Standard molar entropy
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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.