Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/494

Rh 482 CHEMISTRY [OZONX. consequent on the electric action, which may be done by placing fragments of ice in the interior of the tube and also in the water contained in the external cylinder in which the induction tube is immersed. But it is not possible in this manner to convert more than about 15 per cent, of the oxygen into ozone, and Brodie s experiments prove that there is a fixed limit prescribed by the conditions of experiment, beyond which the forma tion of ozone cannot pass. The explanation of this appears to be that the formation of ozone belongs to the class of actions termed dissociation phenomena ; that is to say, not only is oxygen converted into ozone by the electrical action, but ozone is also reconverted into oxygen, the amount of ozone actually obtained under given conditions of experi ment being dependent upon the extent to which these two opposite kinds of change take place. We may there fore expect that the amount of oxygen finally obtained in the form of ozone will be greater the less the ozone is exposed to the electric action ; and that this is actually the case is proved by the fact that no advantage is gained by submitting the oxygen more than once to the electric action or by passing it slowly through the induction tube, and also by the behaviour of carbon dioxide. When this gas is submitted to the action of the electric discharge, it is partially resolved into carbon monoxide and oxygen, and the latter is in part transformed into ozone. Brodie has shown that it is not difficult, by passing a rapid current of carbon dioxide through the induction tube, to convert 75 per cent, of the oxygen eliminated from the carbon dioxide into ozone, and has even succeeded in converting as much as 85 per cent. Pure ozone has never yet been obtained, however. Ozone is also formed in small quantity when water is de composed by the electric current, employing plates of platinum or gold as electrodes; the amount produced is greater the smaller the electrodes. The slow oxidation of phosphorus in moist air is said to be attended with the production of small quantities of ozone, and it is probable that ozono is formed in other slow oxidations ; it appears that in all such cases the formation of ozone is accompanied by that of hydrogen dioxide, a fact which is also true of electrolytic ozone. Traces of ozone are usually present in the atmosphere, especially in the open country ; and it no doubt plays an important part in the removal of organic impurities from the atmosphere. Ozone is a colourless gas of peculiar unpleasant odour; 100 volumes of water at C. only absorb about - 5 volume, so that it is considerably less soluble than oxygen. Air charged with ozone exerts an irritating action upon the respiratory organs. Ozone is chiefly remarkable, however, on account of its oxidizing power. Thus, dry mercury, which is not in the least affected by ordinary oxygen, un less heated to near its boiling point, is at once oxidized by ozone, and dry iodine absorbs ozone and is oxidized by it. It bleaches a solution of indigo, which is oxidized by it. It also rapidly corrodes organic substances, such as cork and caoutchouc, which therefore cannot be employed in experiments with ozone. Paraffin, however, is not attacked by it, and an excellent air-tight joint between two glass tubes may be made by means of it. A piece of glass tube, into which they exactly fit, is slipped over the two tubes, which are placed close together, and a fragment of pure paraffin is placed at the external junction of the tubes ; on geutly heating the paraffin it melts, and the liquid being extremely limpid runs into and fills up the narrow space between the tubes. In the conversion of oxygen into ozone the volume con tracts by one-third, three molecules of oxygen furnishing two molecules of ozone. 30 2 = 20 3 ; Oxygen. Ozone. but when ozone is reconverted into oxygen the volume increases to the same extent that it diminishes when oxygen is converted into ozone, since 2O 3 = 3O 2. Ozone. Oxygen. The conversion of ozone into oxygen may be effected by heat. Thus, when ozonized oxygen is passed through a glass tube heated to 110 C. slight decomposition of the ozone takes place ; at 200 C. the decomposition is very rapid, about 97 per cent, being converted into oxygen ; and below 300 C. decomposition is complete. Similarly, ozone is decomposed to an unlimited extent by contact with metallic silver, and by manganese dioxide, lead dioxide, and copper oxide, without the substances undergoing more than an excessively minute increase of weight in the reaction. These decompositions afford in stances of recurrent action, the active substance being alternately oxidized and reduced by the ozone; in the case of silver, supposing silver monoxide is formed, although the oxide produced is probably a higher oxide, the reaction may be represented in the following manner : 2Ag + 3 = 2 + A g2 0; Silver. Ozone. Oxygen. Silver oxide. Ag z O + 3 = 20. 2 + 2Ag. Silver oxide. Ozone. Oxygen. Silver. Many reactions are known which prove that ozone has this power of effecting deoxidation, although itself a most powerful oxidizing agent ; thus, by its action on a solution of sodium dioxide, sodium monoxide and oxygen are ob tained : Na 2 2 = 20. Na 2 O. Sodium dioxide. Ozone. Oxygen. Sodium oxide. The decomposition of ozone in this manner by silver, &amp;lt;fec., is accompanied by the development of a considerable amount of heat ; thus, according to Berthelot, no less than 29,600 units of heat are evolved in the reaction 20 3 = 30 2 . A corresponding amount of energy must therefore be expended in the formation of ozone from ordinary oxygen. But the conversion of ozone into ordinary oxygen is to be regarded as occurring in two stages, in the first the mole cule is resolved into O 2 + O, and in the second two atoms of oxygen from two molecules of ozone unite forming a molecule of ordinary oxygen. The splitting iip of the ozone molecule into O 2 + O probably requires an expendi ture of energy, so that the energy developed in the reaction in the form of heat is due to the combination of the atoms of oxygen to form molecules, but is less than that actually developed by their combination by the amount expended in the decomposition of the ozone molecules. As already pointed out, the fact that so large an amount of heat is developed by the combination of oxygen with oxygen indi cates that the affinity of the oxygen atoms for each other is very considerable. This being the case, we are enabled also to understand how it is that ozone has such superior power as an oxidizing agent as compared with ordinary oxygen, for it is evident that, in reactions into which oxy gen enters, for every 32 grammes employed an amount of energy corresponding to at least 29,600 heat units must be expended in order to separate- the two atoms forming the oxygen molecules from each other ; whereas, mostly, when ozone is employed, the amount of energy to be expended is only that required to effect the decomposition of the ozone molecules into O 2 + O, which is probably very small. The amount of oxidation effected by the same amount of ozone varies, however, according to the nature of the body