Formation of Water

Formation of Water occured by the oxidation of hydrogen by free or combined oxygen. The relative volumes of free oxygen and hydrogen at 0° and 760 mm. which undergo combination are 1:2.00288, or approximately 1:2. Although a mixture of the two gases in these proportions, known as "detonating gas," is stable under ordinary conditions, no change being observable even after prolonged periods of storage in the absence of light at room temperature, the interaction of the gases can be accelerated in various ways.

1. Sunlight can produce a slow but appreciable combination between the gases, the action being due to the ultra-violet rays. Ultra-violet radiation affects both detonating gas and water-vapour tending in each case to produce an equilibrium between the vapour and the constituent elements; with increase in intensity of the illumination the equilibrium shifts in favour of further dissociation, but the proportion of dissociated vapour at the position of equilibrium is very small. The process of combination probably proceeds by the stages
   
   H + O₂ = H₂O₂;
   H₂O₂ + H₂ = 2H₂O.
2. Radioactive substances can induce the combination of hydrogen and oxygen; the effect being possibly due in part to a primary conversion of the oxygen into ozone, but this cannot represent the sole mechanism as once more the change leads only to an equilibrium, and water-vapour under similar conditions becomes partially resolved into its elements. The α-rays are most active in this respect, although it is possible that β and γ radiations also exert a subordinate influence. A mixture of hydrogen and oxygen may even explode under the influence of radium emanation.
   
   The silent electric discharge appears to be relatively inactive towards detonating gas.
3. Rise in temperature is, in practice, the simplest process for inducing chemical action between hydrogen and oxygen. It is supposed by some chemists that the absence of chemical action in detonating gas at the ordinary temperature in the absence of light or of radioactive substances is only apparent, the actual rate of combination merely being too small for detection by the usual methods; with increase in temperature the combination is accelerated so that it becomes perceptible or even explosive. The necessary heat can be applied in various manners, for example by sudden compression, by an electric spark, by a flame, or by an incandescent solid. The ignition temperature in detonating gas at which self-maintaining combustion is initiated is variable on account of the influence of several factors, but is generally between 500° and 600° C. Although ordinarily "dry " detonating gas will explode readily, a very carefully dried mixture, such as that obtained by prolonged exposure to phosphorus pentoxide, is much less prone to chemical change. A silver wire may be heated to fusion in this gas without causing more than local combination of hydrogen and oxygen; however, an electric spark will induce explosion equally well in the dried and undried gas, probably on account of the higher degree of heat applied. Combination can occur below the ignition temperature, but will then be continuous (in the absence of catalysts) only as long as the temperature is maintained by external sources of heat.
   
   It is of interest to note that the presence of foreign gases has a considerable inhibiting effect on the explosiveness of a mixture of hydrogen and oxygen. The following numbers giving the volume of various gases required to prevent the explosion of one volume of detonating gas by an electric spark are only of relative and not of absolute value, because rather wide divergencies may be observed with different conditions:
   
   

   Hydrogen sulphide	0.5
   Ethylene	1
   Ammonia	1
   Sulphur dioxide	2
   Carbon dioxide	3
   Carbon monoxide	4
   Hydrogen chloride	4
   Nitrogen oxide	9
   Hydrogen	9
   Air	12
   Oxygen	14

   
   
   This is due probably in part to the unequal effect of the surface of the reaction vessel.
   
   The effect of the walls of the containing vessel on the combination of hydrogen and oxygen is merely a special case of a general phenomenon which has already received mention. Hydrogen streaming on to a warm platinum spiral in air will raise the temperature of the spiral to redness by the heat of its combustion on the surface of the metal, and indeed may even inflame. Platinum wire or foil will not cause the ignition of detonating gas unless previously warmed to above 50° C., but in a finer state of division such as the "sponge" or "black" obtainable by decomposing ammonium chlorplatinatc or chlorplatinic acid, platinum, even without previous warming, will induce such vigorous combination of the two gases as to cause an explosion. Such finely divided platinum in a short time loses its activity, especially if brought into contact with certain substances such as ammonia, hydrogen sulphide, or carbon disulphide vapour. It can, however, be revivified by moistening with nitric acid and drying at 200° C. It appears probable that the presence of moisture is necessary to the catalytic activity of finely divided platinum towards detonating gas at the ordinary temperature.
   
   Other metals than platinum also can affect the rate of formation of water from gaseous hydrogen and oxygen. Palladium resembles platinum in activity, but many other metals are less effective, examples being osmium, iridium, gold, and silver; mercury appears to be without influence even at its boiling-point. Reduced copper, when heated in detonating gas, commences to oxidise near 250° C., the oxide becoming vigorously reduced with incandescence at a somewhat higher temperature; copper oxide exerts a marked catalytic effect at 300° C. without undergoing visible reduction. Such behaviour suggests that the catalytic action of the nobler metals cited above may depend on a primary combination with one of the constituents of the gaseous mixture followed by regeneration of the metal with formation of water. The activity of the noble metals may, however, be, at least in part, analogous to that possessed by all heated solid surfaces, especially porous ones, for example, pumice, glass, porcelain, carbon, although these have the power only in a much less degree. In the colloidal condition platinum and palladium accelerate the union of hydrogen and oxygen.
   
   The combination of hydrogen and oxygen is accompanied by the liberation of a large quantity of energy generally in the form of heat; this accounts for the vigour of the explosion of a mixture of the gases
   
   2(H₂) + (O₂) = 2H₂O + 2×68,360 calories.
   
   A calculation of the temperature produced by the combustion of hydrogen in oxygen or air, based on the above number and the specific heat of the steam produced and of the nitrogen present in the case of air, gives a figure greatly in excess of that actually observed. The discrepancy is due to a combination of several causes. Thus the process is not instantaneous but gradual, so that there is time for loss of heat, radiation especially being an important factor. The increased specific heat of steam at such high temperatures and a somewhat incomplete change on account of a slight dissociation of steam at the temperature attained, are other factors.
   
   The water produced by the direct combination of hydrogen and oxygen frequently contains traces of hydrogen peroxide and of nitric acid. The occurrence of the former indicates the possibility that the primary change may be a simple coupling of molecules with formation of hydrogen peroxide, this compound subsequently decomposing into the more stable substances, water and oxygen; by continuous repetition of the process the oxygen is at last completely converted into water. This possibility receives some confirmation from the fact that an equimolecular mixture of oxygen and hydrogen has a lower ignition temperature than electrolytic detonating gas, although the evidence is not final. Traces of nitrogen as impurity in the gases used explain the frequent occurrence of small quantities of nitric acid in the water produced.
4. Water may be produced by the union of hydrogen with combined oxygen, as, for example, during the reduction of oxides. Advantage has been taken of this fact to determine gravimetrically the composition of water, as witness Dumas' classical researches on the formation of water by the reduction of copper oxide.
5. Water may also be formed by the decomposition of a complex molecule containing hydrogen and oxygen atoms or by the interaction of two complex molecules. As an example of the former may be mentioned the action of heat on a hydroxylated substance. Thus, sulphuric acid when dropped on to a red-hot plate decomposes to water, sulphur dioxide, and oxygen.
   
   
   
   and copper hydroxide yields the anhydrous oxide
   
   Cu(OH)₂ = CuO + H₂O.
   
   An illustration of the latter type is afforded by the interaction of sulphur dioxide and hydrogen sulphide:
   
   SO₂ + 2H₂S = 2H₂O + 3S.