Spontaneous processes result in the lowering of the total Gibbs Free Energy (G).
For a chemical reaction to be spontaneous, ΔG must be negative.
It is possible to have both exothermic (ΔH < 0) as well as endothermic (ΔH > 0) spontaneous reactions.
ΔG = ΔH – T * ΔS
Where ΔG is the change in the Gibbs Free Energy, ΔH is the change in enthalpy, ΔS is the change in entropy, and T is the temperature the reaction occurs at (in Kelvin).