A reaction is spontaneous when:

The spontaneity of a chemical reaction at constant temperature and pressure is determined by the change in Gibbs Free Energy ($\Delta G$), defined by the Gibbs-Helmholtz equation:
$$\Delta G=\Delta H-T\Delta S$$

According to the second law of thermodynamics:

* A reaction is spontaneous if $\Delta G<0$ ($\Delta G$ is negative).
* A reaction is non-spontaneous if $\Delta G>0$ ($\Delta G$ is positive).
* A system is at equilibrium if $\Delta G=0$.
While exothermic reactions ($\Delta H=-\text{ve}$) and increases in entropy ($\Delta S=+\text{ve}$) favor spontaneity, the ultimate criteria that guarantees spontaneity under all conditions is a negative value of $\Delta G$.