Ferric bromide, FeBr3, results when iron is heated in excess of bromine vapour, when ferrous bromide is heated from 170° to 200° C. with twice its weight of bromine, and when sulphur monobromide, S2Br2, is passed over ferric oxide at 450° to 650° C. It is a dark red, crystalline but deliquescent substance, yielding a red solution in water. At boiling-point the solution dissociates to ferrous bromide and free bromine. On heating away from air ferric bromide partly sublimes and partly dissociates. When dry the salt is reduced by nitric oxide with the formation of ferrous and nitrosyl bromides.
The aqueous solution upon concentration in the ordinary way decomposes with the precipitation of insoluble basic bromides.
The hexahydrate, FeBr3.6H2O, separates as dark green needles when the dark brown solution obtained by the action of bromine under water is slightly evaporated and concentrated in a desiccator over sulphuric acid. It is soluble in alcohol and ether, and melts at 27° C. without decomposition.
The heat of formation is
[Fe] + (Br3) + Aq. = FeBr3. Aq. + 106,050 calories. |