Gunpowder - Combustion Characteristics

Combustion Characteristics

A simple, commonly cited, chemical equation for the combustion of black powder is

2 KNO₃ + S + 3 C → K₂S + N₂ + 3 CO₂.

A balanced, but still simplified, equation is

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

Although charcoal's chemical formula varies, it can be best summed up by its empirical formula: C₇H₄O

An even more accurate equation of the decomposition of regular black powder with the use of sulfur can be described as:

4 KNO₃ + C₇H₄O + 2 S —> 2 K₂S + 4 CO₂ + 3 CO + 2 H₂O + 2 N₂

Black powder without the use of sulfur:

6 KNO₃ + C₇H₄O —> 3 K₂CO₃ + CO₂ + 6 CO + 2 H2O + 2 N₂

The burning of gunpowder does not take place as a single reaction, however, and the byproducts are not easily predicted. One study's results showed that it produced (in order of descending quantities) 55.91% solid products: potassium carbonate, potassium sulfate, potassium sulfide, sulfur, potassium nitrate, potassium thiocyanate, carbon, ammonium carbonate and 42.98% gaseous products: carbon dioxide, nitrogen, carbon monoxide, hydrogen sulfide, hydrogen, methane, 1.11% water.

Black powder made with less-expensive and more plentiful sodium nitrate (in appropriate proportions) works just as well but is more hygroscopic than powders made from Potassium nitrate—popularly known as saltpeter. Because corned black powder grains made with saltpeter are less affected by moisture in the air, they can be stored unsealed without degradation by humidity. Muzzleloaders have been known to fire after hanging on a wall for decades in a loaded state, provided they remained dry. By contrast, black powder made with sodium nitrate must be kept sealed to remain stable.

Gunpowder contains 3 megajoules per kilogram, and contains its own oxidant. For comparison, the energy density of TNT is 4.7 megajoules per kilogram, and the energy density of gasoline is 47.2 megajoules per kilogram. Gunpowder is a low explosive and as such it does not detonate; rather it deflagrates.