Gibbs free energy

Measured in J/mol, it whether a chemical reaction releases or requires energy. It measures the change in free energy.

ΔG = GP - GR

where:

GP is the free energy of the products.

GR is the free energy of the reactants.

If ΔG is negative, then GP < GR, then the reaction releases energy.

If ΔG is positive, then GP > GR, then the reaction requires an input of energy.

ATP hydrolysis: Gibbs free energy at Khan Academy.