The hydrolysis of ATP into ADP and inorganic phosphate
:ATP(aq) + (l) = ADP(aq) + HPO(aq) + H(aq)
releases  of enthalpy. This may differ under physiological conditions if the reactant and products are not exactly in these ionization states. The values of the free energy released by cleaving either a phosphate (P) or a pyrophosphate (PP) unit from ATP at standard state concentrations of 1 mol/L at pH 7 are:
:ATP +  → ADP + P  ΔG°' = −30.5 kJ/mol (−7.3 kcal/mol)
:ATP +  → AMP + PP ΔG°' = −45.6 kJ/mol (−10.9 kcal/mol)
These abbreviated equations at a pH near 7 can be written more explicitly (R = adenosyl):
:[RO-P(O)-O-P(O)-O-PO] +  → [RO-P(O)-O-PO] + [HPO] +  H
:[RO-P(O)-O-P(O)-O-PO] +  → [RO-PO] + [HOP-O-PO] +  H
At cytoplasmic conditions, where the ADP/ATP ratio is 10 orders of magnitude from equilibrium, the ΔG is around −57 kJ/mol.
Along with pH, the free energy change of ATP hydrolysis is also associated with Mg concentration, from ΔG° = −35.7 kJ/mol at a Mg concentration of zero, to ΔG° = −31 kJ/mol at [Mg] = 5 mM. Higher concentrations of Mg decrease free energy released in the reaction due to binding of Mg ions to negatively charged oxygen atoms of ATP at pH 7.