CH353 - Physical Chemistry I
Spring 2013 Unique 52575

Lecture Summary, 4 February 2013

Entropy of phase transitions: Phase transitions are to a good approximation reversible processes.  Therefore, we can estimate the heat involved in a phase transition, q(trans) = deltaH(trans).  The change in entropy of the system is therefore

  deltaS(trans) = deltaH(trans)/T

Entropy of Mixing:  If two gasses at the same T and P are separated by a partition, and if that partition is removed, the gasses will mix spontaneously.  The change in entropy of the system is

  deltaS(mix) = -n(2)Rln(V2/Vf) + -n(1)Rln(V1/Vf)

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.