The spontaneity of a process is determined by two parameters i.e. $\Delta H=-ve$  and  $\Delta S=+ve$. By combining these two parameters we get a new thermodynamic function $\Delta G$ . The sign of  $\Delta G$ decides the spontaneity of a process at a given temperature.
   $\Delta G=\Delta H-T\Delta S$
If    $\Delta G=0$     system is at equilibrium
     $\Delta G=-ve$     spontaneous
     $\Delta G=+ve$     Non spontaneous