The same is true for a chemical reaction. For a reversible reaction Y ↔ X, a large excess of Y over X will tend to drive the reaction in the direction Y → X. Therefore, as the ratio of Y to X increases, the ∆G becomes more negative for the transition Y → X (and more positive for the transition X → Y).
The amount of concentration difference that is needed to compensate for a given decrease in chemical-bond energy (and accompanying heat release) is not intuitively obvious. In the late nineteenth century, the relationship was determined through a thermodynamic analysis that makes it possible to separate the concentration-dependent and the concentration-independent parts of the free-energy change, as we describe next.
The standard free-energy Change, ∆G°, makes it possible to Compare the energetics of Different Reactions
Because ∆G depends on the concentrations of the molecules in the reaction mixture at any given time, it is not a particularly useful value for comparing the relative energies of different types of reactions. To place reactions on a comparable basis, we need to turn to the standard free-energy change of a reaction, ∆G°. The ∆G° is the change in free energy under a standard condition, defined as that where the concentrations of all the reactants are set to the same fixed value of 1 mole/liter. Defined in this way, ∆G° depends only on the intrinsic characters of the reacting molecules.
For the simple reaction Y → X at 37°C, ∆G° is related to ∆G as follows: where ∆G is in kilojoules per mole, [Y] and [X] denote the concentrations of Y and X in moles/liter, ln is the natural logarithm, and RT is the product of the gas constant, R, and the absolute temperature, T. At 37°C, RT = 2.58 J mole–1. (A mole is 6 × 1023 molecules of a substance.)
A large body of thermodynamic data has been collected that has made it possible to determine the standard free-energy change, ∆G°, for the important metabolic reactions of a cell. Given these ∆G° values, combined with additional information about metabolite concentrations and reaction pathways, it is possible to quantitatively predict the course of most biological reactions.
The free energy of Y is greater than the free energy of X. Therefore ˜G < 0, and the disorderENERGETICALLY of the universe increasesFAVORABLE during the reactionREACTION Y
X.
If the reaction X
Y occurred, ˜G would be > 0, and theENERGETICALLY universe would
REACTION ordered.
this reaction can occur only if it is coupled to a second, energetically favorable reaction
Figure 2–28 The distinction between energetically favorable and energetically unfavorable reactions.
the energetically unfavorable reaction X
Y is driven by the energetically favorable reaction C
D, because the net free-energy change for the pair of coupled reactions is less than zero
Figure 2–29 How reaction coupling is used to drive energetically unfavorable reactions.
FOR THE ENERGETICALLY FAVORABLE  REACTION Y °X, Figure 2–30 Chemical equilibrium. When a reaction reaches equilibrium, the forward and backward fluxes of reacting molecules are equal and opposite.
when X and Y are at equal concentrations, [Y] = [X], the formation of X is energetically favored. In other words, the ˜G of Y °X is negative and the ˜G of X °Y is positive. But because of thermal bombardments, there will always be some X converting to Y.
THUS, FOR EACH INDIVIDUAL MOLECULE, conversion of
Y to X will occur often.
Conversion of X to Y will occur less often than the transition Y °X, because it requires a more energetic collision.
Therefore the ratio of X to Y molecules will increase with time
EVENTUALLY, there will be a large enough excess of X over Y to just compensate for the slow rate of X °Y, such that the number of Y molecules being converted to X molecules each second is exactly equal to the number of X molecules being converted to Y molecules each second. At this point, the reaction will be at equilibrium.