Hydrogen photochemical reaction with chlorine

Bromine, like chlorine, also undergoes a photochemical chain reaction with hydrogen. The reaction with bromine, however, evolves less energy and is not explosive. 

In the photochemical reaction between hydrogen and chlorine, the slow step is usually assumed to be the reaction between hydrogen molecules and chlorine atoms. The experimental data used in obtaining information about the transition state are the relative rates of reactions of H, and HT with Cl atoms.87 Experimentally one measures R, which is defined by the following relations ... 

Figure 6.46 Photochemical reaction of hydrogenated diamond films with 1-chloro-5-hexene, followed by a substitution of the terminal chlorine atoms.

Baly ECC, Barker FB (1921) The photochemical reaction between hydrogen and chlorine and its variation with the intensity of light. J Chem Soc 119 653-664... 

Some reactions occur much faster if the reacting system is exposed to incident radiation of an appropriate frequency. Thus, a mixture of hydrogen and chlorine can be kept in the dark, and the reaction to form hydrogen chloride is very slow however, if the mixture is exposed to ordinary light, reaction occurs with explosive rapidity. Such reactions are generally called photochemical reactions. 

Chlorine combines with hydrogen forming hydrogen chloride, HCl. The reaction occurs rapidly when exposed to hght, involving a photochemical chain initiation step. 

The reaction is explosive with fluorine and occurs under all conditions. With chlorine and bromine reaction occurs rapidly when exposed to light, undergoing a photochemical chain reaction. With iodine, the reaction is very slow, even at elevated temperatures. Hydrogen is a strong reducing agent. At high temperatures, the gas reduces many metal oxides to lower oxides or metals ... 

There are many photochemical reactions in which a large number of molecules may undergo chemical change by the absorption of one photon of light. In such reaction, the quantum yield has been found to vary from 104 to 10s. At moderate gas pressures and with comparable proportion of chlorine and hydrogen photochemical reaction takes place. 

Another example of a solvent-dependent atom-transfer reaction is hydrogen abstraction by chlorine atoms during the photochemical chlorination of hydrocarbons with molecular chlorine for an excellent review, see reference [571]. Russel reported that in the photochlorination of 2,3-dimethylbutane, according to reaction scheme (5-68), certain solvents do not have any effect on the selectivity of the reaction as measured by the rate ratio whereas other solvents increase this ratio significantly (c/. 

O3 is photochemically destroyed through direct photolysis (9), reaction with O (10), and through a series of catalytic cycles involving the oxides of nitrogen, hydrogen, and chlorine ... 

Potts and Rollefson studied the photochemical reaction between H2 and CI2 over the temperature range 140-300 °K. At the lower end of this range solid or liquid chlorine was always present in the system so that the reaction rate could be calculated from measurements of the change in pressure with time using a sulfuric acid manometer. At the higher temperatures where the CI2 pressure decreased as the reaction proceeded, the rates were calculated from measurement of hydrogen pressures before and after the reaction. At temperatures below 172 °K, Potts and Rollefson found the rate of HCl formation to depend upon the square root of the light intensity whereas above 200 °K the rate was proportional to the first power of the intensity. The rates were proportional to [H2] at all... 

This channel reduces significantly the calculated ClO/HCl ratio in the upper stratosphere and, hence, the sensitivity of ozone to chlorine. There are also other slow processes that yield hydrogen chloride. Like HNO3, HC1 provides a relatively inert reservoir which sequesters a photochemically active species and hence reduces the rate of its catalytic reaction with odd oxygen. 

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