Biochemistry - The Chemical Reactions of Living Cells Volume 1 - D. Metzler 1980

Bioenergetics of Biochemical Reactions
Thermodynamics
∆G0 and Equilibrium Constant

Let us consider the following generalized chemical equation describing a reaction in which $a$ moles of substance A react with $b$ moles of substance B to yield products C, D, etc.:

Class="center">аА + bВ + ... = cC + dD + .... (3-26)

The standard free-energy change for this process is given by

denotes the Free energy of substance A, and so on. The value of ∆G for any given concentrations of reactants and products can be obtained from ∆G⁰ by applying equation (3-23) to each component individually. As a result, we obtain

Here, $a_C$ is The activity of component C, and so forth. This useful relation allows us to calculate ∆G at low concentrations of substances, which is typically the case in biochemical systems (most often, these concentrations are in the millimolar range and significantly lower than the concentration of the hypothetical standard solution, which is 1 M). In practice, concentrations are usually substituted directly into equation (3-28)

¹ Here, the subscript $i$ refers to a specific component of a solution, which may contain other substances alongside the solvent. To be precise, is the partial molar free energy, i.e., The change in the total free energy of a very large volume of solution upon The addition of 1 mole of the given component.

² These same equations can also be expressed in terms of mole fractions.

Equation (3-28) is also valuable in another respect. If the system is at equilibrium, then ∆G = 0, and the ratio is simply the Equilibrium Constant $K$. It follows that

Note that although ∆G⁰ is expressed in kJ∙mol^-1, the free-energy change determined by equation (3-30) pertains to the reaction involving $a$ moles of substance A, $b$ moles of substance B, and so forth.