Note that the value for the standard absolute entropy for gaseous water (188.7 J K -1 mol -1) is greater than for liquid water (69.9 J K -1 mol -1). Note that water exists in two different states at 298.15 K and atmospheric pressure, as a liquid (H 2O (l)) and as a gas (H 2O (g)). S° (diamond) = 2.38 J K -1 mol -1 (3-dimensional covalent lattice) S° (graphite) = 5.74 J K -1 mol -1 (2-dimensional covalent lattice) S° (NaCl (g)) = 72.4 J K -1 mol -1 (3-dimensional ionic lattice) Note the (generally) large positive values of S° for gaseous substances in which the molecules are chaotically and randomly distributed:Īnd the (generally) smaller positive values of S° for solid substances in which intermolecular forces act to keep in the particles in a more structured and ordered array: Some examples are given in the table below: The values of standard absolute entropy (S°) have been tabulated for many substances. The entropy of a substance reflects the energy distribution (joules, J) at a specific temperature (kelvin, K) for a specific amount of substance (moles, mol), so the units of standard absolute entropy are J K -1 mol -1. Standard absolute entropy is given the symbol S° Standard absolute entropy refers to the absolute entropy of a substance in its standard state (that is, its state at 298.15 K and atmospheric pressure). S T is then referred to as the absolute entropy of this crystal at temperature T K. We can substitute 0 for S 0 in the equation to get: Since the entropy of a perfect crystal at 0 K is zero: Then, the increase in the entropy of the crystal when heated from 0 K to a higher temperature of T K is: If ΔS° reaction is negative (ΔS° reaction 0 K then S > 0 If ΔS° reaction is positive (ΔS° reaction > 0), the entropy of the system increased.
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