Ferrous bromide, FeBr₂

Ferrous bromide, FeBr₂, may be prepared in the anhydrous form as a yellow crystalline mass by heating iron to dull redness in an atmosphere of bromine vapour. When the reaction is complete, the bromine is replaced by carbon dioxide or air, heating being discontinued. The dry salt is rapidly transferred to a glass tube and hermetically sealed. It readily absorbs ammonia at the ordinary temperature, yielding a white hexammoniate, FeBr₂.6NH₃. This dissociates on heating, yielding a darker salt, the diammoniate, FeBr₂.2NH₃; whilst at higher temperatures the monammoniate, FeBr₂.NH₃, is obtained as a dark grey mass.

The following data have been obtained for these compounds: -

	Temperature. °C.	Dissociation Pressure, mm.	Heat of Dissociation. Cals.
FeBr2.6NH3	107	126	14.9
FeBr2.2NH3	215	11.5	21.9
FeBr2.2NH3	230	23.5	21.9
FeBr2.2NH3	277	126	22.3
FeBr2.NH3	215	5.7	22.6
FeBr2.NH3	230	9.7	22.8
FeBr2.NH3	277	61.5	23.1

On dissolving in water and subsequent crystallisation in the cold, the hexahydrate, FeBr₂.6H₂O, is obtained in bluish green, rhombic prisms. The same salt may be more readily prepared by dissolving iron in hydrobromic acid.

At 50° C. the salt loses two molecules of water, yielding green crystals of the tetrahydrate, FeBr₂.4H₂O. Upon further heating more water is expelled.

Both the aqueous and alcoholic solutions of ferrous bromide absorb nitric oxide, the limit of absorption being reached with one molecule of NO to each atom of iron. The compound FeBr₂.NO has not been isolated.

Anhydrous ferrous bromide absorbs dry nitrogen peroxide, yielding a stable compound, 4FeBr₂.NO₂, which retains its nitrogen peroxide even in vacuo. In the presence of moisture more peroxide is absorbed, but the reaction is complex, bromine being evolved.

Ferrous bromide unites with ethylene and acetylene.

The solubility of ferrous bromide in water is given as follows: -

Temperature, °C	-21	-7	+10	21	37	50	65	95
Grams FeBr2 in 100 grams solution	47.0	48.3	52.3	53.7	56.0	58.0	63.3	63.3

The heat of formation is

[Fe] + (Br₂) + Aq. = FeBr₂.Aq. + 78,070 calories.

A few double salts with the bromides of the alkali metals have been prepared.