A chemical reaction is strictly spontaneous when:

According to the laws of thermodynamics, the spontaneity of a reaction at constant temperature and pressure is dictated by the change in Gibbs Free Energy ($\Delta G$), as defined by the Gibbs-Helmholtz equation:
$$\Delta G=\Delta H-T\Delta S$$

For a process or reaction to occur spontaneously, the overall free energy of the system must decrease. Therefore:

* If $\Delta G<0$ ($\Delta G$ is negative), the reaction is spontaneous.
* If $\Delta G>0$ ($\Delta G$ is positive), the reaction is non-spontaneous.
* If $\Delta G=0$, the system is in a state of dynamic equilibrium.