The fifth of November,
The Gunpowder treason and plot;
I know of no reason
Why the Gunpowder treason
Should ever be forgot!
-English folk rhyme ca. 1870
The story of Guy Fawkes and the gunpowder plot is pretty well known around the internet. On November 5th 1605, Guy Fawkes was caught with 36 barrels of gunpowder, in an attempt to blow up the British parliament. The story of the Gunpowder Treason isn’t particularly scientific, but it makes a great tie in to a truly explosive topic (forgive the pun). In honor of Guy Fawkes Day, I bring you the chemistry of explosives.
What is an explosive?
Simply put, an explosive is a substance that can undergo a very rapid, spontaneous chemical reaction that, once initiated, is driven by both a large exothermic change and a large positive entropy change. Okay, that wasn’t simple at all. Basically, it’s a really fast chemical reaction that can keep itself going and produces a lot of heat and gas. The hot gas expands and creates a pressure wave which is responsible for the actual explosion.
I should mention here that I am referring to chemical explosives. There are also physical and exotic (nuclear) explosives, but those are topics for another time.
Gunpowder seems like the best place to start. It is the oldest known chemical explosive after all. Gunpowder is a mixture of charcoal, potassium nitrate, and sulfur. When a source of heat is applied to it, you get potassium sulfate, potassium carbonate, carbon dioxide, and nitrogen. So, what’s actually going on there? Let’s take a look at the chemical equation:
This is a decomposition reaction, and a really fast one, but that’s not what causes the boom. The substances on the left (the reactants) are all solids, but some of the ones on the right (the products) are gasses. That positive entropy change I mentioned earlier refers to the fact that you have more gas molecules after the reaction than you did before. Immediately after the reaction all of that gas is compressed right around where the reaction took place, but it doesn’t stay that way for long. As the gas expands, you get a compression wave that pushes away everything in its path. The more explosive you have, the more gas is produced, and the more powerful the resulting explosion. That’s also responsible for the sound that you hear, since sound waves are caused by compressed air.
Gunpowder is a pretty inefficient explosive since it leaves behind lots of solid material, soot, (which contains unreacted carbon), and a lot of intermediate products. Over the years and centuries chemists have developed more efficient compounds that react more completely and leave behind less solid material. They also use additives that contribute oxygen to get a more complete reaction
Take a look at TNT (trinitrotoluene). The reaction looks like this:
Compare the reactants and products from the two reactions. With gunpowder 21 reactant molecules produced 16 reactant molecules (and only 11 gas molecules). With TNT 2 reactant molecules produce 22 product molecules (and 15 of those are gas). TNT gives you a lot more bang for your buck (sorry).
Gunpowder is considered a low explosive. That means that the reaction front proceeds slower than the speed of sound (what a chemist would call a deflagration reaction). Think of the classic cartoon image of the trail of gunpowder burning towards the powder keg (and an unsuspecting coyote). The portion of powder that is actually burning is the reaction front. The reaction progresses because the heat from burning powder is enough to ignite the powder next to it. Such a small amount of powder burning in an open area is not enough to explode, since the gas produced will immediately dissipate. Once it gets to the keg; KABOOM. The deflagration spreads quickly in every direction producing lot of gas in a contained area. At that point it’s something like a balloon that gets overinflated and pops… only bigger and much louder. In order to get an explosion you need to have a lot of a low explosive in one place (like in our powder keg), otherwise it will simply burn.
If a low explosive has a reaction front that is subsonic (slower than the speed of sound), then it follows that a high explosive (like TNT) has a supersonic reaction front. Not only does that sound cool, but it makes for a much bigger boom (which also sounds cool). In the gunpowder example the reaction moves along by simple diffusion of heat (the burning powder heats up the powder next to it). With a high explosive, the reaction progresses by the wave of superheated and compressed gas which is moving faster than the speed of sound (what a chemist would call a detonation). The entire mass decomposes at nearly the exact same moment, so the gas is at much higher pressure.
In addition to classifying explosives by the speed of the reaction, you can also look at the stability. Unstable (or primary) explosives will explode with very little input. Nitroglycerine, for example, can explode if its container gets shaken up. Nitrogen triiodide can be set off after exposure to alpha radiation (making it impossible to store, transport, or use for any sort of practical purpose). More stable substances are referred to as secondary explosives because they need a primary explosive to set them off. Even more stable compounds are tertiary explosives which need a secondary explosive to get going. The more stable the explosive, the easier and safer to transport.
Explosives are good for more than you might think. There are the obvious military applications and industrial uses (mining, demolition, etc.). Fireworks (like those used to celebrate Guy Fawkes Day) combine different compositions of gunpowder and other additives to fly into the air, explode, and produce all of the pretty colors we like to see. Speaking of things that fly into the air, solid rocket boosters are essentially explosives mixed with binders that slow down the reaction rate so that it can be directed through a rocket nozzle. Some rockets also use explosive bolts to separate the booster stages after launch because they are more reliable than mechanical releases. Explosives can also save lives. Automobile airbags have to inflate in a fraction of a second in order to provide the most protection. Compressed air isn’t quite fast enough, so airbag makers use a small explosive charge as an inflator.
Next time you're watching a fireworks show, a rocket launch, or building demolition you can tell your friends what is really going on in there. Hopefully they won't be too blown away (last one I promise).