The overall energy needed or released comes from adding up all of the bond break

Question

The overall energy needed or released comes from adding up all of the bond breaking and bond making steps for the overall reaction. We do this by calculating a Ho value. Thus, the fact that hydrolyzing the phosphate bond in ATP to form ADP releases energy, is the result of all bond changes, not just breaking of the phosphate bond. The overall reaction for the hydrolysis of ATP to ADP is:

ATP4- (aq) + 2 H2O (l) ADP3- (aq) + HPO42- (aq) + H3O+ (aq)

Using the data provided in the table, what is the Horxn for the hydrolysis of ATP to form ADP?

Compound Hfo (in kJ/mol) ATP (aq)
(adenosine triphosphate) - 2982 ADP (aq)
(adenosine diphosphate) - 2000 H2O (l) - 285.8 H3O+ (aq) - 285.8 HPO42- (aq) - 1299

Explanation / Answer

ATP4- (aq) + 2 H2O (l) -----------------> ADP3- (aq) + HPO42- (aq) + H3O+ (aq)

Horxn = Hoproducts - Horeactants

            = ( HoADP + HoHPO42- + HoH3O+)- ( HoATP + HoH2O)

             =(-2000 -1299 -285.8) - (-2982-285.8)

             = -3584.8 -   (- 3267.8)

             = - 317 kJ /mol

Horxn = - 317 kJ / mol

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