For the reaction 2H(g)⟶H2(g), the sign of ΔH and ΔSrespectively are

ΔH=−ve(H−H bond formed )ΔS=−veΔng=−12H→H2  the entropy of products is less than the reactants .so the ∆S=-ve from Gibbs equation ∆G=∆H-T.∆S ∆G=∆H+T.∆S(∆S=-ve)for spontaneous reaction  ∆G=-veso the above reaction ∆H is -ve