Oxidation

A few elements yield oxides only with difficulty, and such oxides are frequently unstable at high temperatures, i.e. Oxidation is is very difficult. For example, when platinum foil or sponge is heated in dry oxygen, the monoxide, PtO, is produced as a superficial blackening. At higher temperatures, however, this oxide dissociates to metallic platinum and free oxygen. Oxidation of Gold cannot be performed directly by gaseous oxygen, although several oxides can be prepared by decomposition of compounds containing gold atoms in their molecules.

Fluorine appears quite incapable of yielding an oxide under any conditions.

When oxygen combines with an element, the resulting product is termed an oxide.

The oxides of the various elements show marked differences in their chemical behaviour towards water and also towards acids. These differences form a convenient basis for the classification of the oxides and therefore can be divided into Types of Oxides.

Basic Oxides

It is characteristic of the metallic elements that each forms at least one oxide which will react with acids producing salts, the valency of the metal remaining unaltered. Such oxides are termed basic oxides or sometimes merely bases. Some metals yield more than one oxide, and it is then generally observed that the oxide richer in oxygen possesses a more feebly basic character. In the case of the most electro-positive metals, for example the alkali metals, the oxides will combine with water, producing soluble hydroxides which are strongly alkaline, and in fact constitute the typical alkalies; the less electropositive the metal, the smaller the tendency of the oxide to combine with water, and the greater the tendency of the hydroxide, which can then generally be obtained by precipitation methods, to eliminate water with formation of the oxide.

The suggestion has been made that the hydrates of the metals of Groups I. and II. do not contain hydroxyl groups, but are true hydrates of the corresponding oxides. In other words, molecules of water are assumed to be associated with the metallic oxide in a similar manner to the so-called water of crystallisation of salts. Thus sodium hydroxide would be written Na₂O. H₂O, and not as NaOH. The constitutions of the hydroxides of Groups III. and IV. will depend, according to this theory, upon the electrochemical conditions under which they are produced. Thus Al(OH)₃ and Al₂O₃. H₂O + 2H₂O are both regarded as capable of existing. The hydroxides of the metalloids and non-metals - the acidic oxides (see 2 below) - are true hydroxides containing hydroxyl groups.

Acidic oxides

Acidic oxides often termed acid anhydrides, are generally derived from the non-metals, but may also be higher oxides of certain of the metals. It is characteristic of such oxides that they combine with water, producing hydroxides, which are acids. Unlike the basic oxides, an acidic oxide rarely adds sufficient water to convert all its oxygen atoms into hydroxyl groups, e.g.

SO₃ → SO₂(OH)₂, N₂O₅ → 2NO₂(OH), Mn₂O₇ → 2MnO₃(OH).

It is, however, sometimes possible to prepare organic derivatives of the completely hydroxylated (often termed "ortho-") acids; thus although the aqueous solution of carbon dioxide appears to contain no other acid than an unstable one of the formula CO(OH)₂, ethyl ortho-carbonate, C(OEt)₄, is known, corresponding with the hypothetical ortho-carbonic acid, C(OH)₄.

3. Mixed Oxides. - In some cases so-called mixed acidic oxides are known which combine with water, producing a mixture of two acids; nitrogen tetroxide is an example of this class, as also is chlorine dioxide -

N₂O₄ + H₂O = HNO₂ + HNO₃.

Some basic oxides behave as mixed oxides, yielding with acids two salts derived from the constituent oxides, e.g. magnetic oxide of iron yields a ferrous and a ferric salt. A few oxides may be regarded as a special class of "Mixed Oxides" basic oxides or as salts derived from a basic and acidic oxide of the metal according to the point of view; thus red lead and lead sesquioxide behave as feeble compounds of lead monoxide and lead dioxide (2PbO.PbO₂; PbO.PbO₂), and chromium dioxide (CrO₂) as a compound of chromic oxide with chromic anhydride (Cr₂O₃.CrO₃).

Neutral Oxides

There is yet a small class of oxides, the neutral oxides, which do not belong to any of the classes before named; they do not combine with water to form acids, nor do they neutralise acids. This class has tended steadily to decrease as chemical knowledge has extended. Carbon monoxide, nitrous oxide, and nitric oxide were once included in this class, but each of these can be obtained by the loss of the elements of water from a corresponding acid, so that in a wide sense they might be classed with the acidic oxides. Hydrogen peroxide, on account of its relationship with the peroxides, can hardly in the strictest sense represent a neutral oxide, and the only common oxide which may be regarded as representing this class is water; this must be, in its total behaviour, a neutral oxide, because, as the formula H.OH indicates, any tendency to acidic properties must be accompanied by an equal tendency towards basic properties.

It is worthy of note that, although the behaviour is by no means general, many oxides give indications that they are polymerised, i.e. that their simple molecules have combined together in certain numbers to form more complex aggregates. Water is a well-recognised example. The high melting-points of certain other oxides, e.g. silica and stannic oxide, is also attributed by some to this cause.

Amphoteric oxides

Amphoteric oxides are capable of functioning either as acidic or as basic oxides. Thus tin dioxide, SNO₂, functions as a basic oxide in stannic sulphate, Sn(SO₄)₂, but as an acidic oxide in sodium a stannate, Na₂SnO₃. Lead dioxide yields the tetrachloride, PbCl₄, and sodium metaplumbate, Na₂PbO₃, respectively Similarly aluminium oxide, Al₂O₃, yields the trichloride, AlCl₃, and the aluminate Na₂Al₂O₄. All these examples are Amphoteric oxides.

Suboxides

Suboxides are of frequent occurrence amongst the metals, but are less well known amongst non-metals. The element combined with the oxygen is admittedly unsaturated. Thus when lead is gently heated a little below its melting-point, the suboxide Pb₂O is formed. In the case of nickel, three suboxides, Ni₄O, Ni₃O, Ni₂O, have been postulated. One of the best known non-metallic suboxides is that of carbon, C₂O₃.

Dioxides

Dioxides contain two atoms of oxygen combined with usually one atom of the metal. They usually yield up a portion of their combined oxygen with relative ease, but are distinguished from isomeric peroxides in that they do not yield hydrogen peroxide on treatment with dilute acids. Familiar examples are manganese dioxide, MnO₂; lead dioxide, PbO₂; and nickel dioxide, NiO₂ - usually incorrectly referred to as Ni₂O₃ in the literature.

Marino has directed attention to the fact that sulphur dioxide and manganese dioxide react, yielding the dithionate:

2SO₂ + MnO₂ = MnS₂O₆,

whilst lead dioxide yields a mixture of sulphite and sulphate:

SO₂ + PbO₂ = PbSO₃ + O;
H₂O + SO₂ + O = H₂SO₄; PbSO₃ + H₂SO₄ = PbSO₄ + H₂O + SO₂.

He therefore suggests that the structural formulae of these oxides are

and

respectively.

Peroxides

Peroxides are frequently isomeric with dioxides, but in acid solution react like hydrogen peroxide. Sodium peroxide, Na₂O₂, is a typical example. The peroxides of divalent metals are usually regarded as having a cyclic structure. Thus nickel peroxide, obtained by the action of hydrogen peroxide upon the well-cooled hydroxide, Ni(OH)₂, is written as



Other peroxides are those of manganese and mercury, obtained in an analogous manner to the above. Sometimes these oxides are known as peroxydates.