# Is H2S a lewis acid or a lewis base?

Jan 17, 2016

$\text{H"_2"S}$ is usually a Lewis acid, but it can also act as a Lewis base.

#### Explanation:

$\text{H"_2"S}$ is a weak acid.

${\text{H"_2"O:" + "H"_2"S" ⇌ "H"_3"O"^+ + "SH}}^{-}$

$\text{H"_2"S}$ 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 $\text{O-H}$ bond in hydronium ion.

$\text{H"_2"S}$ has two lone pairs on the $\text{S}$, so it can also act as a Lewis base.

In the presence of a strong acid like ${\text{H"_2"SO}}_{4}$, the $\text{H"_2"S}$ becomes protonated.

${\text{H"_2"S:" + "H"_2"SO"_4 → "H"_3"S"^+ + "HSO}}_{4}^{-}$

Here the $\text{H"_2"S}$ is donating an electron pair to form a bond to the $\text{H}$ of ${\text{H"_2"SO}}_{4}$, so it is acting as a Lewis base.