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A halogen lamp (also called tungsten halogen, quartz-halogen, and quartz iodine lamp) is an incandescent lamp consisting of a tungsten filament sealed in a compact transparent envelope that is filled with a mixture of an inert gas and a small amount of a halogen, such as iodine or bromine.
The hydrogen-halogen reactions get gradually less reactive toward the heavier halogens. A fluorine-hydrogen reaction is explosive even when it is dark and cold. A chlorine-hydrogen reaction is also explosive, but only in the presence of light and heat. A bromine-hydrogen reaction is even less explosive; it is explosive only when exposed to flames.
The relative rates at which different halogens react vary considerably: [citation needed] fluorine (108) > chlorine (1) > bromine (7 × 10 −11) > iodine (2 × 10 −22).. Radical fluorination with the pure element is difficult to control and highly exothermic; care must be taken to prevent an explosion or a runaway reaction.
Halogenation of saturated hydrocarbons is a substitution reaction. The reaction typically involves free radical pathways. The regiochemistry of the halogenation of alkanes is largely determined by the relative weakness of the C–H bonds. This trend is reflected by the faster reaction at tertiary and secondary positions.
A similar reaction like that of MoO 2 is used in halogen lamps. The tungsten is evaporated from the tungsten filament and converted with traces of oxygen and iodine into the WO 2 I 2, at the high temperatures near the filament the compound decomposes back to tungsten, oxygen and iodine. [6] WO 2 + I 2 ⇌ WO 2 I 2, ΔH rxn < 0 (exothermic)
Like other gas-discharge lamps such as the very-similar mercury-vapor lamps, metal-halide lamps produce light by ionizing a mixture of gases in an electric arc.In a metal-halide lamp, the compact arc tube contains a mixture of argon or xenon, mercury, and a variety of metal halides, such as sodium iodide and scandium iodide. [7]
It can also be formed by the displacement reaction of tungsten(II) chloride and iodine: [1] [W 6 Cl 8]Cl 4 + 12 I → [W 6 I 8]I 4 + 12 Cl. It can also be formed by the direct reaction of tungsten and iodine, which is a reversible reaction. This reaction can be used in halogen lamps. [3] W + I 2 ⇌ WI 2
This organic reaction is typical of aromatic compounds and a very useful method for adding substituents to an aromatic system. Halogenation of benzene where X is the halogen, catalyst represents the catalyst (if needed) and HX represents the protonated base.