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The third law of thermodynamics (Nerst
heat theorem) and absolute entropies
The third law of thermodynamics (Nerst heat theorem) staes that the energy of a perfect crystal at 0K is 0. Note  This perfection can never be reached (e.g. isotopes, different
nuclear spins) and gives rise to the so called residual entropy.
Standard/Absolute entropies Def'n: The standard entropy
of a compound is the molar entropy at 298.15K and 1 Bar pressure.
The entropy at a temperature T (and 1 Bar pressure) is calculated as follows (see figure below) : (1) All the properties (except S(0) can be measured experimentally. Note that if we assume that S(0) = 0, then the entropy takes the name of absolute (or third law) entropy.(2) The integrals can be either calculated graphically from the curve 'c_{p}/T' vs. 'T' (see fig. below), by fitting a polynomial and then integrating the polynomial analytically, or by evaluating the area of a plot of c_{p} vs ln(T). (3) T here is a problem in evaluating the integral:
(a) measure c_{p} to as low a temperature as possible; 
