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[jefrs]

Les H - hydrogen is very abundant so where is the free hydrogen on earth? - it exists as hydrocarbons and water etc.



Combustion is the thermal runaway state of an exothermic reaction producing products that are ionised (the flame). A chemical reaction requires an energy source to kick it off. Combustion also requires a concentration of fuel and oxidising agent to produce the chain reaction, otherwise the fire goes out. The fuel and oxygen molecules have to be brought close enough together to react, which is why our motors compress fuel/air to some 8.5 times atmospheric pressure.

Substances have a vibrational energy state given by their absolute temperature that is usually great enough to support a chemical reaction. It does not have to burst into flames to perform a chemical reaction, most of the time stuff does not do that.



The battery is vented to allow gases to escape easily, some the O and H will probably combine to water back into the battery but they evolve separately from +&- plates then disperse into the air within and then outside the battery. The molecules have to be close together to react (not burn) so as they're dispersed they don't and why we don't want a concentration build up. Eventually an oxygen might slam into a hydrogen and produce water, the probability of that is quite high as oxygen is is about 20% of air but is an isolated reaction not supporting combustion. The point rather being that hydrogen does not have to burn to produce water with oxygen.

[papasmurf]

Jefrs I am more than aware of the danger of static charges building up, the point remains with only a 4% presence of hydrogen in air at atmospheric pressures it only takes a spark 10% of the strength of that needed to ignite a petrol/air mix for and explosion to happen.

Such explosions can and do happen and they can be easily replicated for demonstration purposes as I have already shown in a referenced video.
The safety advice is not there for fun. No naked flames or sparks any where near a lead acid battery being charged and don't charge batteries in a confined space.
I have seen such an explosion and luckily the person in the "firing line." Had protective clothing in the form of a welding mask, gloves, and overalls which protecting him enough from the shower of battery acid and battery casing bits to prevent any acid burns, and the fact he took them off rather rapidly and got under a shower.

[jefrs]

Hydrogen is a bit weird - but note - this only applies to like letting it out of a gas bottle.

It exhibits a reverse-adiabatic or Joule-Thompson/Kelvin effect whereby the damned stuff gets hot on expansion whereas most gasses like the good old CO2 extinguisher get very cold. That heating up can be enough to ignite it, and one of the big scary drawbacks of carrying hydrogen around as a fuel (See the HSE paper)

It's why I never liked playing with hydrogen gas in the lab.



N.B. No problem coming out of a battery because there's no difference in pressure. !!!



Googling stuff on the internet is a bit like a greek parliament, six people seven opinions. Especially a lot of higher scientific stuff, a lot of it out there is dumbed-down or just plain wrong: you have to know where to look and what is reliable and verified, NASA, NPL, CERN, universities, and the HSE or PHE (Public Elf England).

[jefrs]

papasmurf - I fully support that safety advice - no sparks nor naked lights neither near battery on charge !!! I never implied otherwise.



Big point is do it in a *well ventilated area* - so the H2 does not build up to even 2%



Hydrogen will burn between 2% and 98% but explode between 4% and 96%, the difference between 'burn' and 'explode' being rather arbitrary and both best avoided. It takes very little to set it off. Fortunately (cof) its combustion heat and force of explosion is quite low, but then so is that of methane/natural gas. But is needs to be concentrated to burn, let's assume the battery is all hydrogen and is 1 litre and that the room is this one 10x3x5 metres = 150 cubic metres - 150,000 litres, you've got a maximum concentration of 1/15000 = 0.006%

Ok, so the battery won't suddenly release that much hydrogen and most rooms are smaller but ... do the sums.

Do not confuse conflagration with the odd molecule of hydrogen having a little chemical reaction with an oxygen off in the corner in private. When the gases are dispersed (diluted) they can't support combustion and gases disperse through a room very quickly indeed, within seconds; add in typical flow rate in a workshop, we estimated 7 to 10 cubic meters per second if a door was open or forced convection (the lab aircon) ... (we had to do the risk assessment 'cos we had an asphyxiant gas in the room). Sheds are normally draughty.

