Gunpowder - the Chemistry Behind the Bang

Two of the best-known explosives must be dynamite and TNT; however, it is gunpowder that has a much older and more interesting history. In addition, the chemistry of gunpowder is very simple and it is this simplicity that led to its being the first and only chemical explosive until the 19th century.

History of Gunpowder at a Glance

The known history of gunpowder is generally attributed to China, known to some as Cathay. It is associated with the Mongol invasion of China, during the 13^th century. It appears to have been first discovered at least 300 years before that. China fell to the Mongols in 1276 and was completely subjugated by the year 1279. This placed gunpowder in the hands of the Mongols, who later used it in invading Japan. It is easy to see how gunpowder became known to the world.

Gunpowder - Its Composition

Gunpowder ordinarily consists of three components-potassium nitrate, also called saltpeter (KNO₃), charcoal (C), and sulfur (S). Proportions of these components may vary, and typically are 65 to 75 percent potassium nitrate, 15 to 20 percent charcoal, and 10 to 15 percent sulfur.

Interestingly, stoichiometrically,¹ the reaction proportions are 84 to 8 to 8. The reaction is considered to generally be,

10 KNO₃ + 8 C + 3 S → 2 K₂CO₃ + 3 K₂SO₄ + 6 CO₂↑ + 5 N₂↑

Gunpowder - What Produces the Bang?

Most chemical reactions do not include a bang. Why does the ignition of gunpowder-after all, simply a chemical reaction-do so? It is because of the sudden generation of pressure within an enclosed space. Gunpowder in a closed tube creates enough pressure it violently shatters the container (perhaps made of steel) and generates a "shock wave."² How?

The chemical reactants-all solid-do not take up much volume. On the other hand, the products generated include 11 parts of carbon dioxide and nitrogen-gases. Now the solid products are slightly less in volume than the solid reactants, so it might seem the pressure should be decreasing, not increasing. However, the volume of the gas products immensely greater, since the atoms in a gas are very far apart. It is the sudden conversion of a small amount of solid material into gas that produces tremendous pressure--suddenly bursting the container-that produces the characteristic explosion with its associated Bang! Anyone who has observed a fireworks display knows that at the end of a beautiful aerial flower his ears may be assaulted by an occasional series of sharp Bangs. Bang-bang-bang-bang-bang-BANG!

¹stoichiometry: "the relation between the quantities of substances that take part in a reaction or form a compound..."

² shock wave: "a region across which there is a rapid pressure, temperature, and density rise, usually caused by a body moving supersonically in a gas or by a detonation."

References and Resources:

Drexel University - "Flash! Bang! Whiz!" by Dr. James B. Calvert

Royal New Zealand Artillery Association - "The Gun - Gunpowder"

Silkroad Foundation - "Gun and Gunpowder"