Fire(thermite) and Ice explosion. What causes it?

[EvanF]

So when you combine thermite and ice there is a reaction that results in a powerful explosion...Here is a Mythbusters episode where they experiment with it...

They say that it is a 'mystery' why this explosion happens...Does anyone here have an explanation?

Edited November 14, 2016 by EvanF

[Fuzzwood]

Very, VERY, rapid expansion of gaseous water due to the heat. That pressure has to go somewhere, hence the explosion.

[EvanF]

Very, VERY, rapid expansion of gaseous water due to the heat. That pressure has to go somewhere, hence the explosion.

So why doesn't Thermite cause an explosion when dropped in water?

[Sensei]

They say that it is a 'mystery' why this explosion happens...Does anyone here have an explanation?

 

Molecule of water could be separated to gaseous Hydrogen, and gaseous Oxygen, and then react together. It's just a matter of temperature.

To split single water molecule there is needed ~1.23 eV,

Which according to Wien's displacement law

http://hyperphysics.phy-astr.gsu.edu/hbase/wien.html

(you can use calculator below) is reached at 2600 C of black body radiation (peak at ~1.23 eV = ~1008 nm wavelength).

According to wikipedia Iron oxide Thermite can have up to 2500 C, very close to what is needed.

https://en.wikipedia.org/wiki/Thermite

Edited November 14, 2016 by Sensei

[Strange]

I assume the water would just boil away: as steam was created it would push the water out of the way and escape. Therefore no explosion.

 

In the case of ice, presumably the pressure of the steam builds up until it shatters the ice: an explosion.

[sethoflagos]

There's a lot going on there, isn't there?

 

My guess (some of this I'm pretty confident of) is that there are at least three phases to this blast.

 

1) Standard steam explosion. The liquid thermite mixture falling through the ice block tower produces steam faster than it can escape subsonically. A type of BLEVE.

 

2) As the presenter suggests, the steam explosion could (would?) turn the thermite mixture into an aerosol, so the steam + aerosol aluminium reaction is definitely in play, though the kinetics ask a couple of questions.

 

3) Choice of a galvanised bucket is interesting. The outside of the bucket is clearly getting very reactive even before the blast. Since zinc boils at 907 deg C, a steam + zinc vapour reaction has less reaction kinetics issues.

[John Cuthber]

 

Molecule of water could be separated to gaseous Hydrogen, and gaseous Oxygen, and then react together. It's just a matter of temperature.

To split single water molecule there is needed ~1.23 eV,

Which according to Wien's displacement law

http://hyperphysics.phy-astr.gsu.edu/hbase/wien.html

(you can use calculator below) is reached at 2600 C of black body radiation (peak at ~1.23 eV = ~1008 nm wavelength).

According to wikipedia Iron oxide Thermite can have up to 2500 C, very close to what is needed.

https://en.wikipedia.org/wiki/Thermite

The heat of combustion of hydrogen s about 286 KJ/mol corresponding to 2.96 eV which means thermite isn't anywhere near hot enough to seriously reverse the reaction.

 

The standard electrode potentials for H2 and O2 are not relevant here, and so the "standard" cell potential for electrolysis is not the place to start.

 

There may be some hydrogen created from the reaction of Al with water.

The big reason for the explosion is that the vapour is created faster than it can escape- because the ice is in the way and the heat isn't dissipated by convection.

[Sensei]

The heat of combustion of hydrogen s about 286 KJ/mol corresponding to 2.96 eV which means thermite isn't anywhere near hot enough to seriously reverse the reaction.

..... gosh....

 

237.5 kJ/mol (2.46 eV) is per H₂ not per H,

or in some other sources:

286 kJ/mol (2.96 eV) is per H₂not per H.

(because it's combustion of gaseous Hydrogen)

 

Per two particles H₂ means you don't need to have SINGLE particle (blackbody radiation peak) with 2.96 eV in the first place..

You can have two particles with 1.48 eV (or two with 1.23 eV).

 

The standard electrode potentials for H2 and O2 are not relevant here, and so the "standard" cell potential for electrolysis is not the place to start.

Why are you writing something like this?

Do you know how to calculate it?

It's just unit conversion from quantum physics electron Volts, to chemistry kJ/mol...

