Is H2S a lewis acid or a lewis base?

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

##"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.