Editing Standard molar entropy (section)

{{Layman|date=February 2025}}{{short description|Standard entropy content of one mole of a substance under a standard state}}
In [[chemistry]], the '''standard molar entropy''' is the [[entropy]] content of one [[mole (unit)|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 = <math>P^0</math> is usually given the symbol {{mvar|S°}}, and has units of [[joule]]s per [[Mole (unit)|mole]] per [[kelvin]] (J⋅mol<sup>−1</sup>⋅K<sup>−1</sup>). Unlike [[Standard enthalpy of formation|standard enthalpies of formation]], the value of {{mvar|S°}} is absolute. That is, an element in its standard state has a definite, nonzero value of {{mvar|S}} at [[room temperature]]. The entropy of a pure [[crystalline]] structure can be 0{{nbsp}}J⋅mol<sup>−1</sup>⋅K<sup>−1</sup> only at 0{{nbsp}}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 defect]]s, [[dislocations]], and/or incomplete rotational quenching within the solid, as originally pointed out by [[Linus Pauling]].<ref>{{cite book |last1=Pauling |first1=Linus |title=The Nature of the Chemical Bond |date=1960 |publisher=Cornell University Press |location=Ithaca, NY |edition=3rd}}</ref> 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]].