By definition, #"chemical spontaneity"# corresponds to the SIGN of #DeltaG_"rxn"^@#.....#DeltaG="Gibbs' Free energy...."#
Explanation:
For #A + B rightleftharpoonsC+D#
#DeltaG=-RTln{K_"eq"}#
If #DeltaG=0#, then the reaction is at equilibrium; if #DeltaG# is positive, then the REACTANTS are favoured at equilibrium; if #DeltaG# is NEGATIVE, then the PRODUCTS are favoured at equilibrium, and the reaction is said to be #"spontaneous"#, i.e. #DeltaG<=0#...........