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
= 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. 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. |