Standard molar entropy

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In chemistry, the standard molar entropy is the entropy content of one mole of substance, under conditions of standard temperature and pressure. The standard molar entropy is usually given the symbol S^o, and the units J mol⁻¹ K⁻¹ (joules per mole kelvin). Unlike standard enthalpies of formation, the value of S^o is an absolute. That is, an element in its standard state has a nonzero value of S^o at room temperature. Only at 0 K is the entropy of an element 0 J mol⁻¹ K⁻¹, the third law of thermodynamics.

By comparing the entropies S and enthalpies H of products and reactants in a chemical reaction, we can determine whether the reaction will go forward or backwards. If the Gibbs free energy change ΔG (ΔG = ΔH − TΔS) is negative, the forward reaction will take place, that is, the reaction can be regarded as being spontaneous. Care must always be taken in that ΔG and ΔH are usually in units of kJ mol⁻¹, and the units must be consistent for the equation ΔG = ΔH − TΔS to be used. If ΔH is positive (an endothermic reaction), then ΔS must also be positive if ΔG is to be negative. In short, for an endothermic reaction to be spontaneous, there must always be an increase in entropy.

See also

• Gibbs free energy
• Third law of thermodynamics
• Entropy

References

External links

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