Sulfur Hydrogen Compounds

Hydrogen sulfide, H₂S, is a colorless gas that is responsible for the offensive odor of rotten eggs and of many hot springs. Hydrogen sulfide is as toxic as hydrogen cyanide; therefore, it is necessary to exercise great care in handling it. Hydrogen sulfide is particularly deceptive because it paralyzes the olfactory nerves; after a short exposure, one does not smell it.

The production of hydrogen sulfide by the direct reaction of the elements (H₂ + S) is unsatisfactory because the yield is low. A more effective preparation method is the reaction of a metal sulfide with a dilute acid. For example:

It is easy to oxidize the sulfur in metal sulfides and in hydrogen sulfide, making metal sulfides and H₂S good reducing agents. In acidic solutions, hydrogen sulfide reduces Fe³⁺ to Fe²⁺, MnO⁴⁻ to Mn²⁺, Cr²O⁷²⁻ to Cr³⁺, and HNO₃ to NO₂. The sulfur in H₂S usually oxidizes to elemental sulfur, unless a large excess of the oxidizing agent is present. In which case, the sulfide may oxidize to SO₃²⁻ or SO₄²⁻ (or to SO₂ or SO₃ in the absence of water):

This oxidation process leads to the removal of the hydrogen sulfide found in many sources of natural gas. The deposits of sulfur in volcanic regions may be the result of the oxidation of H₂S present in volcanic gases.

Hydrogen sulfide is a weak diprotic acid that dissolves in water to form hydrosulfuric acid. The acid ionizes in two stages, yielding hydrogen sulfide ions, HS⁻, in the first stage and sulfide ions, S²⁻, in the second. Since hydrogen sulfide is a weak acid, aqueous solutions of soluble sulfides and hydrogen sulfides are basic: