If a reaction is endergonic in one direction (e.g., converting products to reactants), then it must be exergonic in the other, and vice versa.
As an example, let’s consider the synthesis and breakdown of the small molecule adenosine triphosphate (ATP), which is the “energy currency” of the cell.
ATP is made from adenosine diphosphate (ADP) and phosphate (Pi) according to the following equation:
ADP + Pi → ATP + H2O
This is an endergonic reaction, with ∆G = +7.3 kcal/mol under standard conditions (meaning 1 M concentrations of all reactants and products, 1 atm pressure, 25 degrees C, and pH of 7.0).
In the cells of your body, the energy needed to make ATP is provided by the breakdown of fuel molecules, such as glucose, or by other reactions that are energy-releasing (exergonic).
The reverse process, the hydrolysis (water-mediated breakdown) of ATP, is identical but with the reaction flipped backwards:
ATP + H2O → ADP + Pi
This is an exergonic reaction, and its ∆G is identical in magnitude and opposite in sign to that of the ATP synthesis reaction (∆G = +7.3 kcal/mol).
This relationship of same magnitude and opposite signs will always apply to the forward and backward reactions of a reversible process.