Page 55 - Thermodynamics of Biochemical Reactions

3.8 Chemical Thermodynamic Tables Including Biochemical Species 49

equation for the standard transformed Gibbs energy of formation of a species is
given by
RTa = 9.20483 x 10-3T - 1.28467 x 10-’T2 + 4.95199 x 10-*T3 (3.7-4)

This equation reproduces the second column of Table 3.1 to 0.1% accuracy. The
coefficient RT2(da/dT), in the equation for the standard transformed enthalpy of
formation of a species is given by

RT’ (g) -1.28466 x 10p5T2 + 9.90399 x 10-’T3 (3.7-5)
=
P
This equation reproduces the third column of Table 3.1 to 1% accuracy. The
calculations of these three functions are shown in Problem 3.5, and they are used
in the calculation of standard Gibbs energies of formation and standard enthal-
pies of formation of species at other temperatures in Problems 3.6 and 3.7.
Thermodynamic properties in dilute aqueous solutions are taken to be
functions of ionic strength so that concentrations of reactants, rather than their
activities can be used. This also means that pH, = -log[H+] has to be used in
calculations, rather than pH, = -log{a(H+)}. When the ionic strength is different
from zero, this means that pH values obtained in the laboratory using a glass
electrode need to be adjusted for the ionic strength and temperature to obtain the
pH that is used to discuss the thermodynamics of dilute aqueous solutions. Since
pH, = -logy(H+) + pH,, the use of the extended Debye-Huckel theory yields
a 11’2
(3.7-6)
ln(10) 1 + 1.61”’
pH, - pH, = ~
These adjustments, which are tabulated in Section 1.2, are to be subtracted from
the pHa obtained with a pH meter to obtain pH,. pH, is lower than pH, because
the ion atmosphere of H+ reduces its activity (see Problem 3.7). In the rest of the
book, pH is taken to be pH,.

H 3.8 CHEMICAL THERMODYNAMIC TABLES
INCLUDING BIOCHEMICAL SPECIES

A useful way to store data on equilibrium constants and enthalpies of chemical
reactions is to use equations 3.2-4 and 3.2-13 to calculate standard Gibbs energies
of formation and standard enthalpies of formation of species and to tabulate these
values. Since there are more species than independent chemical reactions between
them (remember N, = R + C), this can only be done by adopting some conven-
tions. The major convention for the construction of chemical thermodynamic
tables is that A,GP and A,HP for each element in a specified reference state is taken
as zero at each temperature. The reference state for the elements that are gases at
room temperature is the ideal gas state at 1 bar. For each solid element, a
particular state has been chosen for the reference state; this is generally the most
stable state at room temperature. In order to treat the thermodynamics of
electrolyte solutions, it is necessary to adopt an additional convention, and that
is that AfGP = AfHP = C&(i) = 0 at zero ionic strength for H+(aq) at each
temperature. Since the thermodynamic properties of ions depend on the ionic
strength, the convention of tabulating values at zero ionic strength has been
adopted. These arbitrary conventions make it possible to have tables of standard
Gibbs energies of formation Af GP, standard enthalpies of formation AfHF, and
molar heat capacity C&,,(i) of species at 298.15K and zero ionic strength. The
NBS Tables (1982) summarize a very large body of standard thermodynamic
properties of species obtained by chemical methods. However, this table does not
contain very much information on biochemical metabolites because it includes