# Are non-exothermic explosions possible?

Explosives like picric acid and trinitrotoluene are unstable due to the close packing of so many $\ce{-NO2}$ groups onto a benzene ring. These highly strained bonds require only a relatively small amount of energy to cause the molecule to rip itself apart, forming a lot of more stable gas molecules, and releasing a lot of heat in the process.

My question is: is it thermodynamically feasible to have a reaction that rapidly evolves a lot of gas without being extremely exothermic? Would this still be considered an explosion?

It doesn't seem to me that this could be possible, but I never cease to be amazed at how little I know about chemistry. If it is feasible, and the gas molecules are more stable, then where does the energy go? If the gas molecules aren't more stable, then what's the driving force?

• There is a precise term for this: entropic explosions. I don't know to what extent they are "low temperature", though. – Nicolau Saker Neto Dec 10 '15 at 9:42
• Sodium azide evolving nitrogen "explosively" (not a detonation) in an automotive air bag might be an example of a not highly exothermic but highly gas-producing reaction. – iad22agp Dec 10 '15 at 14:59
• @NicolauSakerNeto - I think I'd post your comment as an answer. – Geoff Hutchison Dec 10 '15 at 19:23
• If you pour liquid nitrogen into a "weak" container and seal it, or pack dry ice into a "weak" container and seal it, then you're going to get an "explosion." In such a case the physical change is purely endothermic. – MaxW Dec 10 '15 at 22:05

As a sidenote, explosions actually don't necessarily produce gas. There are some rare examples, such as cuprous acetylide ($\ce{Cu2C2}$) and silver acetylide ($\ce{Ag2C2}$) where all the products are solids. Also, there are examples of non-chemical explosions which are only slightly exothermic or even endothermic. As MaxW mentions, a pressure explosion from a liquefied gas will be endothermic, as energy is consumed to evaporate the liquid. Another interesting example of an explosion which is probably only slightly exothermic is a Prince Rupert's drop being set off.