CH353 - Physical Chemistry I
Spring 2015, Unique 51170

Lecture Summary, 17 February 2015


Third Law of Thermodynamics:  For any substance at any temperature Tf, the entropy of the system can be accounted for by 5 processes:

  1) Heating from T = 0 K to T = T(fus)
  2) Phase transformation from the solid to the liquid
  3) Heating from T = T(fus) to T = T(vap)
  4) Phase transformation from the liquid to the gas
  5) Heating from T = T(vap) to T = Tf

The third law of thermodynamics defines the entropy of a pure crystalline substance at absolute zero to be 0 J/K.  Thus, any substance at any temperature has a nonzero value of entropy that can be quantified by counting up the contributions from these 5 transformations.  This entropy, called the molar or 3rd law entropy, is given the term Sm, and can be determined for any substance.  Molar entropies are tabulated in the standard state of 298 K and 1 bar for convenience. 

Molar entropy:  Today we looked at patterns for the magnitude of molar entropy for similar compounds at the standard state.  We made a few conclusions:

  1) Molar entropy increases with FW
  2) Molar entropy increases with additional phase transformations.
  3) Molar entropy increases with conformational degrees of freedom from molecular structure.

Microstates:  All of this implies that there is a molecular mechanism for entropy.  Today we quantified this by defining microstates: experimentally indistinguishable configurations that all lead to a single experimentally distinguishable outcome.  The more microstates available to a system, the higher its entropy.  A system will move spontaneously in the direction of increasing microstates.