Ferrous oxide, FeO

Ferrous oxide, FeO, does not occur free in nature on account of the ease with which it absorbs oxygen, yielding ferric oxide. It is exceedingly difficult to prepare in a pure state. It results in a more or less impure condition when ferric oxide is heated to 300° C. in a current of hydrogen, and is then pyrophoric, becoming incandescent upon exposure to air, and reverting to its original ferric condition. After keeping in an atmosphere of hydrogen for some twelve hours, however, it loses this property. This method of preparation is not altogether satisfactory, however, since the product is liable to be contaminated with ferroso-ferric oxide, Fe₃O₄. Moissan obtained a fairly pure product on heating ferric oxide in hydrogen for twenty minutes at about 500° C.

A better method consists in reducing ferric oxide in a current of pure, dry carbon monoxide at 500° C., or with a mixture of equal parts of carbon monoxide and dioxide at red heat.

When hydrogen mixed with water vapour is passed over heated ferric oxide, ferrous oxide is formed in increasing amount with rise of temperature. Thus at 700° C. the product contains 85 per cent, of FeO, and at 800° C. some 92 per cent. FeO. Perfectly pure ferrous oxide is not obtained, however, even at 1100° C. Neither does it seem possible to obtain the pure oxide from reduced iron by oxidation with a mixture of steam and hydrogen.

Ferrous oxide is also formed, together with metallic iron, when ferrous oxalate is heated to 150°-160° C., and allowed to cool in an atmosphere of carbon dioxide. To avoid separation of carbon or the formation of iron carbide, it is advantageous to heat ferrous oxalate gradually up to 520° in a current of nitrogen. The temperature is finally raised to about 900° C. The product is grey in colour, and dissolves completely in hydrochloric acid, evolving hydrogen, indicating the presence of some free iron. Metallic iron may be oxidised to ferrous oxide by ignition in carbon monoxide at 1000° C.; or by heating with ferric oxide, according to the reaction: -

Fe₂O₃ + Fe = 3FeO.

When nitrous oxide is passed over iron at 200° C. ferrous oxide is produced, and also when iron is heated in steam to 350° C.

Iron amalgam yields ferrous oxide upon slow oxidation by exposure to air.

According to Moissan, ferrous oxide can exist in two polymorphic modifications, according as it is produced at high or at lower temperatures. The variety obtained at the lower temperatures, namely below 600° C., is more chemically reactive, uniting with oxygen, upon exposure to air, with such rapidity that the whole mass becomes incandescent. It decomposes water, slowly in the cold but with considerable rapidity at the boiling-point. It is readily soluble even in dilute acids, such as acetic acid, and easily displaces ammonia from its salts.

When obtained at 1000° C., or if heated to this temperature after production at lower temperatures, ferrous oxide is no longer pyrophoric; it does not decompose water, neither is it soluble in dilute acetic acid.

Both varieties of ferrous oxide are reduced by hydrogen gas to the free metal. If the oxide has not previously been heated above 400° C., the reaction begins at about 280° C.; but if it has been previously heated to about 1200° C., it cannot be reduced by hydrogen below 330° C.

The foregoing observations, however, do not necessarily prove that two polymorphic forms of ferrous oxide can exist. The observed chemical differences may well be due to the variation in the state of subdivision of the oxide, that produced at the lower temperatures being the more finely subdivided.

Ferrous oxide is reduced to metallic iron at 850° C. by a mixture of carbon monoxide and dioxide in the proportion of 10 parts CO to 4 parts CO₂.

Chlorine, dissolved in carbon tetrachloride, reacts even at - 18° C. with violence upon ferrous oxide, yielding ferric oxide and chloride. Thus: -

6FeO + 3Cl₂ = 2Fe₂O₃ + 2FeCl₃.

Ferrous oxide dissolves in acids to yield well-defined series of ferrous salts, which are pretty stable when dry or present in acid solution,, but readily oxidise in moist air and in neutral or alkaline solution. They are usually colourless or pale green.

Ferrous oxide obtained by slow reduction of ferric oxide in hydrogen at 300° C. readily reacts with sulphur dioxide on warming, the mass becoming incandescent. Clouds of sulphur and sulphur dioxide are evolved, a solid residue of sulphide and oxide remaining.

When ferrous oxide is heated with pure forms of carbon, no reaction takes place below 650° C. Above this temperature reduction occurs, but the rate of reaction depends greatly upon the mode of preparation of the carbon. Thus sugar charcoal is relatively inert, a vigorous reduction not taking place until about 800° C. Carbon obtained by burning acetylene in chlorine reacts vigorously at 650° C.

The heat of formation of ferrous oxide is as follows: -

[Fe] + (O) = [FeO] + 64,600 calories,[Fe] + (O) = [FeO] + 64,300 calories,

and at 680° C. and under constant pressure,

[Fe] + (O) = [FeO] + 67,350 calories.