Chemical Properties of Oxygen
|Chemical properties of oxygen are mostly related with that the oxygen is capable of uniting to form simple compounds with all the elements save fluorine and the noble or inert gases. Such combination is termed oxidation and can in general be produced by the direct union of the two elements, as for example in the oxidation of mercury when heated in air, although certain of the non-metals, particularly the halogen elements, show little tendency to direct combination in this manner. Compounds also are capable of uniting with oxygen, sometimes yielding a stable oxidation product of higher molecular weight in consequence of addition of one or more atoms of oxygen; or the molecule of the compound may be disrupted upon oxidation into two or more products. As an example of the former type of reaction, the oxidation of sodium sulphite in aqueous solution may be quoted, sodium sulphate resulting. Thus |
2Na2SO3 + O2 = 2Na2SO4.
The latter type of reaction is illustrated by acetylene which, when ignited, burns in air to form water and carbon dioxide.
2C2H2 + 5O2 = 4CO2 + 2H2O.
Oxidation both of elements and compounds may be effected in the absence of free oxygen through the action of substances containing oxygen. Thus, for example, iron is oxidised by steam and potassium by carbon dioxide at high temperatures. Chemical properties of oxygen, however, is concerned more particularly with oxidation through the direct action of free elementary oxygen.
Most cases of oxidation are exothermic, that is to say they are accompanied by the evolution of heat, although a few cases are known which are endothermic in character. Such, for example, are the oxidation of water to hydrogen peroxide:
2H2O + (O2) = 2H2O2Aq. - 23059 calories,
and the production of ozone from oxygen,
3O2 = 2O3 - 2×34000 calories,
both of which reactions are accompanied by an absorption of heat.
It does not necessarily follow, however, that reactions involving the exothermic oxidation of substances are accompanied by a sensible rise in temperature. The rate of oxidation may be so slow that the heat is dissipated almost as rapidly as it is liberated, so that the rise in temperature is infinitesimal. This is well illustrated by the oxidation of iron upon exposure to air, a reaction commonly known as rusting.
On the other hand, combination with oxygen is often accompanied by the rapid liberation of so much energy that heat and light are emitted. The term combustion is then applied.
The majority of substances require to be raised in temperature before they can combine with oxygen to any appreciable extent. Thus electrolytic gas - a mixture of two volumes of hydrogen with one of oxygen - is very stable at the ordinary temperature; combination, however, begins to be appreciable at temperatures slightly above 400° C., and at higher temperatures proceeds with explosive violence. On the other hand, some substances rapidly combine with oxygen when brought into contact with the gas at the ordinary temperature. Such bodies are said to be spontaneously oxidisable, and include the pyrophoric metals, phosphorus, coal dust, nitric oxide, ferrous and manganous hydroxides, liquid phosphine, silicon hydride, and many organic substances. Under suitable conditions 'most of these steadily rise in temperature as oxidation proceeds until rapid combustion, accompanied by light, ensues. This is termed spontaneous combustion, a familiar example on a large scale being afforded by the firing of hayricks. Autoxidation is a term frequently used to designate spontaneous oxidation.
Chemical properties of oxygen with the rate of oxidation of any particular substance is dependent upon various factors, to wit, its own physical condition as well as that of the oxygen; the presence of moisture or of a catalyser; and the application of light, heat, and pressure. Thus, liquid oxygen does not affect phosphorus or the alkali metals; neither does it combine with solid nitric oxide, although a small jet of burning hydrogen will continue to burn below the surface of liquid oxygen, the water produced being removed as ice and a considerable amount of ozone being formed. Similarly graphite and diamond, when once ignited, will burn on the surface of liquid oxygen, the carbon dioxide produced being frozen and some ozone passing into solution. Oxidation is usually facilitated to a considerable extent by increasing the superficial area of the substance to be oxidised. Well-known illustrations are supplied by the pyrophoric forms of iron, lead, etc. When a solution of phosphorus in carbon disulphide is poured on to a sheet of filter paper, the solvent rapidly evaporates, leaving the phosphorus within the pores of the paper in an excessively fine state of subdivision, so that vigorous combustion ensues.
Moisture plays an important r61e in many cases of oxidation. Its presence is necessary, for example, to effect the spontaneous combustion of pyrophoric metals. At ordinary temperatures, also, the majority of metals are stable in dry oxygen, although readily attacked by the moist gas. Small quantities of many foreign substances are capable of catalytically assisting the rate of oxidation of certain substances. Thus a trace of platinum black introduced into electrolytic gas causes the gases to instantly unite with explosive violence; and the passage of a mixture of sulphur dioxide and oxygen over platinised asbestos effects their union to form sulphur trioxide.
Light frequently exerts a considerable influence on the rate of oxidation; thus phosphorus trichloride when illuminated undergoes more rapid conversion into the oxychloride than in the dark; and iron likewise corrodes more rapidly in similar circumstances. Oxidation may be catalytically accelerated by radio-active substances. Thus thorium-X has been found to assist the oxidation of adrenaline and morphine.
Many metals on being heated in dry air or oxygen yield an' adhering coat of oxide which tends to protect the underlying metal from attack. The rate at which oxidation proceeds at any temperature is given by the expression
θ = aeby - a,
where a is a constant independent of the temperature; b is a constant depending on the temperature and on the delay of film thickening during time θ. y is the thickness of the oxide layer.
The time required for a visible film of oxide to form on the surface of some of the more common metals at 15° C. has been calculated to be as follows:
|Metal.||Time in Years.||Initial Velocity of Oxidation. Thickness of layer μμ per second.|
Oxidation processes are, as a general rule, greatly accelerated by a rise in temperature; the first effect of the application of heat may be merely to initiate a slow oxidation which soon ceases on the removal of the source of heat; but a higher temperature may cause so marked an increase in the rate of the chemical action that the heat produced suffices to maintain the temperature, and the oxidation or combustion will proceed unaided. This temperature at which the process of rapid combustion becomes independent of external supplies of heat is termed the ignition temperature of the substance. Phosphorus does not commence rapid combustion until a temperature of 60° C. is attained; hydrogen will combine, albeit excessively slowly, with oxygen already at 180° C., but the reaction is not very appreciable below 400° C., and continuous inflammation does not occur until near 530° C.; a red-hot glass rod will cause the ignition of carbon disulphide vapour, but not of ether vapour.
By flame is generally understood a mass of gas raised to incandescence. Flame is produced only in those cases of combustion in which gases or vapours are present, which become more or less luminous or incandescent on account of their high temperature. But a visible flame does not always accompany rapid gaseous combustion, a striking exception being afforded by the rapid oxidation of hydrogen or coal-gas mixed with air on a surface of platinised asbestos or porous firebrick. Such combustion is termed surface combustion, and is utilised commercially in a variety of ways.
Pressure exerts an important influence upon the rate of oxidation. Thus silicon, ethane, phosphorus, arsenic, and several other substances, are found to oxidise more readily at low oxygen pressures; on the other hand, the rates of the rusting of iron and the oxidation of ferrous sulphate solution are accelerated by increase of pressure.
|It is possible to prepare an Active Oxygen analogous to active nitrogen by subjecting the dry, ozone-free gas to the influence of an electric discharge. It yields a weak, bluish-green afterglow, which is less persistent than that of hydrogen. When mixed with active nitrogen, this active oxygen yields oxides of nitrogen. Both oxygen and ozone are unaffected by active nitrogen. Hence active oxygen is different from these, but is capable of existing for only a short time.|