GROUP VII: THE HALOGENS 321
occur with explosive violence. A chain mechanism is likely for the
reaction.
Mixtures of chlorine and hydrogen react only slowly in the dark
but the reaction proceeds with explosive violence in light. A
suggested mechanism for the photochemical chain reaction is:
C1 2 + hv -» 2Cr
Cl* + H 2 ->HC1 + H-
H' + C1 2 -> HC1 4- Cl* and so on.In the presence of charcoal, chlorine and hydrogen combine
rapidly, but without explosion, in the dark. A jet of hydrogen will
burn in chlorine with a silvery flame and vice versa.
The affinity of chlorine for hydrogen is so great that chlorine will
react with many compounds containing this element, for example
hydrocarbons (a wax taper burns in chlorine).
Chlorine substitutes the hydrogen of methane giving successively
the chlorides CH 3 C1, CH 2 C1 2 , CHC1 3 and CC1 4. It is to be noted
that if a hydrocarbon is unsaturated, chlorine atoms will first add
to the double or triple bond after which substitution may occur.
Chlorine will also remove hydrogen from hydrogen sulphide,
liberating sulphur, and from ammonia, liberating nitrogen:H 2 S + C1 2 -> 2HC1 4- S
8NH 3 + 3C1 2 -> 6NH 4 C1 + N 2
Bromine, like chlorine, also undergoes a photochemical chain
reaction with hydrogen. The reaction with bromine, however,
evolves less energy and is not explosive.
Like chlorine, bromine can displace hydrogen from saturated
hydrocarbons, though not as readily, and adds on to unsaturated
ones.
Iodine and hydrogen react reversibly to give hydrogen iodide:H 2 + I 2 ^ 2HI
This equilibrium has been extensively studied by Bodenstein.
Unlike the other halogen-hydrogen reactions, it is not a chain
reaction but a second order, bimolecular, combination.
Iodine does not replace hydrogen from saturated hydrocarbons
directly, as do both chlorine and iodine.WITH ELEMENTS OTHER THAN HYDROGEN
Fluorine is exceedingly reactive and combines vigorously with most
elements. Some ignite spontaneously in gaseous fluorine at room