Page 166 - Chemical equilibria Volume 4

142 Chemical Equilibria
– the value of the specific heat capacity standard at constant pressure;
– the standard entropy;
– the value ( g− T 0 − h 0 298 ) / T which is, in fact, the difference between the
standard entropy at temperature T and the standard entropy at the reference
temperature;
– the difference h − h 0 between the standard enthalpy of formation at
0
T 298
temperature T and that at the reference temperature;
0
– the standard enthalpy of formation at temperature T: h ;
T
– the standard Gibbs energy of formation at temperature T: g ;
0
T
– the logarithm of the equilibrium constant for the formation at
(I)
temperature T: ln K .
f
In fact, it is imperative, before using a thermodynamic table or a
databank, to carefully read the accompanying notice, which usually specifies
the definitions of the values given and the units used. Indeed, the symbols of
the different values are not always the same from one table to another, and it
is possible for confusion to arise. We also need to beware of confusing the
decimal logarithm and the Napierian logarithm. The values given may be
molar, gravimetric or volumetric.

When the data are provided for specific values of the temperatures, linear
interpolations between two values or parabolic interpolations with three
values can be used to obtain non-tabulated data.

4.9. Estimation of thermodynamic data

In practice, we can see that we actually have very scant data on the
standard enthalpies and standard entropies of compounds, found by reliable
measurements, and that those data are not known for a large number of
compounds. Hence, users have sought to estimate, with the greatest possible
degree of accuracy, values for those data when they have not had values
gleaned from calorimetric experimentation or the examination of the
adsorption spectrum.