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Chemistry LibreTexts

7: Entropy, Part II

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    • 7.1: Calculating Entropy Changes
      Entropy changes are fairly easy to calculate so long as one knows initial and final state. For example, if the initial and final volume are the same, the entropy can be calculated by assuming a reversible, isochoric pathway and determining an expression for dq/T. That term can then be integrated from the initial condition to the final conditions to determine the entropy change.
    • 7.2: Trouton's rule
      Trouton's rule  says that for many liquids the entropy of vaporization is almost the same, around 85 J/K. The (partial) success of the rule is due to the fact that the entropy of a gas is considerably larger than that of any liquid.
    • 7.3: The Third Law of Thermodynamics
      One important consequence of Botlzmann’s proposal is that a perfectly ordered crystal (i.e. one that has only one energetic arrangement in its lowest energy state) will have an entropy of 0. This makes entropy qualitatively different than other thermodynamic functions. For example, in the case of enthalpy, it is impossible have a zero to the scale without setting an arbitrary reference (i.e., the enthalpy of formation of elements in their standard states is zero.) But entropy has a natural zero!

    Thumbnail: Image used with permission (Guy vandergrift [CC BY-SA 3.0 (])