Question

Is H2S a lewis acid or a lewis base?

Answer 1

`"H"_2"S"` is usually a Lewis acid, but it can also act as a Lewis base.

Explanation:

`"H"_2"S"` is a weak acid.

`"H"_2"O:" + "H"_2"S" ⇌ "H"_3"O"^+ + "SH"^-`

`"H"_2"S"` is a Brønsted acid, because it is donating a proton to the water.

It is also a Lewis acid, because it is accepting a pair of electrons to form the `"O-H"` bond in hydronium ion.

`"H"_2"S"` has two lone pairs on the `"S"`, so it can also act as a Lewis base.

In the presence of a strong acid like `"H"_2"SO"_4`, the `"H"_2"S"` becomes protonated.

`"H"_2"S:" + "H"_2"SO"_4 → "H"_3"S"^+ + "HSO"_4^-`

Here the `"H"_2"S"` is donating an electron pair to form a bond to the `"H"` of `"H"_2"SO"_4`, so it is acting as a Lewis base.