# What is Gibbs energy?

$\text{Gibbs Free Energy}$ is the single unequivocal criterion for the spontaneity of the chemical change.
$\Delta {G}^{\circ} = \Delta {H}^{\circ} - T \Delta {S}^{\circ}$
The defining expression thus includes an enthalpy term, and an entropy term. If $\Delta {G}^{\circ}$ is negative, then the reaction is spontaneous as written. It can be related to the equilibrium constant by the expression, $\Delta {G}^{\circ} = - R T \ln {K}_{\text{eq}}$. Clearly, only negative values of $\Delta {G}^{\circ}$ will give positive equilibrium constants.