CH353
- Physical Chemistry I Spring 2012, Unique 52135 Lecture Summary, 10 February 2012 |

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 delta S(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 isdelta S(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
= TfThe 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. |