How is a high explosive even possible?

Take something like http://en.wikipedia.org/wiki/Nitroglycerin. Simplifying, you pour glycerine in Nitric acid and Nitroglycerin is formed. My question is: explosives are chemicals which are in an energetically unfavorable state - with the correct activation (shock or heat), they decompose into simpler molecules and release a great deal of stored energy. But, how are they formed in the first place? It seems peculiar that mixing two chemicals would create a third that was so unstable - it’s like having two boxes full of mousetrap parts and pouring them together, and getting cocked mousetraps as the end result. It just seems strange to me.

Nitroglycerin is formed by the combination of highly concentrated sulphuric acid and extremely pure nitric acid (typically a grade called white fuming nitric acid). The sulphuric acid strips electrons from the nitric acid, making it preferable to the nucleophilic oxygen atoms of glycerol by displacing the terminal hydrogen atoms. This creates a metastable state in which the oxygen atoms are sufficiently bound not to be immediately reactive, but can be debonded and available for reaction by a relatively small impulse. This is very temperature sensitive: below a certain temperature (less than 50 deg F) pure nitroglycerin is almost completely insensitive, and almost no shock will detonate it, despite having a very negative oxygen balance, and above about 120 deg F it will spontaneously decompose (though not explosively unless impurities are present).

The energy from detonation comes essentially from the combination of oxygen species which are normally kept separated by the nitrogen atoms to which they are bonded; it is similar to taking an tank of metastable monoatomic oxygen and letting it spontaneously recombine. This is also partially why nitroglycerin is so useful at putting out oil well fires; even after solid state detonation you end up with oxygen poor species (mostly N and NO) that will eagerly consume free oxygen needed to support combustion. The primary effect, though, is the displacement of atmospheric oxygen with relatively inert gases produced by the detonation.

In simple terms, nitroglycerin is like hauling a large boulder up a hill, and then bracing it in place with a small but secure and sturdy post with a tripwire. It won’t move until you pull the post, but just a small pull on the tripwire will jerk the post out of the way and allow the boulder to fall with all of its weight.

Stranger

The details are slightly off here. The sulphuric acid protonates nitric acid. The nitric acid loses water to make the NO[sub]2[/sub]+ ion. This then gets attacked by the glycerin oxygen.

Most of the “hill” has already been climbed before the synthesis of nitroglycerin. Nitric acid is itself pretty high energy. This is the fate of molecules with lots of nitrogen and oxygen. Also, glycerin itself has plenty of energy to give up. What you are really doing in the case of high explosives is making compounds with lots of excess oxygen and nitrogen. The oxygen reacts with carbon to make really low energy carbon dioxide and the Nitrogen reacts to make really really low energy Nitrogen gas. The fact that these products are both gasses helps create a shockwave that detonates the rest of the material.

The reason these compounds don’t spontaneously explode on their own is because it requires activation energy. Its like building a house of cards. You can build it, but a slight breeze will make the whole thing catastrophically fail.

I can assure though, that people researching these types of explosives in the lab have to change their shorts occasionally. Even with small quantities behind a blast shield, they are sweating bullets working with that stuff. From what I understand, they work in special glove boxes with very keen eye on static. Anyone will tell you that static is real issue in a glove box.

My high school teacher described an explosive as anything that generates a large amount of gas, and or a large amount of heat, from a small sample. That’s a kinda crappy definition, it doesn’t really tell what will be an explosive, just why something is an explosive, when you think about it. What’s explosive is a little surprising sometimes, for example, a small amount of dry ice, in a sealed container, will soon explode. CO2 isn’t a high energy molecule like nitro, but frozen, you can pack a lot of gas, into a small space. Also, people are often suprised by various explosive powders – some powdered metals, some powderd plastics (but not all) or even finely dispersed wheat flour.

That’s not an explosive, that’s a bomb. A bomb explodes, but it is not necessarily made from explosives. If you attempt to contain a system where the pressure increases rapidly, you will get what I call a bomb. Bombs made from high explosives don’t need to be contained obviously.

There’s a difference between a generic explosive device (what you’re describing), and a high explosive chemical (what the OP is describing). Whereas an explosive device merely relies on rapid release of gases, a high explosive produces its gases by a chemical reaction that progresses through the explosive material at supersonic speeds. No confinement is required: the gases are produced so rapidly that they can’t get out of their own way fast enough, and a shock wave is produced.

Note also that high explosives don’t contain a particularly large amount of energy. For example, nitroglycerin contains 6.38 megajoules per kg; gasoline, for comparison, contains around 47. The behavior of high explosives is of interest because the energy they do contain is released with extreme rapidity.

The analogy I’ve always liked for how explosives can be (relatively) stable is that of a swimming pool atop a mesa. The mesa is 500 feet above the desert floor, and any water that goes over the edge will fall – hard. But the water in the pool will remain there (ignoring evaporation) because to fall to the desert floor it must first be elevated to the level of the mesa, instead of being down in the pool. Water splashed out of the pool will (again ignoring evaporation) end up falling to the desert floor – but it takes energy to move the water up to where it will flow over the edge of the mesa. (The technical term is, I think, a “metastable” state.)

I get activation energy, but I still don’t grok the answer to the OP’s question.

Are glycerin and/or nitric acid at a higher energy level than nitroglycerin?

If so, do they not have explosive reactions themselves?
If not, how does that reaction occur?

For some fascinating reading about the early days of nitro, I suggest Tales of Destruction.

In terms of heat of formation, they are probably pretty similar mass wise. remember that one of the products of synthesis is very low energy water, so nitroglycerin could be higher in energy. That’s not really what makes the difference though. Nitric acid isn’t really explosive because it doesn’t have anything to produce on it’s own. Sure you can get gaseous nitrogen, but then your left with oxygen, and oxygen itself is still pretty high energy. Glycerin has lots of energy, but it needs oxygen to burn. Combine the two and you have a perfect storm.

I just wanted to point out that there are several different “energies” that a chemist may talk about. When I’m talking about heat of formation, that is enthalpy. In terms of synthesis and explosions, a more important term is Gibbs free energy. It is possible to push the energy of the products higher than the energy of the reactants, because you can control Gibbs free energy somewhat. It’s a little bit difficult to explain, but my point is that products don’t always have to be lower energy than the reactants.

Can it be made at home as a byproduct of soap manufacture, as we saw in Fight Club?

Off-topic, but how are the nitroglycerin pills that one takes for heart problems different from the explosive form of nitroglycerin?

Glycerin is a byproduct of soap manufacture. If you have nitric acid and sulfuric acid as well as the right equipment, then it is a fairly elementary reaction for a trained chemist carry out. The biggest trick is in purifying and concentrating it.

Chemically ,there is no difference. The highest dose I could find though was 0.3 mg. Even if you could detonate it, it would be barely audible. Since the pills are made with so much filler, the filler likely helps stabilize it. You probably couldn’t detonate it if you tried.