I have a container with 3% hydrogen and the rest is air. How fast will the hydrogen get oxidized by air?
How much will the reaction speed up with increased temperature?
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8$\begingroup$ The combustion of hydrogen is famous for being an extremely thermodynamically favourable process held back by a very large kinetic barrier. I've heard that mixtures of oxygen and hydrogen can last centuries without appreciable reaction, but I'd be pleased to see a calculation. $\endgroup$– Nicolau Saker NetoApr 2, 2015 at 14:12
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3$\begingroup$ The 3% is below the lower explosive limit (LEL), so it won't go BOOM!... $\endgroup$– Jon CusterApr 2, 2015 at 14:28
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$\begingroup$ @NicolauSakerNeto Thanks for the info. I wonder, what temperature is necessary for them to react reasonably fast? $\endgroup$– SergeyApr 2, 2015 at 15:31
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1$\begingroup$ @JonCuster Yes, LEL for hydrogen is 17% and LFL is 4% according to environmentalchemistry.com/yogi/periodic/H.html $\endgroup$– SergeyApr 2, 2015 at 16:02
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$\begingroup$ UV can also speed up the reaction. In fact, a mix of hydrogen and chlorine can be exploded with a tiny UV light -- see youtube.com/watch?v=NN82GoBG98s. $\endgroup$– DrMoishe PippikApr 2, 2015 at 22:16
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1 Answer
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According to the Encyclopedia Britannica, to somewhat paraphrase this non-technical source, the reaction rate of $\ce{O2(g)}$ and $\ce{H2(g)}$ at atmospheric pressure is not measurable at any mixing ratio at temperatures below $\ce{300^oC}$. Unfortunately that source does not report what kind of experimental detection limits define what "not measurable" means.
These class notes from the University of Toronto Institute for Aerospace Studies, AER1304 course state this in more dramatic fashion:
The reaction between hydrogen and oxygen is a good example of a multicomponent kinetic system. To describe the system properly, we should consider eight major species and at least 16 reactions. [The overall] reaction is exothermic, but mixtures of gaseous hydrogen and oxygen are quite stable at atmospheric conditions. Any conceivable direct reaction between the two gases is zero.
- The reaction half-time at atmospheric [pressures] has been estimated to be much larger than the age of the universe.
- If the reaction is initiated by some free-radical species, then the reaction proceeds very rapidly and violently.
So the manner in which the reaction rate of $\ce{O2(g)}$ with $\ce{H2(g)}$ increases with temperature is very complex, though basically negligible in the absence of a catalyst until you reach the auto-ignition temperature.
According to this Wikipedia article:
The hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is 500 °C.
Notice that this says air not oxygen. The $\ce{H2}$ concentrations for the basis of this statement range from 4% to 74%. This seems like quite a large range to give a single result for autoignition temperature, but given the great complexities of the $\ce{H2}$ + $\ce{O2}$ reaction, some estimations are clearly necessary in order to discuss this question in a reasonably concise fashion.
One note about the question regarding the composition and contents of the container; I'd be sure that it never contained any high surface-area platinum powder or be coated with any sort of perchlorate residues or any such ridicules thing! The whole issue with the difficulty in getting this highly exothermic reaction to proceed is kinetics. As eluded to in some of the comments, there are chemical catalysts that can help the reaction get over this large energy barrier, it doesn't have to be a spark!
