Ferrous chloride, FeCl₂

Ferrous chloride, FeCl₂, may be obtained in the anhydrous condition in a variety of ways. A convenient method consists in heating to dull red heat excess of iron filings in a current of chlorine, of ammonium chloride, or of gaseous hydrogen chloride, the letter methods being preferable, as the first-named generally yields small quantities of the ferric salt. Ferrous chloride also results when ferric chloride is heated in a current of pure hydrogen; but care must be taken that the temperature does not rise too high, or the ferrous chloride will itself be reduced.

When a current of low density is passed through dilute ethereal solutions of ferric chloride, reduction to ferrous chloride takes place at the cathode.

If the hydrated salt, FeCl₂.4H₂O, is dissolved in ammonium chloride solution, evaporated to dryness, and subsequently heated in the absence of air, the water and ammonium chloride escape, leaving the anhydrous ferrous chloride behind.

Anhydrous ferrous chloride as obtained by these methods is a white scaly crystalline substance when pure, but usually it possesses a yellow colour in consequence of the presence of traces of the ferric salt. Its density is 2.53. It readily dissolves in alcohol and in water, with the latter yielding a colourless solution which turns green upon exposure to air.

When heated strongly in air ferric chloride and oxide are produced, the former volatilising. Thus: -

12FeCl₂ + 3O₂ = 8FeCl₃ + 2Fe₂O₃.

When heated in steam, ferrous chloride yields hydrogen chloride and ferroso-ferric oxide: -

3FeCl₂ + 4H₂O = Fe₃O₄ + 6HCl + H₂.

In the absence of air, ferrous chloride fuses at red heat and volatilises at yellow heat. Its vapour density has been determined under various conditions, both in an atmosphere of nitrogen and of gaseous hydrogen chloride. At bright yellow heat in the latter gas the densities obtained in two experiments corresponded to molecular weights of 192.1 and 183.8 respectively - values that lie midway between those for the simple molecule FeCl₂ (namely 126.8) and the double molecule Fe₂Cl₄ (namely 253.6), respectively. This suggests that the vapour consists of a mixture of double and single molecules in equilibrium with each other, as represented by the equation

Fe₂Cl₄ ⇔ 2FeCl₂.

At 1300° to 1500° C. dissociation into single molecules of FeCl₂ is complete.

In boiling solutions of organic liquids such as pyridine, ferrous chloride appears to have the simple formula FeCl₂.

An ethereal solution of ferrous chloride heated in a sealed tube with ethylene absorbs the gas, yielding a compound, FeCl₂.C₂H₄.2H₂O, in the form of colourless, needle-shaped crystals, readily soluble in water.

Ferrous chloride unites with bromine to yield an unstable chloro-bromide, FeCl₂Br.

Ferrous chloride readily absorbs ammonia, yielding a voluminous white mass, which on warming evolves some of the gas again, whilst on heating to above 350° C. reduction takes place, iron nitride being produced. Two definite compounds have been obtained, namely the di-ammoniate, FeCl₂.2NH₃, and the hexammoniate, FeCl₂.6NH₃; whilst the existence of the monammoniate, FeCl₂.NH₃, has been detected. Between the temperatures of -18° and 350° C. the reaction is reversible, the equilibrium being represented as follows: -

xNH₃ + FeCl₂ ⇔ FeCl₂.xNH₃.

The hexammoniate is formed at the ordinary temperature, and the diammoniate by heating the hexammoniate to 100° C. in hydrogen.

The dissociation pressures and heats of dissociation of these compounds have been determined as follow: -

	Temperature. °C.	Dissociation Pressure, mm.	Heat of Dissociation. Cals.
FeCl2.6NH3	90	270	13.65
FeCl2.2NH3	230	121	20.3
FeCl2.2NH3	277	555	20.7
FeCl2.NH3	214.5	5.8	22.5
FeCl2.NH3	277	54.4	23.1

With methylamine, ferrous chloride yields two compounds: namely, a white voluminous compound containing six molecules of methylamine, FeCl₂.6CH₃NH₂; and a grey compound, FeCl₂.2CH₃NH₂.

