##”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.
