# L21 - The second law For an isolated system a spontaneous...

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Dr. P. Lucas U of A MSE 110 Intro to thermo The second law : For an isolated system, a spontaneous process is always associated with an increase in entropy S . Hence, S is always maximum for an isolated system at equilibrium. •T h e entropy is a measure of the degree of disorder of a system and. The entropy is quantified as follow: S = k·ln . •T h e statistical weigh is the number of possible configurations (arrangement of atoms) corresponding to a macroscopic state (example: gas in volume V). There is always more ways to arrange atoms in a disordered state than in an ordered state, hence the most disordered macroscopic state always have a much larger number of possible atomic configurations, hence larger and larger S . The number of possible configurations of the most disordered state being overwhelmingly larger, statistically it is the most probable state and is therefore the state of equilibrium. This is why S is maximum at equilibrium. The third law : There is only one configuration for a perfect crystal at 0ºK, hence S=0. For a non-isolated system, the entropy is not the only driving force of spontaneous processes, instead, the f ree energy : G = H - TS is the variable that predict the direction of processes in non-isolated systems. G<0 for spontaneous processes, G=0 at equilibrium,

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Dr. P. Lucas U of A MSE 110 Free energy and phase change INTRO TO THERMODYNAMICS The free energy G = H - TS is helpful to predict the stable phase (solid, liquid, gas) of a material at a given temperature. For example, during melting, a solid absorbs heat H f as atomic bonds are broken and the solid liquefies. ( H f = H liquid -H solid ) Hence for the process of melting a solid G f = H f –T S f . At the fusion point T
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## This note was uploaded on 11/19/2009 for the course MSE 110 taught by Professor Lucas during the Spring '08 term at Arizona.

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L21 - The second law For an isolated system a spontaneous...

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