The hydrazinate, FeCl₂.2N₂H₄, has also been prepared.

A study of the vapour tensions of hydrated ferrous chloride indicates the existence of four hydrates - namely the hexahydrate, FeCl₂.6H₂O; tetrahydrate, FeCl₂.4H₂O; dihydrate, FeCl₂.2H₂O; and monohydrate, FeCl₂.H₂O. Of these, the best known are the tetra- and di-hydrates, the former being the usual hydrate under normal conditions.

The tetrahydrate, FeCl₂.4H₂O, is readily obtained in the form of bluish monoclinic crystals when iron is dissolved in aqueous hydrogen chloride and the solution evaporated out of contact with air. The density of the crystals is 1.937. They deliquesce upon exposure to air, acquiring a greenish colour consequent upon slight oxidation.

The tetrahydrate is converted into the dihydrate, FeCl₂.2H₂O, on gentle heating, the transition temperature for the phases

FeCl₂.4H₂O ⇔ FeCl₂.2H₂O + 2H₂O

being 65.2° C. From a saturated solution it separates out at 72.6° C. In a saturated solution of ferrous magnesium chloride the dihydrate forms at 43.2° C. The dihydrate is also formed by allowing the tetrahydrate to effloresce at the ordinary temperature in a vacuum; and by passing a current of dry hydrogen chloride through a saturated solution of ferrous chloride, when it is obtained as fine, transparent pale-green needles, which do not alter in a vacuum. The heats of formation of the anhydrous and hydrated salts are given as follow: -

[Fe] + (Cl₂) + Aq. = FeCl₂. Aq. + 99,950 calories,
[Fe] + (Cl₂) = [FeCl₂] + 82,050 calories,
whence [FeCl₂] + Aq. = FeCl₂.Aq. + 17,900 calories.
[Fe] + 2(HCl) = [FeCl₂] + (H₂) + 60,000 calories,
[Fe] + 2HCl.Aq. = FeCl₂Aq. + H₂ + 21,320 calories,
[FeCl₂] + 2H₂O = [FeCl₂.2H₂O] + 6920 calories,
[FeCl₂.2H₂O] + 2H₂O = [FeCl₂.4H₂O] + 2520 calories,
[FeCl₂] + 2[H₂O](solid) = [FeCl₂.2H₂O] + 6400 calories,
[FeCl₂2H₂O] + 2[H₂O](solid) = [FeCl₂.4H₂O] + 2600 calories.

The heat of solution of the dihydrate at 20° C. is 8700 calories.

A monohydrate, FeCl₂.H₂O, has been described by Leseoeur.

The solubility of ferrous chloride in water is as follows: -

Temperature, °C.	16	18	25	28	43	50	53	72	89	96	118
Grams FeCl2 per 100 grams solution	40.5	40.9	41.0	42.5	44.4	45.0	45.9	49.2	51.3	51.0	51.7

The specific gravities of solutions of ferrous chloride at 15.5° C. at various concentrations are as follow: -

Grams of FeCl2 in 100 c.c. of Solution.	Specific Gravity at 15.5° C.
5.19	1.0459
10.27	1.0892
19.73	1.1688
30.16	1.2528
44.65	1.3674
54.20	1.4439

Ferrous chloride in acid solution is partially oxidised by sulphur dioxide to the ferric salt, sulphur being deposited. Solutions of ferrous chloride in various solvents such as water, alcohol, ethylacetate, pyridine, etc., absorb nitric oxide, the extent of absorption depending upon the concentration of the iron salt, the temperature, gaseous pressure, and the nature of the solvent. The limit of absorption is reached with one molecule of NO to one atom of iron. Presumably the compound FeCl₂.NO exists in solution, most probably combined with the solvent. It has not as yet been isolated.

Dry ferrous chloride also absorbs nitrogen peroxide, yielding a fairly stable compound 4FeCl₂.NO₂, which does not lose its nitrogen peroxide content even in vacuo. In the presence of moisture more of the gas is absorbed, but the reaction is complicated.