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March 6, 2012

Nitrogen as an element is stubborn and uncommunicative. It’s rudeness stems from the triple bond between the identical twin atoms. The bond is so strong it can’t be broken easily which excludes reactions with all other elements. For all intents and purposes nitrogen behaves like a noble gas. It makes up around 78% of the Earth’s atmosphere and every breath we take and our lungs don’t even notice. A room filled with nitrogen and no oxygen can kill anyone who enters it in a few breaths with no apparent distress to the victim because our body, unable to react with nitrogen, can’t detect that anything is wrong.

The official discovery of nitrogen is credited to Daniel Rutherford (apparently no relation of Ernest Rutherford of atomic structure fame) in 1772. Discovery is perhaps a bit strong to describe what Rutherford achieved. Rutherford found that there was a portion of air that did not allow a flame to burn and killed any mice placed in a jar containing this strange new gas (most of this portion of air was later proved to be nitrogen). Many other eminent scientists were carrying out similar experiments at the time but no one could figure out what was going on. Theories such as ‘de-phlogisticated air’ and ‘mephitic air’ or azote (meaning ‘lifeless’) were bandied about but a name ‘nitrogen’ was finally given to this gas in 1794 by Jean-Antoine Chaptal after nitre as nitric acid could be shown to contain the mysterious gas.

An unreactive nature and relative abundance means nitrogen is often used to protect substances from reactive oxygen. The gas inside crisp packets is nitrogen to stop the crisps going soft and soggy and petrol is stored under a blanket of nitrogen to stop unwanted reactions with oxygen. When cooled down to -196 degree Celsius nitrogen becomes a liquid similar in appearance to water. Using liquid nitrogen to freeze food preserves it better than putting it in a standard freezer. The extreme cold freezes the water molecules in food quickly and goes some way to prevent the ice expanding and breaking apart the cell structures within the food which would normally turn it mushy when warmed up again. The nitrogen won’t react with the food to spoil it and simply evaporates leaving no residue when warmed up.

The character of nitrogen changes completely when combined with other elements to form compounds but to achieve this is no mean feet. To split up nitrogen twins there are two approaches. Plants use method one; quiet, calm negotiation through nitrogen fixing enzymes. These enzymes are actually found in a nitrogen fixing bacteria that lives in a symbiotic relationship in the root nodules of certain plants. Their subtle persuasion gently prizes apart the three strong bonds between the two nitrogen atoms.

Method two is rather more dramatic and pretty much comes down to brute force. By piling in a huge amount of energy the bonds can be ripped apart and forced in to forming new bonds with other elements. Nature does this through lightning. Thousands of volts of power ripping through the air pull apart the nitrogen twins and the free nitrogen atoms join up with oxygen and hydrogen to form nitrates and ammonia that gets washed down in the rain.

Original Harber Reactor at BASF

Humans like a challenge and faced with an abundant but unreactive element tried their damnedest to get the twins to play with the other kids. The problem was solved by Fritz Haber in typical human style – punish, beat and torment nitrogen until it has no other choice (similar sentiments are often expressed by students learning the Haber Process for chemistry exams). Nitrogen and hydrogen molecules are ‘kettled’ in to a tiny space (high pressure) and bated (heated) into a seething mass of anger. The molecules are pinned to an iron catalyst (adsorption) where the bonds between the atoms start to break and reform. Some nitrogen atoms will react with the hydrogen atoms and, one hydrogen atom at a time, nitrogen becomes ammonia (NH3). Despite all of this only around 15% of the nitrogen can be persuaded to react. Any unreacted nitrogen simply gets cycled round again and again until it relents.

The Haber Process is now used to produce 500 million tonnes of ammonia and ammonium nitrate worldwide every year and is almost identical to the 1909 process developed by Haber himself. This has allowed the manufacture of fertilizers on such a scale and has lead to the huge global population growth of the 20th Century as we are now able to feed far more people than ever before. Haber’s great innovation created headlines like ‘Bread from air’. All of this should be fairly familiar to GCSE students. What isn’t discussed as much is the use of ammonia in the production of explosives but I’ll talk about that more in another post on Fritz Haber.

Next week we get radical, it’s oxygen!


Image from Dr Ian Hamerton via @SciCommStudios

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