[PeteF]

As I stated in my last post, I am fully aware of the dangers of believing everything I read on the web. In this case, however, its not a Greek parliament situation as I can't find ANY evidence that the reaction occurs at NTP; rather the reverse.
Perhaps I shouldn't believe ANYTHING I read on the web. I could start with this forum

[PeteF]

If anyone's still interested. I think there is some confusion here between atomic hydrogen and molecular hydrogen. I'm certainly confused
Now, whether a battery gives of H or H2 I don't know.

Quote:

One molecule of hydrogen dissociates into two atoms (H2 → 2H) when an energy equal to or greater than the dissociation energy (i.e., the amount of energy required to break the bond that holds together the atoms in the molecule) is supplied. The dissociation energy of molecular hydrogen is 104,000 calories per mole—written 104 kcal/mole (mole: the molecular weight expressed in grams, which is two grams in the case of hydrogen). Sufficient energy is obtained, for example, when the gas is brought into contact with a white-hot tungsten filament or when an electric discharge is established in the gas. If atomic hydrogen is generated in a system at low pressure, the atoms will have a significant lifetime—e.g., 0.3 second at a pressure of 0.5 millimetre of mercury. Atomic hydrogen is very reactive. It combines with most elements to form hydrides (e.g., sodium hydride, NaH), and it reduces metallic oxides, a reaction that produces the metal in its elemental state. The surfaces of metals that do not combine with hydrogen to form stable hydrides (e.g., platinum) catalyze the recombination of hydrogen atoms to form hydrogen molecules and are thereby heated to incandescence by the energy that this reaction releases.
Molecular hydrogen can react with many elements and compounds, but at room temperature the reaction rates are usually so low as to be negligible.

[Les H]

jefrs why don't you just admit you got it wrong? Trying to bludgeon your way out with massive amounts of scientific "cut and pastes" and talking testacles isn't going to change the laws of science in our universe just to suit you. Remember the story of King Canute?....just call it a day on this one and get back to talking about Royal Enfield motorbikes please....nobody cares.

[Scalyback]

well, we learnt all this at school, and that wa sin the days that things were demonstrated, not 'wiki'ed'.



I was told that two thirds hydrogen gas to one third oxygen gas would be H2O. The science Master then took a metal biscuit tin, punched two small holes in the top, filled it with hydrogen and covering the holes, took it outside. He lit one of the holes leaving the air to be pulled into the other. Obviously, it was not pure oxygen, but as we all hid behind a wall, there was suddenly quite a bang! when we retrieved the lid of the biscuit tin, there were indeed drops of presumably water that had not been there before.



This was interesting to me, where as most of my classmates seemed more bothered about where the biscuits had all gone before the experiment.



so today we learnt that a mixture of 2 thirds hydrogen and one third oxygen, goes bang quite well when lit and produces....H2O. YAY!



Next week, 'Fun with Thermite'!

[Beezabryan]

My head hurts !
I think, after another mug of tea, I will go to the peace and tranquility of my garage. After contemplating my navel and counting the scars thereabouts I may, without the aid of any pseudoscientific flummery, try firing up a mess of inorganic and organic matter that is also known as the infamous Bullet that has lain silent for so many months.
I will report back in a little while

[Thack]

jefrs writes: "The fuel and oxygen molecules have to be brought close enough together to react, which is why our motors compress fuel/air to some 8.5 times atmospheric pressure. "



Hey, hang on! That it simply not true. Motors compress the fuel/air mix for reasons to do with the laws of thermodynamics, not so the reaction will take place. Petrol vapour and air will combust without any compression at all. Indeed, the first internal combustion engines - before Otto invented his cycle - ran like that, with no compression. Compression increases the efficiency of the overall cycle.



Also, jefrs, I'm with the others on this: hydrogen and oxygen gases will NOT react at normal temperature and pressure. When they are made to react by increasing the temperature (e.g. with a spark), the reaction is always exothermic (i.e. a flame or a bang). The reaction of hydrogen with oxygen is always combustion, by definition. It is described in this quotation from Wikipedia:



A simple example can be seen in the combustion of hydrogen and oxygen into water vapor, a reaction commonly used to fuel rocket engines. This reaction releases 242 kJ/mol of heat and reduces the enthalpy accordingly (at constant temperature and pressure):



2H2(g) + O2(g) → 2H2O(g)