Chlorine : Photochemical Union Of Hydrogen And Chlorine

A mixture of practically equal volumes of hydrogen and chlorine, containing a trace of oxygen, is obtained by the electrolysis of hydrochloric acid (sp. gr. 1.1). The washed gas is passed through a series of very thin glass bulbs,

Fig: Filling glass bulbs with a mixture of chlorine and hydrogen

the whole operation being performed in a dark room lighted by a ruby lamp. After the gas has passed for at least half an hour, the two ends of the bulbs are closed with wax, and the capillaries separating them carefully sealed off with a small flame. The combustion does not usually spread from the heated part. The bulbs are preserved in a dark box.

According to Faraday (1833) concentrated hydrochloric acid, diluted with nine to fifteen parts of water, gives "only a little oxygen with much chlorine at the anode," but experiments by Bunsen showed that the evolution of oxygen begins to be recognisable with 23 per cent, acid; with stronger acids the chlorine is practically pure. With more dilute acids, chloric acid is also formed at the anode as well as oxygen. The amount of oxygen liberated with 0.1N acid is very appreciable, according to Haber and Grinberg (1898).

Expt. 22. - If a bulb containing a mixture of hydrogen and chlorine, protected by a screen of plate glass,

Fig: Explosion hydrogen and chlorine

Explosion of a mixture of hydrogen and chlorine by exposure to strong light of burning magnesium.

is exposed to the light of burning magnesium flash-powder, a sharp explosion occurs, and the glass is shattered. A similar bulb filled with gas dried by passing over P₂O₅ does not usually explode, but the gases combine, as may be shown by opening under litmus solution.

Heat is evolved in the reaction, hence the action of light consists only in initiating the reaction, which when once started goes on spontaneously. Photochemical reactions which involve an absorption of energy, and which stop when the light is cut off, are the formation of ozone from oxygen, and the decomposition of hydrogen chloride and ammonia, by ultraviolet light.

According to Thos. Thomson, the explosion of a mixture of hydrogen and chlorine on exposure to sunlight was discovered by Dalton, who communicated it to him by letter before the announcement of the experiments of Gay-Lussac and Thenard in 1809.

Pringsheim in 1887 found that if the mixed gas was dried with phosphorus pentoxide before passing into the bulb, and the latter exposed to magnesium light, there was no explosion, but the bulb became hot and the gases combined completely. Following experiments of H. B. Baker (1894), Coehn and Tramm (1923) found that a carefully purified and dried mixture of hydrogen and chlorine underwent no change on exposure to visible light (λ > 4000 A.U.); a partial pressure of water vapour of 10^-6 mm. was sufficient to initiate the reaction. With (ultraviolet) light of wavelength smaller than 3000 A.U., however, reaction occurred even with very dry gases. Other experimenters report that the rate of combination is not affected by moisture. Dixon and Harker (1890) found that the velocity of the detonation wave in carefully dried hydrogen and chlorine was 1795 m. per sec.; in the moist gas it was only 1770 m. per sec. Moisture, although assisting the initiation of the reaction, therefore appears to retard it once it has begun.

J. W. Draper (1843) investigated and confirmed an effect noticed by Dalton (1809), that a mixture of hydrogen and chlorine did not begin to contract at once when exposed over water to diffused daylight. There was an initial "hesitation," called the period of photochemical induction, or Draper effect. Bunsen and Roscoe (1857-59) used the apparatus shown in,

Fig: Actinometer of Bunsen and Roscoe

called an actinometer, to investigate the reaction. The mixed gases were confined in the half-blackened flat bulb I by chlorine water. On exposure to light, contraction occurred, the HCl formed dissolving, and the rate of combination could thus be estimated by the movement of the thread of liquid in the horizontal tube k. It was found that the rate of combination was proportional to the intensity of the light. These experimenters also noticed the photochemical induction period.

Burgess and Chapman (1904) showed that the period of photochemical induction was not really peculiar to the reaction H₂ + Cl₂ = 2HCl, but was due to traces of impurities, ammonia or nitrogenous organic matter, in the water used to confine the gases. If this water was first boiled with chlorine, these substances were destroyed, and the gases then began to combine the instant they were exposed to light. Traces of oxygen retard the velocity of combination but do not give rise to a period of induction.

With very pure gases the rate of combination is approximately proportional to the square root of the light intensity.

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Chlorine
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