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the 'carriers of oxygen.' As the oxidation of sulphurous acid by the oxygen of the air takes place very slowly, nitric oxide is used as a carrier of oxygen in the leaden chamber of the vitriol works. The nitric oxide, NO, is oxidised at the expense of the oxygen of the atmosphere to the peroxide, NO,, which oxidises the sulphurous acid and is reconverted into nitric oxide. The reaction is not quite as simple as it is here depicted. In the first place nitro-sulphonic acid (the crystals of the leaden chamber) HO-SO,—NO, is first formed from nitric oxide, oxygen, steam, and sulphur dioxide. This compound decomposes, yielding sulphuric acid and oxides of nitrogen

2

2HO-SO,—NO2 + H2O = 2H0—SO2—OH + NO2 + NO.

2

The sulphates and other salts of manganese, copper, iron, and other metals act as carriers of oxygen to aqueous solutions of sulphurous acid, as they are reduced by the sulphurous acid and reoxidised by the atmospheric air. A cold solution of oxalic acid is not oxidised by chromic acid, except in the presence of a manganese salt, which reduces the chromic acid and oxidises the oxalic acid. Manganese sulphate also acts as a carrier of oxygen when oxalic acid is oxidised by permanganic acid. Indigo effects the oxidation of an alkaline solution of grape sugar by acting as a carrier of atmospheric oxygen, for the indigo is reduced to indigo-white by the grape sugar, but at once unites with oxygen and forms indigo-blue again.

The formation of ethyl ether from alcohol and sulphuric acid was formerly considered to be an example of catalytic or contact action

2C2H ̧.OH = (C2H ̧)2O + H2O.

But Williamson proved that the sulphuric acid takes part in the

The sulphuric acid does not act merely by its presence, but takes part in the interaction and is formed again. It is probable

that something similar takes place in the case of the apparently simple decomposition mentioned in § 108. The decomposition of cane sugar into dextrose and lævulose is probably preceded by the formation of corresponding ethereal salts with the inverting acid, which are in their turn decomposed by water.

Emmerling observed and explained a very remarkable case of apparent contact action. Oxalic acid scarcely attacks crystalline calcium carbonate, e.g. marble. A thin insoluble crust of calcium oxalate forms on the surface of the marble, which prevents any further action taking place, but on the addition of a very small quantity of nitric acid or a nitrate the marble is rapidly attacked and converted into calcium oxalate. Apparently it is the nitric acid or the nitrate which incites the oxalic acid to attack the calcium carbonate, but in reality it is the nitric acid which was added or was liberated from the nitrate by the oxalic acid that attacks the marble, forming calcium nitrate. The oxalic acid interacts, forming calcium oxalate and liberating nitric acid.

Interactions which apparently take place between two substances but require the presence of a third body, are of frequent occurrence. Many metals remain unaltered in dry air which rust or oxidise in a damp atmosphere. M. Traube has shown that water takes part in this process of oxidation; a metallic hydroxide and hydrogen peroxide are first formed according to the equation

Zn + 2HOH+ O2 = ZnO2H2 + H2O2.

2

The metal decomposes the water, forming hydroxyl (HO—) and hydrogen, but the latter unites with a molecule of oxygen. The molecule of oxygen consists of two atoms, and at the low temperature the hydrogen is unable to effect their separation. The hydroxide loses water and passes into oxide, the hydrogen peroxide loses oxygen, forming water. In this way the water which took part in the interaction reappears as water when the reaction is completed.

Certain bodies act as carriers of chlorine in a similar way to the oxygen carriers. Pure nitro-benzene is practically unattacked by chlorine, but in the presence of anhydrous ferric chloride substitution of the hydrogen by chlorine takes place;

KINETIC NATURE OF AFFINITY

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but the ferric chloride remains unchanged in quantity and has apparently taken no part in the reaction.

When bromine is substituted for chlorine the following reaction takes place, according to A. Scheufelen :

Br2 + C¿H¿NO2 + FeCl3 = Br.CH.NO, + HCl + FeCl2Br

and the analogous change in the case of chlorine is

Cl2 + C¿H¿NO2 + FeCl3 = Cl.C2H1.NO2 + HCl + FeCl2.Cl.

The chlorine of the ferric chloride unites with an atom of hydrogen of the benzene to form hydrochloric acid, and both atoms are replaced by free chlorine or bromine. All three bodies consequently take part in the interaction.

The numerous and varied forms of fermentation were formerly regarded as cases of catalytic action. It was believed that simple contact with the ferment, e.g. yeast, brought about the decomposition of the fermentable substance, sugar. Fermentation is now considered to be due to the action of minute living organisms, which devour the fermentable substance for their nourishment and excrete the decomposition products. The yeast ferment feeds on sugar and produces carbon dioxide and alcohol. The process is much more complicated than was formerly supposed to be the case.

This example, taken from a large class of similar phenomena, clearly shows that those changes which were formerly described as the result of contact action must be generally regarded as the interaction of three or more different bodies, or are even much more complex reactions. Berzelius explained this class of phenomena by the hypothesis of a special catalytic force contained in those substances which apparently take no part in the interaction. This hypothesis is now known to be unnecessary and superfluous.

§ 123. Kinetic Nature of Affinity.-An examination of the various forms of chemical change as described in the preceding paragraphs leads of necessity to the conclusion that the hypothesis of an attractive force known as affinity, such as was formerly accepted and even survives to the present time, is of little or no use in explaining chemical phenomena.

We might conclude from the fact that cupric sulphate is

reduced to a cuprous salt by sulphurous acid that sulphurous acid has a stronger affinity for oxygen than copper has in the form of cuprous oxide; we should consequently expect that cuprous oxide will take up oxygen from the air less readily than sulphurous acid does. As observation shows that the reverse is the case, we come to the conclusion that this view of the matter is incorrect; nor is the explanation satisfactory in many other cases. We have gradually receded from the idea of a static state of equilibrium of the atoms brought about by their powers of affinity, and we now consider the atoms, and the molecules which are built up of atoms, as particles in an active state of movement. Their relations to each other are essentially determined by the magnitude and form of their movements. Chemical theories grow more and more kinetic, and although, partly from habit and partly from want of a better expedient, the existence of an attractive force between the atoms is frequently used in explaining chemical phenomena, this only happens in the conviction that this hypothetical affinity is merely an expression for the real though imperfectly known cause of the internal cohesion of chemical compounds.