Unlike standard enthalpies of formation, the value of S° is absolute.
That is, an element in its standard state has a definite, nonzero value of S at room temperature.
The entropy of a pure crystalline structure can be 0 J⋅mol−1⋅K−1 only at 0 K, according to the third law of thermodynamics.
However, this assumes that the material forms a 'perfect crystal' without any residual entropy.
This can be due to crystallographic defects, dislocations, and/or incomplete rotational quenching within the solid, as originally pointed out by Linus Pauling.
[1] These contributions to the entropy are always present, because crystals always grow at a finite rate and at temperature.
However, the residual entropy is often quite negligible and can be accounted for when it occurs using statistical mechanics.
is the molar heat capacity at a constant pressure of the substance in the reversible process k. The molar heat capacity is not constant during the experiment because it changes depending on the (increasing) temperature of the substance.