Question

Why is H₂ more acidic than NH₃?

Answer 1

Look at the Lewis structures for H₂ and NH₃ (taken from internet sources). NH₃ has an unshared electron pair on the N-atom. Since H atoms lack vacant p or d orbitals, hence donation of the N unshared electron pair (lone pair) into vacant p or d orbitals of H is not possible. In other words, there can be no partial double bond character between N and H atoms.

So what happens to this lone pair? This lone pair over N atom is entirely available for donation to electron-deficient species (substance which need electrons to complete their octet). In this sense, NH₃ acts as an electron donor. As per Lewis, all electron-donating species are bases and hence NH₃ is a Lewis base. Lewis acids are electron-deficient species which accept electron density from electron-rich compounds like NH₃. Therefore, NH₃ has markedly low acidity.

H₂ on the other hand, has a completely filled duplet and has no extra electron to donate. Therefore, H₂ can never act as a Lewis base. However, H₂ does act as a Lewis acid by accepting an electron and forming the hydride (H^-) ion. Therefore, H₂ shows more acidity than NH₃ (infact, H₂ can never act as a Lewis base while NH₃ can only act as a Lewis base).