Principles of Biochemistry Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
The ATP Cycle and Cellular Bioenergetics
The values of ∆G0' and ∆G differ, and this distinction is of great significance

It is important to clearly distinguish between the Free energy change ∆G and the Standard Free Energy change ∆G⁰'. As we already know, in any spontaneous chemical or physical process, the Free energy of a reaction system always decreases, meaning that ∆G is always expressed as a negative value. However, we also know that every chemical reaction corresponds to a strictly defined standard free energy change ∆G⁰', which can be positive, negative, or zero depending on the Equilibrium Constant of that particular reaction. The magnitude of the standard free energy change ∆G⁰' tells us the direction and extent to which a reaction will proceed before reaching equilibrium under standard conditions—that is, initial concentrations of all components at 1.0 M, pH 7.0, and a Temperature of 25 ˚C. Consequently, ∆G⁰' is a strictly defined constant characteristic of each given reaction. By contrast, the true free energy change ∆G for a given chemical reaction depends on the actual conditions under which the reaction takes place (i.e., the concentrations of the reacting components, pH, and temperature), and these conditions may not coincide with standard conditions. Moreover, the value of ∆G for any reaction proceeding toward equilibrium is always negative and decreases in absolute magnitude (becomes less negative) as equilibrium is approached; at the exact moment equilibrium is reached, it is equal to zero, indicating that no further work can be performed by this reaction.

The values of ∆G and ∆G⁰' for the reaction A + B → C + D are related by the equation

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in which the quantities highlighted in red characterize the true state of the system.

Let us consider a simple example. Assume that the reaction A + B → C + D proceeds at standard temperature (25 °C) and standard pressure (1 atm), but that the initial concentrations of components A, B, C, and D are not equal to one another and none of them equals the standard value of 1.0 M. To determine the true free energy change ∆G for the case where equilibrium is established at these non-standard initial concentrations, we simply substitute the actual initial concentrations of components A, B, C, and D into the equation given above; the values of R, T, and ∆G⁰' naturally retain their usual values. Solving this equation yields ∆G—that is, the free energy change of the given reaction at the component concentrations under which it actually occurs. The value of ∆G will be negative and will decrease over time as the concentrations of A and B decrease and the concentrations of C and D increase as the reaction proceeds. Thus, the value of ∆G for a truly proceeding chemical reaction is always negative and always approaches zero, whereas ∆G⁰' is a constant value.

It is important to remember that ∆G⁰' and ∆G only indicate the maximum amount of free energy that a given reaction can theoretically yield. This amount of energy can be utilized only if There is a highly efficient mechanism available capable of capturing this energy and channeling it to perform useful work. If no such mechanism exists, then at constant temperature and pressure, no work can be performed by that reaction.