Standard molar entropy

In chemistry, the standard molar entropy is the entropy content of one mole of pure substance at a standard state of pressure and any temperature of interest. These are often (but not necessarily) chosen to be the standard temperature and pressure.

The standard molar entropy at pressure = ${\displaystyle P^{0}}$ is usually given the symbol S°, and has units of joules per mole per kelvin (J⋅mol⁻¹⋅K⁻¹). 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.