Page 41 - Thermodynamics of Biochemical Reactions

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

3.1 Derivation of the Expression for the Equilibrium
Constant
3.2 Changes in Thermodynamic Properties in
Chemical Reactions
3.3 Importance of Components
3.4 Gibbs-Duhem Equation and the Phase Rule at
Chemical Equilibrium
3.5 Isomer Group Thermodynamics
3.6 Effect of Ionic Strength on Equilibria in Solution
Reactions
3.7 Effect of Temperature on Thermodynamic
Properties
3.8 Chemical Thermodynamic Tables Including
Biochemical Species

When a chemical reaction occurs in a closed system at constant T and P, the
criterion for spontaneous change and equilibrium is no longer dG d 0 at constant
7; P, and (ni} because the amounts of species change in the reaction. Therefore,
the following question arises: If the amounts of species are not constant during
the approach to equilibrium in a reaction system, what is? The answer is: The
amounts n,, of components are constant in a reaction system. When a chemical
reaction occurs in a closed system, the amounts of atoms of elements and electric
charge are conserved. Atoms of elements and electric charge can be taken as
components, but some of these conservation equations may be redundant and are
therefore not needed. Groups of atoms in molecules can also be chosen as
components. This is important in biochemistry when large molecules are involved
because counting atoms becomes laborious. Various choices of components can
be made for a reaction system, but the number of components is independent of
the set of components chosen. A particular set of components may be especially
useful, depending on the objective of the calculation. In an independent set of
conservation equations, no equation in the set can be obtained by adding and
subtracting other equations in the set. Thus we will see in this chapter that the
criterion for spontaneous change and chemical equilibrium in a closed system is
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