The change in Gibbs Free Energy (#DeltaG#) for any reaction is related to the equilibrium constant #K_(eq)# by the simple equation:
#DeltaG=-RT lnK_(eq)#
where #R# is the universal gas constant, 8.314 J/mol-K and #T# is the temperature of the system in Kelvins.
If #DeltaG# is negative, then #K > 1#, which means that the reaction will be spontaneous in the forward direction when all species are present in standard concentrations (1 bar for gases, 1 M for solutes).
If #DeltaG > 0# then the reverse reaction is spontaneous from standard conditions.