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!

  • $\begingroup$ Im running the experiment for work, I am taking all safety precautions and its very important I find a way to get the two to react. Having said that I appreciate the input, So blue light hey? I still cannot replicate the conditions which lead to explosion, Im wondering are there any possible mechanisms in the electrolysis of brine that could be inhibiting the reaction? for example, moisture? Oxygen Levels? if I turn the voltage up on the electrodes is it possible that I start to electrolyse water instead of salt? releasing H and O2? i need to find a way to get the electrolytic cell to explode. $\endgroup$ – Michael Cousins Oct 7 '16 at 1:07
  • $\begingroup$ @MichaelCousins If you’re attempting it in solution, my first guess is that they just won’t react in solution. And yes, if you apply too strong a current, $\ce{O2}$ will be released due to over-something. $\endgroup$ – Jan Oct 8 '16 at 0:08
  • $\begingroup$ yes, but I haven't filled the container completely with water, there is space for the gas to collect. obviously a lot of chlorine will dissolve into the water but surely some gas is released? is there a way to decrease how much chlorine dissolves? $\endgroup$ – Michael Cousins Oct 11 '16 at 23:39
  • $\begingroup$ A better question might be: why are you doing this for work? HCl gas is available and relatively cheap. There are safer, more controlled ways to create explosions without UV light. $\endgroup$ – Zhe Nov 4 '16 at 20:14

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.

  • $\begingroup$ Welcome to Chemistry.SE! Take the tour to get familiar with this site. I have updated your post with chemistry markup. If you want to know more, please have a look here and here. We prefer to not use MathJax in the title field, see here for details. $\endgroup$ – Martin - マーチン Oct 5 '16 at 6:15
  • $\begingroup$ Yes there is enough H2 and Cl2, however; I think there must be O2 in the mix which is inhibiting an explosive reaction (which is what Im going for in this instance.) $\endgroup$ – Michael Cousins Oct 5 '16 at 7:26
  • $\begingroup$ Nope; the "explosion" of hydrogen gas is due to the formation of water (H2O) while the proper reaction beween hydrogen and chlorine might not be that "explosive" since it may not cause a flame but just some heat and maybe a gleam of light. Oxygen is not required in the reaction $\endgroup$ – user35659 Oct 5 '16 at 13:45
  • $\begingroup$ well i mean detonation, I am running the electrolysis in a closed cell. $\endgroup$ – Michael Cousins Oct 7 '16 at 1:15

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