The free energy change #DeltaG# of a chemical reaction is related with Enthalpy change #DeltaH #, Entropypy change #DeltaS # and the absolute temperature #T# of the reaction as follows
#DeltaG=DeltaH-T DeltaS#
The combustion reaction of hydrocarbon exothermic one.
So #DeltaH <0# again as per given balanced equation the number of moles of gaseous product (12moles) is greater than that of gaseous reactants (9moles). So #DeltaS>0 # and #T>0#
So as a whole RHS #DeltaH-T DeltaS<0#
Hence the free energy change of the reaction
#DeltaG<0->"The reaction is spontaneous"#