What makes substances explosive

Can someone explain clearly to me why some things are explosive?

• It might be better to explain in more detail the type of information you are looking for, and let those who want to answer choose how they would like to answer it. – bobthechemist Jan 1 '14 at 0:47
• Look for molecules with very exothermic decomposition. – f p Jan 2 '14 at 13:11
• – cngzz1 Jan 31 at 0:31

4 Answers

I could say explosives are things that blow up, but this would not a helpful answer.

It is also not that helpful to describe explosives as compounds with highly exothermic decomposition. This characteristic alone does not define an explosive.

What matters is rate of the reaction in the bulk compound. This rate defines two key types of explosive. If the reaction, once started, proceeds through the bulk compound faster than the speed of sound we call it detonation. If the reaction is slower than the speed of sound we describe it as deflagration.

This distinction matters because of the different uses explosives can be put to. Propellants, such as the explosives used to drive bullets, tend to deflagrate (as you want a relatively slow, controlled reaction to drive the shell out of the barrel without blowing up the barrel). But if you want something to really go bang and create a mess (as the military do) or if you want something to break up rock (as quarriers and miners do) then you want something that will detonate.

The structures of some well-known explosives are shown below:

Most well known real world explosive compounds are made from these compounds plus some added materials to make them easier to handle. C-4, a well known military explosive is mostly RDX with plasticisers to make it easy to mould like plasticine. The favourite terrorist plastic explosive is Semtex, similar to C-4 but made from a mixture of PETN and RDX. TNT is often used in high-explosive military shells (partially because it can be melted by steam and poured into shell casings). The first modern explosive, dynamite, is based on nitroglycerine mixed with diatomaceous earth (which makes it much easier to handle than the dangerously shock-sensitive pure compound).

The chemical reactions typically involve the production of CO2, H2O and, sometimes, N2.

Another characteristic that matters is how easy the reaction is to set off. Some compounds are very shock sensitive like mercury fulminate or nitroglycerin. This is only useful in detonators (in small quantities) or for chemists who have a death wish. Many bulk explosives require a detonator to trigger an explosion, making them much safer to handle and transport. C-4 in small quantities will actually burn slowly (and was sometimes used by soldiers in the Vietnam war to warm rations).

One thing that the answers so far have not mentioned is that an explosion is a chemical reaction. So the question is: why does the explosion reaction occur? And like so many chemical reactions, the answer is that the post-explosion state is lower energy than the pre-explosion state. That would seem to be obvious, given all the energy released in an explosion! But what's more interesting is that an explosive compound can remain for extended periods in a metastable condition (literally, in an energy state higher than its lowest possible state). This metastability is precisely why explosive compounds can be useful: they don't just spontaneously blow up.

But this begs the question: why are explosive compounds metastable? This gets to questions of activation energy or introduction of reactants. In the first case, it may be required that more energy is required to break the metastability. This is what is needed for explosive mixtures of hydrogen and oxygen to detonate, for example. In the second case, the introduction of a reactant may be just what is needed for explosive decomposition. For example, the explosive $\ce{XeO_3}$ is triggered by mere contact with organic compounds.

So in summary: explosions are particularly vigorous examples of chemical reactions moving to a lower energy state, but the "why" and "how" can be complex.

Chemical compounds possess bond energy. Except at very high temperatures, there are many chemicals which are not the most stable (for that given set of elements). One (simple, crude) way to look at this, these chemicals are "frozen" into shapes which aren't the most stable.

Cellulose is the major ingredient of wood. It is a polymer with the approximate chemical formula $\ce{C6H10O5}$. While wood can last for decades and centuries, wood dust (saw dust) is the same material and is quite explosive when in a dust cloud mixed with air. So, is wood "explosive" or not?

There are many aspects to this: An explosion (a chemical explosion) is a chemical reaction (often a set of related chemical reactions) which occur over a time frame of fractions of a second. This is called the "rate" of the reaction. If I were making an unstable chemical, and as it flowed out of the reaction vessel it reacted, we'd consider that decomposition, rather than explosion. We'd consider the exact same reaction an explosion if it broke some of the equipment. So, we have to be careful to understand what we mean by "explosive". Generally (to simplify), a chemical or chemical mixture is "explosive" if it is mostly stable (decomposing slowly, taking days, weeks, months, years) and if we can add a small amount of energy (a spark, a flame, hitting it with a hammer, etc.) to start a self-propagating and FAST chemical reaction or decomposition.

"Fast": I've already explained why fast is required, but I've not explained what causes "fast". A fast chemical reaction has to be an "easy" rearrangement of the chemical bonds. That means it has to happen between atoms that are close together. I won't go into any more detail here; it would require you to understand quite a bit about stereochemistry and chemical kinetics. Suffice it to say that wood dust allows oxygen in the air and cellulose in the wood to be close together and makes it easy to self-propagate the reaction. "Self-propagation" is a bit of a misnomer. What the term means is that the energy released in the reaction of one molecule (or set of molecules) is more than enough to "unfreeze" another molecule so that it can and will react. We call this the chemical "activation energy", you can learn more about it here.

I look at (chemical) explosives as either:

• Mixtures of chemicals (such as black powder) which will react fast enough and with enough energy to result in an incredibly accelerating reaction of more of the same. It feeds on itself.

• A single chemical in which its bonds are "strained" and "ready to burst". A very small amount of energy can cause these compounds to detonate, but again the same two requirements of speed and quantity of energy must be met. The same activation energy needs to be overcome and the same propagation needs to happen. But in the case of a single compound, the "ingredients" are naturally "close together" - they are in the same molecule. This is one reason why nitroglycerin is "more dangerous" than black powder, it takes much less to "start".

Note: chemical reactions which don't have "activation energies" happen spontaneously, so you won't be able to make (except fleetingly) a chemical which is both unstable and has no activation energy for reaction/decomposition.

Another way to say this is that all explosives have activation energies (since without activation energy, the reaction would already have happened).

A material that has low reaction rate in bulk are also possible to be explosive when increase its surface area which increase its reaction rate such as dust explosion.