Question

What causes chemical reactions to "explode"?

Answer 1

In thermodynamics there's a quantity called Gibb's Free Energy that determines how spontaneous a reaction is, and is dependent upon several factors, one of which being temperature.

Generally as you increase temperature, the rate of a reaction increases, and if that reaction is exothermic, meaning it releases heat.

There is a threshold of the amount of energy needed before a reaction takes place, but once you heat something up to give it that energy and if it's exothermic, it will cause an almost instantaneous reaction by causing a chain reaction where the reaction takes place very quickly, releasing heat causing other particles to react, releasing even more heat, causing an explosion basically.

Answer 2

so it's all about heat and temperature?

Answer 3

Depends what you mean. If you mean, why would a chemical reaction release a lot of heat energy, it's because the bonds formed are stronger than the bonds broken. Most modern chemical explosives produce nitrogen (N2) as a product, because the N2 triple bond is the strongest known. Producing the strong O2 double bond, or CO triple bond, is also a favorite. But reduction of nitrogen is a favorite, which is why when the TSA snifs your luggage with a mass spec it's nitrogen for which they're looking.

If you mean why would a reaction be fast, the answer is complex, although a highly exothermic reaction tends to be fast, all other things being equal. A key is that any intermediates that must form should be relatively low energy, and not require a lot of unlikely collisions. For this reason some of the best explosives are decompositions, in which a molecule just falls apart, and there is no need for even the collision of oxygen molecules required by a combustion reaction.

If you mean why would a chemical reaction produce a large outward force, this is usually because the reaction produces a lot of gas, e.g the classic decomposition of TNT:\[2 {\rm C}_6{\rm H}_2{\rm CH}_3({\rm NO}_2)_3(s) \rightarrow 3 {\rm N}_2(g) + 5 {\rm H}_2{\rm O}(g) + 7 {\rm CO}(g) + 7 {\rm C}(s)\]This reaction produces 15 moles (say 1000 liters or so) of hot gas for every 450g of TNT that explodes.

If you mean how do you draw the line between "combustion" and "detonation," the general rule is that if the reaction front (the surface on which the reaction is occuring, between the unreacted and reacted mixture) propogates slower than the speed of sound, you have a conflagration, or rapid fire. If, on the other hand, the flame front propogates faster than the speed of sound, you have a detonation. The reason is that the pressure wave builds up ahead of the flame front, because it can't "escape" forward (the speed of pressure waves equals the speed of sound), so you get an unusually strong pressure wave right at the flame front, which of course when it hits something produces a great deal of damage.