Question

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

Answer 1

The overall standard free-energy change for the reaction Phosphocreatine + ADP to form creatine and ATP is the sum of the changes for the individual reactions, resulting in −12.5 kJ/mol.

Step-by-step explanation:

To determine the overall standard free-energy change (Δ G'°) for the reaction where phosphocreatine and ADP react to form creatine and ATP (“Phosphocreatine + ADP → creatine + ATP”), we need to consider the given free-energy changes for the individual reactions:

• Phosphocreatine → creatine + Pi (Δ G'° = −43.0 kJ/mol)
• ATP → ADP + Pi (Δ G'° = −30.5 kJ/mol, which should actually be considered as ADP + Pi → ATP since it occurs in the reverse direction)

By reversing the second reaction, we invert the sign of its standard free-energy change. Therefore, for the reaction ADP + Pi → ATP, the Δ G'° becomes +30.5 kJ/mol.
Now, to find the overall Δ G'° for the reaction Phosphocreatine + ADP → creatine + ATP, we sum up the standard free-energy changes of the two reactions:

Δ G'° (overall) = Δ G'° (Phosphocreatine → creatine + Pi) + Δ G'° (ADP + Pi → ATP)

Δ G'° (overall) = (−43.0 kJ/mol) + (+30.5 kJ/mol) = −12.5 kJ/mol

The result is −12.5 kJ/mol, which indicates that the coupled reaction is also exergonic, releasing energy.

Answer 2

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

Step-by-step 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)