Page 172 - Thermodynamics of Biochemical Reactions

Thernwdyanamics of Biochemical Reactions. Robert A. Alberty
Copyright 0 2003 John Wiley & Sons, Inc.
ISBN 0-471-22851-6

4 10.1 Calorimetric Determination of the Standard
Transformed Enthalpy of Reaction
R 10.2 Calculation of Standard Transformed
Enthalpies of Reaction from the Standard
Enthalpies of Formation of Species
10.3 Calculation of Standard Transformed Entropies
of Biochemical Reactions
%" 10.4 Effect of Temperature

The enthalpy H of a chemical reaction system is of special interest because when
a reaction occurs at constant pressure, the change in enthalpy is equal to the heat
absorbed; AH = qp. The standard enthalpy A,Ho of a chemical reaction can be
obtained by determining the heat of reaction calorimetrically or by measuring the
dependence of the equilibrium constant K on temperature. The standard trans-
formed enthalpy A,H" of a biochemical reaction can be obtained by determining
the heat of reaction at specified pH or by measuring the dependence of the
apparent equilibrium constant K' on temperature. The determination of the
transformed enthalpy of reaction of a biochemical reaction is complicated by the
fact that the biochemical reaction may produce or consume hydrogen ions that
react with the buffer to produce an additional heat effect in a calorimetric
measurement. When the reactants bind metal ions, a similar effect will occur if the
metal ion is bound by a reactant. If these effects are properly handled in the
analysis of calorimetric data, the transformed enthalpies of reaction determined
from equilibrium constants and from calorimetric measurements should agree.
Enthalpies of reaction may be rather constant over narrow temperature ranges,
but over wider temperature ranges, it is necessary to take into account the
dependence of the heat of reaction on temperature. For a biochemical reaction
the effect of temperature on the heat of reaction depends on the pH.
If the change in heat capacity in a chemical reaction is equal to zero, the
enthalpy of the reaction is independent of temperature, and the equilibrium
constant of the chemical reaction can be readily calculated over a range of
temperature without making an integration, as described in Section 3.7. In
general, the enthalpy of a chemical reaction is a function of temperature and ionic
strength. When A,Go and A,Ho are known, the standard reaction entropy A,So
can be calculated
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