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It's well known that undiluted nitroglycerin will explode when subjected to a physical shock. What is happening as a result of the physical shock that sets off the explosive reaction? In particular, why doesn't it explode spontaneously at room temperature without a physical shock?

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    $\begingroup$ Is it the kinetic energy imparted by the shock being converted to a small amount of heat and pushing the reaction just over a very small energy barrier? $\endgroup$ – user137 Jul 7 '15 at 17:55
  • $\begingroup$ @user137, I thought it might be something like this, but then it seems like random (thermal) motion of molecules at room temperature could also set off the reaction. Or is there just not enough energy in room temperature collisions to trigger the reaction? $\endgroup$ – Dan Bryant Jul 7 '15 at 17:58
  • $\begingroup$ I don't know, wikipedia says nitroglycerin boils at 50°C ( which also makes it explode ) so it seems like simply heating the compound isn't enough to make it explode so easily. $\endgroup$ – user137 Jul 7 '15 at 18:08
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My guess would be cavitation, presumably 'non-inertial cavitation':

Non-inertial cavitation is the process in which a bubble in a fluid is forced to oscillate in size or shape due to some form of energy input, such as an acoustic field. Such cavitation is often employed in ultrasonic cleaning baths and can also be observed in pumps, propellers, etc.

The energetics of cavitation are astonishingly out-sized with respect to the tiny size of bubbles, probably leading to more than enough energy to cross the initial activation barrier of the combustion reaction. As one example, the abstract here summarizes other reports' results indicating that the temperature inside collapsing cavitation bubbles may reach between 10,000 K and 100,000,000 K.

I don't have access to the full text to determine whether or not they met with success in attempting to ascertain cavitation's role in nitroglycerin initiation, but these authors apparently investigated the question almost fifty years ago. The abstract (references excised):

It is known that in some circumstances liquid explosives can be initiated with unexpected ease, and in other circumstances only with great diffculty.

Thus, Winning has shown that nitroglycerine (NG) free of gaseous inclusions and poured into a vessel so as to leave no wall surfaces free of liquid is not exploded even by the action of a fairly strong shock wave from a detonator immersed in the NG.

On the other hand, in handling liquid explosives there have been quite a few cases in which relatively weak vibrations or impacts have led to unexpected explosions, which have sometimes had serious consequences. For example, a British report describes an unfortunate accident that resulted from dropping a polyethylene bottle containing NG. Upon hitting the ground the NG exploded.

The initiation of explosion by “hot spots” resulting from the adiabatic compression of gaseous inclusions even before the arrival of the shock wave has been reliably demonstrated in numerous experiments. However, some cases of initiation of liquid explosives simply cannot be attributed to the heating of such gaseous inclusions, since in these cases the adiabatic compression temperatures of the gas are so small that it is not possible to talk of a “hot spot. “Such puzzling cases include, for example, the above-mentioned explosion of NG in a polyethylene bottle. In other experiments the role of gaseous inclusions has been completely eliminated by first subjecting the liquid explosive to a constant high pressure. This so reduced the degree of compression of the gaseous inclusions by a weak shock that strong heating of the gas in the bubbles, if any were present in the liquid explosives, was completely excluded. Nonetheless, there was no reduction in the sensitivity of the explosive to weak shocks.

In attempting to explain such puzzling cases it is usually pointed out that explosion can be initiated by cavitation, which may develop in a liquid even as a result of a weak impact or vibration. So far, however, no one has offered any direct experimental evidence of the possibility of cavitational initiation of explosion in liquid explosives. The object of our research was to fill that gap.

Clearly they published their results, so I figure the odds are good that they reached some sort of affirmative conclusion.

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  • $\begingroup$ I found a reference to the paper in a Cavitation and Bubble Dynamics textbook. Their mention of the paper suggests that cavitation luminescence may be a trigger. So I think your guess is correct. $\endgroup$ – Dan Bryant Jul 7 '15 at 20:05
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In particular, why doesn't it explode spontaneously at room temperature without a physical shock?

According to this review article, the low activation energy barrier for initiating the detonation of nitroglycerin is the culprit in re: hazards in handling the substance. It also provides its own oxygen as fuel (cf. nitrocellulose, also discussed in the article), which is a contributing factor to its instability.

As for random thermal motion of air molecules at room temperature, that's an average kinetic energy described by the Boltzmann distribution and is not the energetic equivalent of imparting even a small amount of energy by jarring/shaking the system for a short time (such as hitting a flask or dropping it).

What is happening as a result of the physical shock that sets off the explosive reaction?

A rapid and exothermic evolution of gaseous species is produced: That's the explosion. The mechanism of explosion is (see here): $$\ce{4C3H5(ONO2)3(l) -> 12CO2(g) + 10H2O(g) + 6N2(g) + O2(g)}$$

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