Can someone explain to a non-chemist...

[Jonti]

There's no such thing as an endothermic chemical, only an endothermic chemical reaction.

Almost all chemical reactions either absorb, or emit heat. The former are called by chemists endothermic reactions; the latter exothermic.

One can turn calcium carbonate into calcium oxide plus carbon dioxide by applying heat, for example. This endothermic reaction is useful, for calcium oxide is highly reactive and does not occur naturally. But if it is heated, it gives off a bright white light. We still use the term limelight. That comes from the light that hot calcium dioxide gives out!

Now take that calcium oxide ("quicklime") and carefully pour a little water on it, and it will seeth and boil. The water combines with the quicklime exothermically to make slaked lime or calcium hydroxide.

Oh, and bubble your breath through a solution of that, and tiny flakes of calcium cabonate, the original substance, will appear suspended in the liquid, and eventually settle out as a layer of fine chalk.

 

[Badger Kitten]

So the flour or black pepper (presumably the fine ground stuff) makes loads of motes which can ignite? Like this sort of thing?
www.youtube.com/watch?v=EpGZG3N0hGI

Well, in theory, but the thing is that the 21/7 peroxide/chappatti flour was semi-liquid - bubbling and 'like cake dough' or in Andy Hayman's book, 'like cheese on the top of a pizza taken out the oven' - and described as being so by witnesses on 21/7 train.

So unless it deteriorated very quickly, from, say, dry crystalline substance mixed with dry (compressed?) pepper, carried, we're told, in a plastic container ( because metal corrodes/reacts) and cooled by icepacks...

...then you have the problem of detonating wet sludge. Or if it crystals/powder, still slightly damp, 'tamped down' sandy stuff.

I must stress that the IEDs used on 7/7 are explained to have puzzled the Govt forensic scientists, who had little residue to go on anyway, because of the unusual nature of the explosions, and that 21/7 leaving failed devices was a huge breakthrough for them. That peroxide bombs are practically a first in 21st century Europe, and that the bombs seemed to have a 'signature', thought to be a Pakistani bombmaker, and interestingly, Mutkar Said Ibrahim ( 21/7) and Mohammed Siddique Khan ( 7/7) were both in Pakistan Nov 04- Feb 05, and thought to be atthe same camp.

( this is totally all public domain stuff btw. Well, maybe apart from Hayman's book quote, but the injunction is not because of Hayman's 7/7 descriptions, it's just a boring legal possible-contempt issue)

I suppose damp cartridges have exploed before, and on 21/7 the mix was made wrong ( peroxide not strong enough) which might explain why it was cake-dough-pizza-cheese -like.

Or am I just wrong and chilled wet sludge can detonate no problem, if close enough to an HMTD exploding detonator?

Scampi flavoured crisps are an abomination.

oh god, GCHQ will think that ^^ is some kind of code. argh.


thanks for all the helpful chemical explanations chaps, knew this was the right forum

 

[Jonti]

I suppose damp cartridges have exploded before, and on 21/7 the mix was made wrong ( peroxide not strong enough) which might explain why it was cake-dough-pizza-cheese -like.

This sounds plausible to me. It is hard to make a "strong enough" solution of hydrogen peroxide as it is a very reactive and unstable liquid; and for the same reason, it is difficult to store it for any length of time without it spontaneously dissociating into into water and oxygen.

 

[SpookyFrank]

Whether a reaction is endothermic or exothermic is judged based on the difference in potential energy between the initial reactants and the products of the reaction. In an explosion, lots of energy is released so what you have left afterwards has less potential energy stored in it. You might need to put a bit of energy in to start the reaction off, this is called the activation energy, but still the reaction as a whole leads to a net loss of energy from the matter involved so the reaction is still exothermic.

 

[ViolentPanda]

To help illustrate an endothermic reaction, bear in mind that most flour mill explosions (caused by static discharge or other electrical events causing ignition of fine particulates in the enclosed environment of the mill) are classed as endothermic explosions. You don't get a fireball, but you can and do get the sort of concussive blast that can shear metal beams and turn triple-course brickwork to rubble.

 

[ViolentPanda]

So the flour or black pepper (presumably the fine ground stuff) makes loads of motes which can ignite? Like this sort of thing?
www.youtube.com/watch?v=EpGZG3N0hGI

Yes. I was only posting recently about having demolished an allotment shed with a bag of cornflour, a 9v battery and some wire wool.

 

[ViolentPanda]

Flavoured crisps were invented in Ireland. Cheese and Onion, as it happens.

Shouldn't that be "Cheese and O'Nions"?

 

[Fruitloop]

It's just a bit of confusing terminology I think, in that it refers to the chemistry of the reactants and not the pressure/volume energy of the explosion.

 

[ibwaheemi]

..the difference between endothermic and exothermic explosions?

An endothermic explosion has been described as 'like a large air bag, creating a rapidly expanding shock wave void of the heat of a conventional bomb', is this right?

An endothemic explsion produces heat, but not fire, is that right? It could produce a flash when it goes off, but not flames? What causes the damage when an endothermic explosion happens? Air moving at a very high speed in a blast wave?

I could really do with understanding this. I am not a chemist or a physicist, and all the sites I'm looking at are very technical. Sorry if this questions sounds dim.

Thanks

no one answered here is the comprehensive answer.

exothermic explosion results in heat being produced

endothermic explosions result in no heat being produced