Standard Molar Entropy, S^o

The standard molar entropy, S^o, is the entropy of 1 mole of a substance in its standard state, at 1 atm of pressure. These values have been tabulated, and selected substances are listed in Table 1.1 “Standard Molar Entropies of Selected Substances at 298 K.”

Table 1.1. Standard Molar Entropies of Selected Substances at 298 K

Several trends emerge from standard molar entropy data:

• Larger, more complex molecules have higher standard molar enthalpy values than smaller or simpler molecules. There are more possible arrangements of atoms in space for larger, more complex molecules, increasing the number of possible microstates.
• Gases tend to have much larger standard molar enthalpies than liquids, and liquids tend to have larger values than solids, when comparing the same or similar substances.
• The standard molar entropy of any substance increases as the temperature increases. This can be seen in Figure 1.1 “Entropy vs. Temperature of a Single Substance.” Large jumps in entropy occur at the phase changes: solid to liquid and liquid to gas. These large increases occur due to sudden increased molecular mobility and larger available volumes associated with the phase changes.

Figure 1.1. Entropy vs. Temperature of a Single Substance

This is a generalized plot of entropy versus temperature for a single substance.