Why does a mixture of hydrogen and chlorine gas stable in the dark, yet is likely to explode in sunlight?
Answer:
Some reactions require so little energy to happen that even the little energy that is given by light can cause the reaction to become violent.
I would expect that it is the UV light that is splitting the Cl2 gas into Cl free radicals. These are highly reactive and will propagate a very strong chain reaction causing the explosion.
This is the propagation stage. Note that the reactions always leave a free radical to continue the reaction.
Cl2 + uv = Cl. + Cl. (note dot is an electron)
H2 + Cl. = HCl + H.
H. +Cl2 = HCl + Cl.
You will also get the free radicals recombine, this is the termination stage of the reaction.
H. + Cl. = HCl
H. + H. = H2
Cl. + Cl. = Cl2 (which can be re-split with uv light. So the reaction will continue until the Cl2 is used up or the light removed.)
This is similar to how CFC's destroy the ozone layer.
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