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 = P⁰ is usually given the symbol S°, and has units of joules per mole 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.

The standard molar entropy of a gas at STP includes contributions from:

• The heat capacity of one mole of the solid from 0 K to the melting point (including heat absorbed in any changes between different crystal structures).
• The latent heat of fusion of the solid.
• The heat capacity of the liquid from the melting point to the boiling point.
• The latent heat of vaporization of the liquid.
• The heat capacity of the gas from the boiling point to room temperature.

References

Relevant Sciencemadness threads

• Molar Entropy/Enthalpy Values of H2SO3