Microsoft Word - WaterChemistry

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Chemical Compounds
Selenium compounds commonly exist in the oxidation states −2, +2, +4, and +6.

Chalcogen Compounds
Selenium forms two oxides: selenium dioxide (SeO 2 ) and selenium trioxide (SeO 3 ). Selenium
dioxide is formed by the reaction of elemental selenium with oxygen:

Se 8 + 8 O 2 → 8 SeO 2

It is a polymeric solid that forms monomeric SeO 2 molecules in the gas phase. It dissolves in water
to form selenous acid, H 2 SeO 3. Selenous acid can also be made directly by oxidizing elemental
selenium with nitric acid:

3 Se + 4 HNO 3 + H 2 O → 3 H 2 SeO 3 + 4 NO

Unlike sulfur, which forms a stable trioxide, selenium trioxide is thermodynamically unstable and
decomposes to the dioxide above 185 °C:

2 SeO 3 → 2 SeO 2 + O 2 (ΔH = −54 kJ/mol)

Selenium trioxide is produced in the laboratory by the reaction of anhydrous potassium selenate
(K 2 SeO 4 ) and sulfur trioxide (SO 3 ).

Salts of selenous acid are called selenites. These include silver selenite (Ag 2 SeO 3 ) and sodium
selenite (Na 2 SeO 3 ).

Hydrogen sulfide reacts with aqueous selenous acid to produce selenium disulfide:

H 2 SeO 3 + 2 H 2 S → SeS 2 + 3 H 2 O

Selenium disulfide consists of 8-membered rings of a nearly statistical distribution of sulfur and
selenium atoms. It has an approximate composition of SeS 2 , with individual rings varying in
composition, such as Se 4 S 4 and Se 2 S 6. Selenium disulfide has been use in shampoo as an anti-
dandruff agent, an inhibitor in polymer chemistry, a glass dye, and a reducing agent in fireworks.
Selenium trioxide may be synthesized by dehydrating selenic acid, H 2 SeO 4 , which is itself produced
by the oxidation of selenium dioxide with hydrogen peroxide:

SeO 2 + H 2 O 2 → H 2 SeO 4

Hot, concentrated selenic acid is capable of dissolving gold, forming gold(III) selenate.

Halogen Compounds
Iodides of selenium are not well known. The only stable chloride is selenium monochloride (Se 2 Cl 2 ),
which might be better known as selenium(I) chloride; the corresponding bromide is also known.
These species are structurally analogous to the corresponding disulfur dichloride. Selenium
dichloride is an important reagent in the preparation of selenium compounds (e.g. the preparation
of Se 7 ). It is prepared by treating selenium with sulfuryl chloride (SO 2 Cl 2 ). Selenium reacts with
fluorine to form selenium hexafluoride:

Se 8 + 24 F 2 → 8 SeF 6