Note 3 bis — Chapter XI · THE HALOGENS: CHLORINE, BROMINE, IODINE, AND FLUORINE

This representation of the process of the reaction is most natural. However, this decomposition is generally represented as if chlorine gave only one degree of combination with manganese, MnCl2, and therefore directly reacts in the following manner—MnO2 + 4HCl = MnCl2 + 2H2O + Cl2, in which case it is supposed that manganese peroxide, MnO2, breaks up, as it were, into manganous oxide, MnO and oxygen, both of which react with hydrochloric acid, the manganous oxide acting upon HCl as a base, giving MnCl2 and at the same time 2HCl + O = H2O + Cl2. In reality, a mixture of oxygen and hydrochloric acid does give chlorine at a red heat, and this reaction may also take place at the moment of its evolution in this case.

All the oxides of manganese (Mn₂O₃, MnO₂, MnO₃, Mn₂O₇), with the exception of manganous oxide, MnO, disengage chlorine from hydrochloric acid, because manganous chloride, MnCl₂, is the only compound of chlorine and manganese which exists as a stable compound, all the higher chlorides of manganese being unstable and evolving chlorine. Hence we here take note of two separate changes: (1) an exchange between oxygen and chlorine, and (2) the instability of the higher chlorine compounds. As (according to the law of substitution) in the substitution of oxygen by chlorine, Cl₂ takes the place of O, the chlorine compounds will contain more atoms than the corresponding oxygen compounds. It is not surprising, therefore, that certain of the chlorine compounds corresponding with oxygen compounds do not exist, or if they are formed are very unstable. And furthermore, an atom of chlorine is heavier than an atom of oxygen, and therefore a given element would have to retain a large mass of chlorine if in the higher oxides the oxygen were replaced by chlorine. For this reason equivalent compounds of chlorine do not exist for all oxygen compounds. Many of the former are immediately decomposed, when formed, with the evolution of chlorine. From this it is evident that there should exist such chlorine compounds as would evolve chlorine as peroxides evolve oxygen, and indeed a large number of such compounds are known. Amongst them may be mentioned antimony pentachloride, SbCl₅, which splits up into chlorine and antimony trichloride when heated. Cupric chloride, corresponding with copper oxide, and having a composition CuCl₂, similar to CuO, when heated parts with half its chlorine, just as barium peroxide evolves half its oxygen. This method may even be taken advantage of for the preparation of chlorine and cuprous chloride, CuCl. The latter attracts oxygen from the atmosphere, and in so doing is converted from a colourless substance into a green compound whose composition is Cu₂Cl₂O. With hydrochloric acid this substance gives cupric chloride (Cu₂Cl₂O + 2HCl = H₂O + 2CuCl₂), which has only to be dried and heated in order again to obtain chlorine. Thus, in solution, and at the ordinary temperature, the compound CuCl₂ is stable, but when heated it splits up. On this property is founded Deacon's process for the preparation of chlorine from hydrochloric acid with the aid of air and copper salts, by passing a mixture of air and hydrochloric acid at about 440° over bricks saturated with a solution of a copper salt (a mixture of solutions of CuSO₄ and Na₂SO₄). CuCl₂ is then formed by the double decomposition of the salt of copper and the hydrochloric acid; the CuCl₂ liberates chlorine, and the CuCl forms Cu₂Cl₂O with the oxygen of the air, which again gives CuCl₂ with 2HCl, and so on.

Magnesium chloride, which is obtained from sea-water, carnallite, &c., may serve not only as a means for the preparation of hydrochloric acid, but also of chlorine, because its basic salt (magnesium oxychloride) when heated in the air gives magnesium oxide and chlorine (Weldon-Pechiney's process, 1888). Chlorine is now prepared on a large scale by this method. Several new methods based upon this reaction have been proposed for procuring chlorine from the bye-products of other chemical processes. Thus, Lyte and Tattars (1891) obtained up to 67 p.c. of chlorine from CaCl₂ in this manner. A solution of CaCl₂, containing a certain amount of common salt, is evaporated and oxide of magnesium added to it. When the solution attains a density of 1·2445 (at 15°), it is treated with carbonic acid, which precipitates carbonate of calcium, while chloride of magnesium remains in solution. After adding ammonium chloride, the solution is evaporated to dryness and the double chloride of magnesium and ammonium formed is ignited, which drives off the chloride of ammonium. The chloride of magnesium which remains behind is used in the Weldon-Pechiney process. The De Wilde-Reychler (1892) process for the manufacture of chlorine consists in passing alternate currents of hot air and hydrochloric acid gas through a cylinder containing a mixture of the chlorides of magnesium and manganese. A certain amount of sulphate of magnesium which does not participate in any way in the reaction, is added to the mixture to prevent its fusing. The reactions may be expressed by the following equations: (1) 3MgCl₂ + 3MnCl₂ + 8O = Mg₃Mn₃O₈ + 12Cl; (2) Mg₃Mn₃O₈ + 16HCl = 3MgCl₂ + 3MnCl₂ + 8H₂O + 4Cl. As nitric acid is able to take up the hydrogen from hydrochloric acid, a heated mixture of these acids is also employed for the preparation of chlorine. The resultant mixture of chlorine and lower oxides of nitrogen is mixed with air and steam which regenerates the HNO₃, while the chlorine remains as a gas together with nitrogen, in which form it is quite capable of bleaching, forming chloride of lime, &c. Besides these, Solvay and Mond's methods of preparing chlorine must be mentioned. The first is based upon the reaction CaCl₂ + SiO₂ + O(air) = CaOSiO₂ + Cl₂, the second on the action of the oxygen of the air (heated) upon MgCl₂ (and certain similar chlorides) MgCl₂ + O = MgO + Cl₂ The remaining MgO is treated with sal-ammoniac to re-form MgCl₂ (MgO + 2NH₄Cl = MgCl₂ + H₂O + 2NH₃) and the resultant NH₃ again converted into sal-ammoniac, so that hydrochloric acid is the only substance consumed. The latter processes have not yet found much application.