Is a reaction spontaneous if the ΔH is negative or positive?

A reaction is not definitively spontaneous based solely on the sign of ΔH (enthalpy change). While a negative ΔH indicates that the reaction releases heat (exothermic), which can favor spontaneity, spontaneity also depends on the entropy change (ΔS) and temperature, as described by the Gibbs free energy equation: ΔG = ΔH - TΔS. A reaction is spontaneous if ΔG is negative, which means both ΔH and ΔS must be considered together.