The explosive chemistry of nitrogen, phosphorus and urine.

Nitrogen is an essential component of living matter. It is present in protein molecules that form all organisms. Unlike oxygen, nitrogen can not be used by animals and plants directly from the air. Even though 78% of the atmosphere is due to nitrogen gas its availability is extremely limited. Plants must absorb nitrogen in the form of ammonium ions (NH4+) and nitrate ions (NO3-) in order to build the complex molecules that we know as amino acids, the building blocks for proteins. Conversion of nitrogen gas into these ions is difficult and can occur only at high temperatures or by bacteria in the soil. This process is called nitrogen fixing.

 
The difficulty with nitrogen fixing lies in the fact that the nitrogen molecule has a strong triple bond holding the two atoms together. This bond is very hard to break which makes nitrogen an inert gas. It takes very powerful collisions to break the triple bond and this can only be achieved with high temperatures or with a catalyst.

 

During lightning strikes enough energy is supplied to break the bonds of the nitrogen molecule and form nitrous oxide according to the equation below.

N2 + O2 => 2NO
The nitrous oxide formed combines with oxygen to form nitrogen dioxide according to the equation below.

2NO + O2 => 2NO2
Nitrogen dioxide readily dissolves in water to produce nitric and nitrous acids which provides a source of nitrates available to plants.

2 NO2 + H2O => HNO3 + HNO2

Nitrogen fixation is also carried out by bacteria in the soil according to the reaction below.

N2 + 8H+ => 2 NH4+ (ammonium ion)

 
Animals can only obtain nitrogen by eating plant material or other animals. They must dispose of the nitrogen from their body when they break down proteins. Nitrogen is excreted in the urine as urea (CO(NH2)2). Bacteria convert, the nitrogen present in urea, into new products such as ammonia (NH3), ammonium ions (NH4+) and nitrate ions (NO3-). Not only are these nitrogen compounds very important as fertilisers but they are also very powerful oxidants.
It is hard to believe that many of the ingredients for explosive mixtures, such as potassium nitrate, ammonium nitrate and phosphorus, were originally manufactured from putrefied urine. Phosphorus was discovered in 1669 by an alchemist, Hennig Brand while searching for a way to convert silver into gold. For his efforts, Brand was able to isolate a white, waxy solid that glowed in the dark and burst spontaneously into flame when exposed to air. The purification of phosphorus involved the evaporation of water from urine and allowing the resultant liquid to be putrefied for several months by bacteria. Brand later evaporated the water from urine and allowed the black residue to putrefy for several months. He then mixed this residue with sand, heated the mixture in the absence of air and collected, under water, the volatile phosphorus that precipitated out of the liquid. Phosphorus was later used commercially in the manufacture of matches.

 

Nitrates, such as potassium nitrate (saltpeter) and sodium nitrate are mixed with fuels to form very explosive mixtures. Evidence of the immense explosive power of mixtures of nitrates and fuel are the ruins of the Alfred P. Murrah Federal Building in Oklahoma City. As seen on the right, the building was destroyed by a truck loaded with fertiliser (ammonium nitrate) and fuel (petrol).

 
Footage of the Beirut explosion, also caused by a huge storage of ammonium nitrate, is shown in the video on the right. Notice the incredible, explosive power of ammonium nitrate. Ammonium nitrate serves as an oxidant and causes a rapid acceleration of a combustion (burning) reaction.

 

 

The use of saltpeter was first documented in ancient Greece where Greek-fire was reported to have originated. Greek-fire was a mixture of saltpeter, sulfur, charcoal and other flammable substances. Once set on fire the mixture burnt fiercely, even under water.

 

In 1245 Friar Roger Bacon mixed saltpeter, carbon and sulfur and made gunpowder. Gunpowder involves a very powerful exothermic reaction as outlined below.
16 KNO3(s) + S8(s) + 24 C(s) => 8 K2S(s) + 24 CO2(g) + 8 N2(g)
The supply of saltpeter was limited and the munitions industry was unable to get enough to meet demand.

Saltpeter is now mined in huge quantities. Before WW1, Europe depended on imports of nitrates (saltpeter) for weapons production. Without its own source of nitrates, Germany found itself in a very weak position. British domination of the seas prohibited Germany to receive imports of nitrates in quantities needed for munitions manufacture. A German chemist, Haber, changed the situation and made Germany self-sufficient in nitrogen compounds through a process of making ammonia from hydrogen and nitrogen gases. This is called the Haber process and the critical reaction is given by the equation below.

N2(g) + 3H2(g) <=> 2NH3(g)

From ammonia, nitric acid is formed according to the equation below in a process called the Ostwald process.

NH3(g) + 2O2(g) => HNO3(aq) + H2O(l)

After the synthesis of nitric acid formation of sodium nitrate, ammonium nitrate and saltpeter was easily accomplished according to the equations below. The Haber process and the Ostwald process revolutionised the explosives industry.

HNO3 + NH3 => NH4NO3

HNO3 + NaOH =>NaNO3 + H2O

HNO3 + KOH => KNO3 + H2O

 

78% of the atmosphere is due to nitrogen gas. Considering this fact, explain why the soil is relatively poor in nitrogen compounds?

Explain how Haber and Ostwald may have contributed to Germany's rise as a military power.

Describe the composition of the mixture known as Greek fire. Why was it difficult to extinguish the burning mixture?

Senior chemistry question
Continue with nitrogen as an ingredient of explosive compounds
Keep your powder dry