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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The Chemistry of War: Chemical Explosives

In the modern world, chemistry is all around us. One of the most spectacular, and most terrifying, uses of chemistry is on the battlefield. Chemistry, in the form of artillery and explosions, can turn whole cities into fields of rubble in a matter of hours, or, in the case of a nuclear bomb, in an instant. The science of military explosives is a fascinating one, so lets begin with the basics, starting with conventional chemical explosives.

Chemical Explosions: The Basics

Chemical explosives are substances that, when exposed to heat or shock, rapidly decompose, releasing large amounts of gas and heat. The most important thing about an explosive reaction is that it is fast. Any combustion reaction, such as the combustion of gasoline, releases relatively large amounts of gas and heat. In fact, most fuels have a higher specific energy (energy stored per unit of volume) than explosives. Burning a kilogram of gasoline produces about 46 MJ of energy, compared to the mere 4.6 MJ produced by detonating a kilogram of TNT. Yet a small package of TNT scares us a lot more than a bottle of gasoline. The answer to this apparent paradox is basic kinetics. The rate of reaction of exploding TNT is several thousand times faster than that of burning gasoline, making it infinitely more dangerous.

Not all reactions that rapidly create large amounts of gas are termed explosions. In order to truly be considered an explosion, the reaction must also be exothermic. One classic example is the reaction of nitrogen and oxygen gas to form NO. This reaction is spontaneous at temperatures greater than 2000 degrees Celsius, and produces gaseous products, but it is endothermic, absorbing 180 kJ per mole from its surroundings. Thus, it is not classified as an explosion.

A diagram of a molecule of TNT (trinitrobenzene). The white atoms represent hydrogen atoms, the black atoms represent carbons atoms, the red atoms represent oxygen atoms, and the blue atoms represent nitrogen atoms.

Detonation vs. Deflagration

Explosives can be divided into two categories: those that work by detonation and those that work by deflagration. Deflagration is basically just the fast, but subsonic, combustion of a material such as gunpowder. Explosives that work by deflagration are usually termed the “low explosives”. Detonation, on the other hand, occurs when an exothermic reaction accelerates through the material at supersonic speeds, creating a shock wave (an advancing area of extreme pressure) in front of it. These are the “high explosives”. Low explosives are easier to control, but are far less powerful.

The power of an explosion can be measured in its explosive velocity. This is the speed at which the reaction, and thus the shock wave, progresses through the material. Deflagration explosions also have an explosive velocity, though it is generally smaller than that of detonation ones. In fact, you can even measure the explosive velocity of a burning match! However, this is usually referred to as flame speed, since few people would refer to a lit match or a burning stove as an explosion. For example, the flame speed of methane in air is 1.3 ft/s, or around 0.4 m/s. Compare that to 6,900 m/s, the explosive velocity of TNT.

Types of explosives

A chemical explosive is usually either a pure, unstable compound, such as nitroglycerin or the RDX found in C-4, or a mix of oxidizer and fuel. The difference is primarily in the sort of reaction the explosive undergoes when it reacts. Most pure compound explosives work by dissociating when shocked, with each dissociating molecule shocking the next one into dissociating (this is what the above-mentioned shock wave is on a molecular level).

A mix of oxidizer and fuel is simply a highly explosive combustion reaction. One example of a fuel-and-oxidizer explosive is gunpowder. Typically gunpowder uses saltpeter, the oxidizer, charcoal, the fuel, and sulfur. The sulfur is optional since it only changes the reaction to reduce the amount of carbon monoxide produced. Using the fuel and oxidizer together provides a stable reaction that can be controlled. In fact, one does not even necessarily need the oxidizer. A combustion-based explosive that does not have any oxidizer is called a thermobaric explosive.

This is “The Father of all Bombs”, a nickname given to the most powerful non-nuclear bomb ever detonated. It was created by Russia and was tested in 2007. It is a thermobaric weapon with destructive power equivalent to 44 tons of TNT.

Thermobaric explosives

Thermobaric explosives are, barring nuclear bombs, the most destructive explosives ever created. Not only do thermobaric explosives create a vicious fireball that incinerates everything nearby, but the blast wave they produce is both stronger and lasts longer than that of a conventional explosive. Since they are completely reliant on oxygen from the air,  they can not function underwater, at high altitudes, or in adverse weather conditions. However, they are particularly effective at destroying tunnels and caves. This is because thermobaric weapons have one terrifying property. Upon detonation, they create large quantities of extremely hot gas. This gas then begins to cool with extreme rapidity. As we all know from the ideal gas law, this leads to an immediate drop in pressure, effectively creating a vacuum. Everything surrounding the explosion is sucked towards it. The effect in a tunnel is particularly devastating: even if those inside survive the initial fireball and shockwave, the resulting vacuum will suffocate them or simply rupture their lungs. This property of thermobaric weapons has given them the nickname “vacuum bombs”.