful if we could predict if a chemical reaction will proceed spontaneously (or naturally) toward the products. C+ D+ A+ B+ ab cd Will the products or the reactants be favored when equilibrium is reached? Equally useful would be to predict the extent to which a reaction will proceed : 2
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Lec-13-Chap18-1-b 9/30/2009 Directionality of Chemical Reactions Entropy Changes. 3 rd Principle. Standard Molar Entropies. 2 nd Principle 2 Directionality of Chemical Processes Questions of this sort are answered by thermodynamics : the criteria for making such predictions is provided by the Second Principle of Thermodynamics . While entropy and the second principle provide a means for answering whether a process is spontaneous or not, they do not give us any information concerning the rate at which a spontaneous process occurs. Entropy S At the heart of the second principle of thermodynamics is a new function of state: 3 Entropy: Thermodynamic Introduction The entropy change in a process in which the system changes by a very small amount is given by   rev is the absolute temperature, in K T S q T where δq rev is the heat exchanged by the system with the surroundings when the small transformation carried in a quasi-static (or reversible) manner. The entropy is a function of state ; changes in entropy can be calculated for any process . The right hand side of the equation provides the means for calculating δS , even if the process is not reversible . 4
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Lec-13-Chap18-1-b 9/30/2009 Directionality of Chemical Reactions Entropy Changes. 3 rd Principle. Standard Molar Entropies. 2 nd Principle 3 Entropy Calculation: Example If the process is at constant pressure   rev constant T P H q HS  If, in addition, the process is isothermal T S H For example, when liquid water and ice are at equilibrium with each other at p =1bar , the standard entropy change of fusion is given by 0 fusion fus 0 ion fusion H T S 5 Entropy: Thermodynamic Introduction Once again, for a process at constant pressure   2 1 rev constant T T q HC T TT C dT T T p S S This equation allows us to calculate entropy changes (at constant pressure) by making calorimetric measurements.
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Lec-13-Chap18-1-b - Lec-13-Chap18-1-b 9/30/2009 Lec-13:...

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