Final answer:
For the reaction A + B ↔ C + D with ΔG = -8.7 kcal/mole, a suitable coupled reaction would have a positive ΔG close to or greater than 8.7 kcal/mole. The coupled reaction should turn the overall ΔG negative to achieve spontaneity.
Step-by-step explanation:
To identify a suitable coupled reaction for the reaction A + B ↔ C + D (ΔG = -8.7 kcal/mole), we should look for a reaction that has a positive Gibbs free energy change (ΔG) close to or greater than 8.7 kcal/mole to make the overall ΔG negative when the two are summed.
According to the concept of coupled reactions, the energy released by an exergonic reaction (such as ATP hydrolysis) is often used to drive an endergonic reaction.
For instance, ATP hydrolysis has a ΔG of approximately -7 kcal/mole, which could be coupled to the glucose phosphorylation reaction with a ΔG of +3 kcal/mole. The sum of these two ΔGs gives an overall ΔG of -4 kcal/mole for the coupled reaction.
Therefore, we can conclude that a reaction with a ΔG that is positive and ideally slightly higher than the magnitude of 8.7 kcal/mole (the negative ΔG of the spontaneous A + B ↔ C + D reaction) could be a suitable couple, making the nonspontaneous reaction proceed.