KNO3 the Chemistry Behind the Boom

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It comes as no big surprise that action films and explosions go hand-in-hand. As most of us have likely experienced, utter obliteration on the silver screen conveys a sense of immediate gratification that other movie-made special effects have trouble measuring up to. In this post, we embrace our inner ten- year old selves and delve into the hugely esoteric field of things going boom. It’s always nice to see the crux of a fast- paced film relate to something other than gunshots and arbitrary detonations, and 21 Jump Street puts forth a noteworthy effort to align a little sense in their big finish. We’d tell you ourselves, but Channing Tatum really said it best in this excerpt from the movie:

“Potassium Nitrate-

Don’t hate.

It’s great.

It can act as an oxidizer.

I didn’t know that,

but now I’m wiser.

It has a crystalline structure.

If you can’t respect that,

you’re a butt-muncher.

It’s a key ingredient in gunpowder.

K-No-Three!”

Tatum plays Jenko, a jock- turned- police officer who goes undercover when assigned to infiltrate a high school in his special police unit. Jenko’s course is switched with that of Schmidt, his brainy partner in crime (or justice), and subsequently is forced to suffer through an AP Chemistry course. His progressive appreciation for the subject pays off, however, when Jenko is able to utilize the knowledge he gained to blow up the limo of a drug dealer in the final chase scene with the creation of an impromptu battery bomb composed of a tequila, potassium nitrate (from shotgun shells) and lithium (from lithium batteries of a camera). Would this concoction have produced the earth-shaking explosion it did?

Lithium batteries and tequila would have produced the exothermic single displacement reaction  Li (s) + H2O (l) → LiOH (aq) +H2 (g). When lithium comes into contact with water a violent reaction occurs, resulting in the release of hydrogen gas inside the tequila bottle, since lithium is higher up in the reactivity series than hydrogen gas due to lithium’s single valence electron in its 2s1 orbital. Furthermore, alcohol is highly flammable and will combust with the hydrogen gas if there is sufficient heat. When Jenko agitated the solution by shaking the bottle, the reaction released the necessary activation energy to combust hydrogen gas and alcohol. Said combustion reaction, 2 H2(g) + O2 (g) → 2 H2O (l), has its oxygen gas provided by the oxidizing agent KNO3.

While the theoretical portion of this analysis has been relatively accurate, the tendency of hollywood to exaggerate now comes into play. Once Jenko placed the lithium inside the bottle, the reaction should have occurred almost instantly with the near immediate release of hydrogen gas, exploding before the bottle left his hands.  Instead, there is a considerable amount of lapse before Jenko throws the battery bomb. As with any reaction, the rate at which the reaction occurs depends on the required amount of activation energy. In this case, the activation energy required to produce the combustion reaction should have been generated from the exothermic reaction between lithium and water. This combined with the shaking of the bottle ultimately would result in the explosion occurring right away. Thus, although the explosion was reasonably designed, its timing was not necessarily as realistic. Moreover, the size of the explosion in the scene is inordinately exaggerated; the use of lithium would not have produced destruction anywhere near that degree. Rather, it would have been more suitable to have used an alkali metal with a more reactive potential in place of lithium to produce an explosion closer to the magnitude of the one shown on the movie screen.

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All in all, screenwriter Michael Bacall’s incorporation of chemistry into an action flick wins high points for creativity, but falls under the standard on the scale of realism. Even Bacall himself acknowledges that in hindsight, he should have “talked to an actual chemist” instead of relying on his fading knowledge back from his own AP chemistry class back in high school. Still, his enthusiasm to portray chemistry as the climatic solution to the problem in the movie is applaudable and exciting.

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