Page 165 - Chemical equilibria Volume 4

Determination of the Values Associated with Reactions – Equilibrium Calculations 141
The old tables usually use the kcal/mol for enthalpies and the
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cal.mol K (or clausius, which is identical) for entropies. The standard state
is chosen for the pressure of 1 atmosphere. Thus, we have the following
conversions to switch to the international system:
3
– for the enthalpies, multiply by 4.184×10 to switch from kilocalories
per mole to joules per mole. The effect of pressure is negligible;
– for the entropies, it is helpful to multiply the values by 4.184 to switch
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from clausius or cal.mol K to J.mol K , and add the term R.ln1.01325 =
0.1094, to take account of the change of pressure for the standard state;
– for the Gibbs energies, we multiply the ancient value by 4.184 to switch
from calories to joules, and we add the term RT.ln1.01325 to take account of
the pressure change for the standard state.
In terms of the detailed data, we distinguish the basic tables and the
detailed tables.

The basic tables, for each substance in a given physical state (crystalline
solid in a given variety, liquid or gas), give us:

– the standard enthalpy of formation at the temperature of 298.15 K
(25°C);

– the standard entropy at the temperature of 298.15 K (25°C);
– an equation showing the variation of the molar specific heat capacities
at constant pressure, at standard pressure, with temperature and the
temperature range of its validity; and

– the temperatures of state change and the enthalpies associated with
those transformations (sometimes called the latent heat of state change) at
that temperature.

Certain tables are supplemented by the standard Gibbs energy of
formation at 298 K and the logarithm of the equilibrium constant of the
reaction of formation of the compound at the temperature of 25°C.

The detailed tables, for certain temperature values, generally at constant
intervals, and a specific physical state of the substance, give us the following
data: