Sciencemadness Discussion Board

What makes a substance capable of detonation?

Belowzero - 6-12-2020 at 08:35

I was pondering over this issue and I hope you people could help me understand this better;

What makes a substance capable of detonating and where lies the boundry if any such thing exists?
Wouldn't all substances be capable of detonation when subjected to a high enough pressure wave and heat?


As far as I understand it; a detonation is basically the supersonic replacement of a substance by a substance that takes in more space, most often a gas.
Following that path I would have to conclude that if the circumstances permit it any substance is capable of undergoing this change.
There are probably exceptions but I can't come up with reasonable examples.

I might be entirely on the wrong track and if so please point out where I go astray

If this is indeed correct than this would imply that any substances can be an explosive and what we consider an explosive is merely a substance that behave a certain way within given parameters.

Thoughts?

clearly_not_atara - 6-12-2020 at 11:20

A pressure wave is never detonation. Think about the words: detonation is faster than sound, a pressure wave is sound, hence a pressure wave is not detonation.

Detonation happens when a chemical reaction moves faster than a pressure wave. In effect, the chemical reaction is inertially confined, leading to a rapid increase in mechanical energy density. This energy is transferred to a shock wave.

In order to do this you need a certain amount of stored chemical energy in a substance and in particular the stored chemical energy must be large compared to the activation energy of decomposition at the temperature of initiation. I don't know the specifics but it isn't "just anything"; usually you get deflagration. You might be able to create similar shock waves by transferring electrical energy, but it won't follow the same infinitesimal dynamics because it is not self-reinforcing.

greenlight - 6-12-2020 at 11:38

Just a couple of things to ponder cause I am quite tired.....

From the second result on Google:

“An explosive is any chemical compound or mechanical mixture that, when subjected to heat, impact, friction, detonation, or other suitable initiation, undergoes rapid chemical change, evolving large volumes of highly heated gases—typically nitrogen or CO2—that exert pressure on the surrounding medium."

But what does it really mean, that still hasn't explained much has it?

So, first thing...

It must contain an oxidizer and fuel to be an explosive:(

If pyrotechnic, the oxidizer and fuel are in powder forms and intimately mixed together which results in fast burning subsonic compositions. Example would be black powder or flash powder.
If high explosive, the oxidizer and fuel are within the same molecule which leads to very fast supersonic burning/decomposition. Example would be TNT where the oxygen on the NO2 groups are the oxidizer and the carbon and hydrogen benzene ring are the main fuel.
So basically, in just about all cases (not going to go into azides, fulminates, etc) you need some strategically placed oxygen in the same molecule usually in form of the NO2 group which can break down from shock.

This brings us to number two......

The compounds structure must prefer to be in a more stable state than it already is;).

Pretty much, it's a solid or some cases liquid, but it would much prefer to live life as a cloud of stable gases.
In other words it's in a higher energy state and would like to move to a lower one where it can exist as nitrogen, water vapour and CO2.
Like a boulder balanced on a cliff, all it needs is a push to get to the lower resting state. The detonator provides this push by supplying heat and shock to begin rupturing the bonds which causes the chain reaction known as detonation. The size of the push required depends on the explosive properties.

Those are the main ones to outline at this hour I think. I hope I've described it somewhat coherently.

So unfortunately in two paragraphs, nature has taken all the fun out of having the majority of objects and substances around us just detonating left right and centre.




Microtek - 6-12-2020 at 23:47

As you point out yourself with azides, a redox reaction is not mandatory. Your second point is a much better criterion, however it is a required but not sufficient one: The reaction must be energetically favoured in order to be possible, but things like N2O can have a positive heat of formation, but be non-explosive.

symboom - 7-12-2020 at 06:35

Energetic materials remind me of this video
https://youtu.be/pAD0cJb6Ouo
Popsicle stick explosion

Stickbomb.gif - 233kB
https://en.m.wikipedia.org/wiki/Stick_bomb

But in this case this would more represent strained bonds
Such as nitrogen sulfide and nitrogen selenide
And of course Benzvalene






[Edited on 7-12-2020 by symboom]

greenlight - 7-12-2020 at 10:13

Quote: Originally posted by Microtek  
As you point out yourself with azides, a redox reaction is not mandatory. Your second point is a much better criterion, however it is a required but not sufficient one: The reaction must be energetically favoured in order to be possible, but things like N2O can have a positive heat of formation, but be non-explosive.


Ahh yes, one of the common trends in science and chemistry... You can group the majority of things into a class of behavior but there's always those few exceptions.

A main part of the second point I forgot to add was indeed it being energetically favored with energy released when the bonds are broken.

unionised - 7-12-2020 at 10:56

Quote: Originally posted by symboom  
Energetic materials remind me of this video
https://youtu.be/pAD0cJb6Ouo
Popsicle stick explosion

...

[Edited on 7-12-2020 by symboom]

How did I not find out about that before?

Laboratory of Liptakov - 8-12-2020 at 14:31

Belowzero........What makes a substance capable of detonation?.....Anvil and big hammer....:cool:

Pyro_cat - 8-12-2020 at 19:48

When glass breaks, the cracks move at detonation speeds. https://www.youtube.com/watch?v=Mv48tq3j4Bc


Prince Rupert's drops kind of detonate too https://www.youtube.com/watch?v=xe-f4gokRBs

foreign maple - 9-12-2020 at 07:09

There are many different reasons something can detonate. High oxygen%, very stable byproducts and a less stable compound, awkward bond stain and positive heat of formation. There are likley other reasons for a compound to detonate but these are the main ones.


[Edited on 9-12-2020 by foreign maple]