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SPONTANEITY: ENTROPY AND FREE ENERGY A spontaneous process is one that can happen without outside intervention. It is a process, in which products are favored over reactants at specified conditions. Examples: sublimation of CO 2 (s) at room temperature, rusting of iron. Many products favored (spontaneous) reactions are exothermic. However, not all exothermic processes are spontaneous. Also, not all spontaneous processes are exothermic. The dissolution of NaCl(s) in water is an endothermic process, yet it is spontaneous. The melting of ice is an endothermic yet spontaneous process at a temperature above 0 ° C. What are the factors that make a process spontaneous? Two factors determine the spontaneity of a process under specified conditions: the enthalpy change and the entropy change. An exothermic process favors but does not imply spontaneity and an increase in disorder favors by does not imply spontaneity either. Therefore, processes are more likely to be spontaneous if the products have lower heat content and greater disorder. ENTROPY The thermodynamic state function entropy, S, is a measure of the disorder of a system. It is one of two factors that affect spontaneity. Entropy is associated with probability. The more ways a certain state can be reached, the higher the probability for that state to occur. The entropy change depends on both, heat (q) and temperature (T). Therefore, the quantitative entropy change for a reversible process
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This note was uploaded on 05/06/2009 for the course CH 302 taught by Professor Holcombe during the Spring '07 term at University of Texas at Austin.

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