Forum

[PeteF]

I always thought that mixing O2 and H2 at NTP did nothing until the the ignition (reaction) temperature was reached (500degC if I recall) Once the reaction starts it is very vigorous of course. I haven't noticed petrol reacting without some form of ignition.
But we live and learn.

[papasmurf]

It only take 4% Hydrogen 96% air mix at atmospheric pressure plus a spark of only 10% the strength of that needed ignite a petrol air mix to leave you needing a clean pair of underpants and/or an ambulance.

[jefrs]

The energy needed to kick off a chemical reaction is given by their absolute temperature. Water freezes at 273K. A lot of stuff will react below that. Burning is more like a chain reaction where you get a lot of energy (heat) released in a short time. It also requires a lot of fuel and oxidant to be together in the same place. If the hydrogen from the battery has been wafted about and dispersed around the room, it is not going to start burning. For much the same reason your room doesn't explode when the gas boiler lights up, the room is not full of gas.



Btw there's never a fire extinguisher handy when you need one - if you have an electrical fire on your bike, not petrol or oil, you can chuck your drinking or ditch water over it. We've only got 12 volts so we won't get an electrical short problem. The fire brigade will jet water onto an electrical (mains) fire knowing that if it shorts the fuse will blow and two tons of water will put it out anyway. I came across a car with an electrical fire, he was trying to blow it out so I emptied his windscreen wash bottle over it: it went out.

[Les H]

To jefrs, perhaps a bit more studying is required on your behalf. Hydrogen does not react with Oxygen UNLESS it is heated sufficiently by a separate source of energy (energy added) to cause the gas to ignite (react)and produce more heat and water.
This can be clearly seen in the fact that the electrolysis of sulphuric acid in a lead acid battery creates the two gases separately and are emitted in their separate states. If what you were saying was true the gases would recombine into water immediately and the electrolyte would never get any lower.

Furthermore Hydrogen is the most abundant gas in the universe but if was so VERY reactive it would not exist as hydrogen.....your turn.....Les

[PeteF]

Jefrs statement that H2 will readily react with 02 at normal temperatures also struck me as odd. I've googled around a bit (I know, don't believe everything on the net) but can't find any evidence for this, indeed, I've come across lots of quotes that this reaction does not occur. Many sources state that free hydrogen migrates to the upper atmosphere and actually escapes the earth's gravity. This could hardly happen if it reacted so readily with oxygen. Hydrogen will react with lots of materials but not oxygen.
Obviously something is amiss here but it's probably me - I only got to A level in chemistry.

[Dennis C]

Pete, if you like odd statements, read back over some of the other threads where our scientist has posted things as "fact", and been proven incorrect.

[papasmurf]

For SPONTANEOUS combustion of hydrogen and oxygen to occur requires VERY high pressures.

http://www.hse.gov.uk/research/rrpdf/rr615.pdf

[Graham43]

................well I suppose they have stopped being mucky Thank you all for some very interesting and helpful replies and the chemistry lesson.

[PeteF]

Sorry Graham, we do rather seem to have drifted off your subject. Threads do that, they develop a life of their own

[jefrs]

papasmurf - that's not what Ashbury and Hawkesworth said, the last paragraph is particularly telling, plus there is a considerable difference in consequence between a simple chemical reaction combining H & O and combustion/explosion - "2.3
SUMMARY OF FINDINGS
To summarise the findings from the paper by Astbury and Hawksworth, they concluded:
•
Hydrogen does not necessarily ignite spontane
ously when released at high pressure.
•
Compression ignition, Joule-Thomson expa
nsion, diffusion ignition and hot surface
ignition are unlikely ignition mechanisms fo
r most accidental releases of hydrogen at
ambient temperature.
•
The postulated mechanisms described in the literature and discussed above do not
account for all the reported ignitions a
nd non-ignitions of hydrogen releases.
•
There is the possibility that when hydrogen
does ignite on release, two or more of the
postulated mechanisms are present together.
•
It is possible that some form of electrosta
tic charging is a part of the mechanism where
spontaneous ignition of leaks of hydrogen
from high pressure has occurred at ambient
temperature.
•
Further work is required to establish the
conditions under which hydrogen releases
ignite, particularly with resp
ect to electrostatic phenomena.