Is it possible to react chlorine with hydrogen under a UV lamp during the electrolysis of salty water?

I have set up an experiment where I am generating chlorine by electrolysing water saturated in NaCl. I know chlorine gas is actually being generated because I hang wet litmus paper in the gas which quickly bleaches. I also know hydrogen is being generated because if I hold a match to the gas it reacts with a popping sound.


If I have this set up in a closed container, how come I cannot get the hydrogen and chlorine to react under a black light? What am I missing?




**I advise you *not* to attempt a reaction of hydrogen and chlorine gas by light!**


**Do not attempt it unless you know *exactly* what you are doing *and* have seen the experiment be performed by others!**


You’re actually rather lucky that one of the following happened:


* the black light gave the wrong wavelength
* there were too small concentrations of hydrogen gas and chlorine gas to react with each other.


The reaction of hydrogen and chlorine is a radical chain reaction and rather exothermic. There is no change in the amount of substance involved, so entropic effects do not play a role. However, it is an *explosive* reaction and was often used in the introductory inorganic chemistry course at my university to give off a loud bang (and send the tin can in which the gases were mixed flying).


The reaction is triggered by light energy interacting with chlorine; the first (initiation) step is the generation of chlorine radicals by the following equation:


$$\ce{Cl2 ->[$h\nu$] 2 Cl^.}\tag{1}$$


This chain then propagates according to reactions $(2)$ and $(3)$.


$$\ce{Cl^. + H2 -> HCl + H^.}\tag{2}$$


$$\ce{H^. + Cl2 -> HCl + Cl^.}\tag{3}$$


The light needed for the first step must supply chlorine’s bond dissociation energy; blue (visible) light, not UV light is actually required. But just for your safety please let me restate:


**The reaction is explosive! Do not attempt it at home under any circumstance! Do not attempt it unless you know *exactly* what you are doing!**


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Yes, the reaction from $$\ce{H2 + Cl2 -> 2HCl}$$ (Hydrogen Chroride) is usually done under the presence of UV light, but did you check if there are enough amount of $\ce{H2}$ and $\ce{Cl2}$ to react?


If the amount of the reacting gas is insufficient, the reaction seldom happens: the probability of collision would significantly decrease.


If you want the reaction to occur, you can collect purer $\ce{H2}$ and $\ce{Cl2}$ (maybe by water displacement) and try the UV lamp on them.


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