1.48 eV * 2 * 1.602176565*10^-19 J/eV * 6.022141*10^23 mol^-1 = 286 kJ/mol

1.23 eV * 2 * 1.602176565*10^-19 J/eV * 6.022141*10^23 mol^-1 = 237.5 kJ/mol

 

H₂O + 1.23 eV -> H+ + OH-

or in some other sources:

H₂O + 1.48 eV -> H+ + OH-

 

To create free protons that could react with Oxygen (from air for instance) you need just 1.23 eV/1.48 eV per single water molecule..

 

According to your "explanation" what happened in this video would require temperature ~6600 C..

https://youtu.be/D1hhgTbtsCs?t=30s

Edited November 14, 2016 by Sensei

[John Cuthber]

Well, first off, lets just remind you that you need to get to grips with your spurious accuracy problem

http://www.scienceforums.net/topic/93083-making-nitrocellulose/?p=904853

http://www.scienceforums.net/topic/95788-how-humans-discovered-the-use-of-metals/?p=926511

http://www.scienceforums.net/topic/87510-sulfuric-acid-in-ppm/?p=850221

 

And now, back to the topic in hand.

Have you not noticed that electrolysis is different from fire?

The fact that different sources give different answers should tell you a lot.

What you are actually talking about is a high temperature reaction so the combustion energy is the right energy change to use.

 

So you are right to say

"286 kJ/mol (2.96 eV) is per H₂not per H."

and since we are talking bout the formation of a molecule of water which takes two hydrogens, two hydrogens is the right number of hydrogens to take.

 

The reaction is well documented. The heat of combustion of hydrogen- as with many fuels- is well known.

https://en.wikipedia.org/wiki/Heat_of_combustion

 

So, why use the cell voltage from a completely unrealistic experiment when you can use the right, measured, value?

 

 

And it doesn't help when you tell lies like this

"According to your "explanation" what happened in this video would require temperature ~6600 C.."

 

What I actually said was "The big reason for the explosion is that the vapour is created faster than it can escape- "

that will happen a little bit above the boiling point of water- perhaps as high as 150C or something.

 

Why are you pretending I said it would take 6600C?

For what it's worth, that is roughly the temperature of a reversible hydrogen flame, which is what this daft idea of "the water gets cracked to hydrogen" is sort of related to.

If you want to dissociate water by heating you need to get to a temperature in that ballpark.

So what?

That's not what happens (not least because it simply isn't hot enough).

 

There's a more fundamental problem with the "it's so hot the water gets split into hydrogen and oxygen". That splitting reaction is enormously endothermic- it would cool the system down.

And when they recombined, the gases would get back to exactly the same temperature they started from- what else could they do? It's the conservation of energy.

 

BTW, this "To create free protons that could react with Oxygen you need just 1.23 eV/1.48 eV per single water molecule.." is also massively wrong unless you do the experiment under water.

 

For example, dissociation of HO–H bond of a water molecule (H₂O) requires 493.4 kJ/mol. The dissociation of the remaining hydroxyl radical requires 424.4 kJ/mol. The bond energy of the covalent O–H bonds in water is said to be 458.9 kJ/mol, the average of these values"

 

from

https://en.wikipedia.org/wiki/Bond-dissociation_energy

Once you have split off teh hydrogen atom, you need to take the electron off it to get a proton.

That takes about 13.6 electron volts of energy.

 

Free protons don't get out much in chemistry.

[sethoflagos]

Perhaps the idea to focus on is why they could find no trace of the bucket.

 

Seems to have the signs of 50 years of rusting condensed into a few milliseconds. The overall reaction is elemental metals become metal oxides. As John Cuthber effectively says: ice in; steam out. It actually subtracts a little from the overall energy release, Maybe steam is simply a catalyst that generates a fast kinetic pathway for the various key reactions.

[Butch]

This is not a true explosion, just a reaction that is accelerated.

An explosion occurs when the speed of the propogating reaction exceeds the speed of the propogating shock wave, the entire reaction occurs almost instantaneously.

[EvanF]

This is not a true explosion, just a reaction that is accelerated.

An explosion occurs when the speed of the propogating reaction exceeds the speed of the propogating shock wave, the entire reaction occurs almost instantaneously.

Sounds like semantic gibberish.

I don't know where you are getting these specific things that qualify as an 'explosion'...

"Explosion" just means a violent reaction that results in a rapid release of energy.

 

Watch the video again, it certainly qualifies as an explosion.

[John Cuthber]

This is not a true explosion, just a reaction that is accelerated.

An explosion occurs when the speed of the propogating reaction exceeds the speed of the propogating shock wave, the entire reaction occurs almost instantaneously.

You are muddling up the technical definition of a detonation (which this isn't) and an explosion (which this is).