The standard free-energy changes for the reactions below are given. Phosphocreatine → creatine + Pi ∆ G'° = - 43.0 kJ/molATP → ADP + Pi ∆ G'° = - 30.5 kJ/mol

What is the overall ∆ G'° for the following reaction?

Phosphocreatine + ADP → creatine + ATP

Gibbs free-energy of the reaction = (-12.5 kJ/mol)

Explanation:

The Gibbs free-energy of a reaction predicts the spontaneity or feasibility of a given chemical reaction.

Given the standard Gibbs free energy changes:

Phosphocreatine → creatine + Pi, ∆G° = - 43.0 kJ/mol ... (1)

ATP → ADP + Pi, ∆G° = - 30.5 kJ/mol ... (2)

Now to calculate the Gibbs free-energy of the given chemical reaction: Phosphocreatine + ADP → creatine + ATP; the equation (2) is reversed to give:

ADP + Pi → ATP, ∆G° = + 30.5 kJ/mol ... (3)

Now the equation (3) and (1) are added, to give:

Phosphocreatine + ADP + Pi→ creatine + ATP + Pi

⇒ Phosphocreatine + ADP → creatine + ATP

Therefore, to calculate the Gibbs free-energy of the reaction, the standard Gibbs free energy changes of the equations (1) and (3) are added similarly:

Gibbs free-energy of the reaction: ∆G° = (-43.0 kJ/mol) + ( + 30.5 kJ/mol) = (-12.5 kJ/mol)

Therefore, the Gibbs free-energy of the reaction = (-12.5 kJ/mol)