Part II of this five-part series is focused on further clarification of entropy and thermodynamics. We emphasize that entropy is a state function with a numerical value for any substance in thermodynamic equilibrium with its surroundings. The interpretation of entropy as a “spreading function” is suggested by the Clausius algorithm. The Mayer-Joule principle is shown to be helpful in understanding entropy changes for pure work processes. Furthermore, the entropy change when a gas expands or is compressed, and when two gases are mixed, can be understood qualitatively in terms of spatial energy spreading. The question-answer format of Part I1 is continued, enumerating main results in Key Points 2.1–2.6.

In Part I, we showed that the entropy of a room temperature solid can be calculated at standard temperature and pressure using heat capacity data from near absolute zero, denoted by T=0+, to room temperature,...

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