Nitrogen vs Oxygen

These two gases make up 99% of the air and behave as if they came from different planets. One is so unreactive that it is used to stop things reacting; the other is the reason iron rusts, food spoils and anything burns at all. They sit next to each other on the table and their difference comes down to a single structural fact.

If you need something to happen, oxygen. If you need something to stop happening, nitrogen. That is close to the whole of it, and the reason is worth understanding because it explains an enormous amount besides.

The bond that makes the sky boring

Nitrogen travels as N₂, its two atoms joined three times over. Prising that apart costs roughly 945 kilojoules for every mole — very little in chemistry is harder to pull apart. The double bond in O₂ gives way at about 498, barely more than half.

Nitrogen is not inert in the thermodynamic sense. Ammonia, nitric acid and every amino acid in your body are perfectly stable nitrogen compounds. The inertness is kinetic: to make any of them you first have to take that triple bond apart, and almost nothing at ordinary temperature can pay the entry fee. An atmosphere four-fifths made of a molecule with a locked door is why the sky does not react with the ground.

The same fact read backwards explains a whole class of energetic materials. Nitroglycerin, TNT and the azides all contain nitrogen in strained, higher-energy arrangements, and what they release when they decompose is largely the energy of forming N₂. Nitrogen's stability is not a small effect; it is the payoff.

Two brakes, and they work nothing alike

Both gases sit in the air doing very little, and it is tempting to file that under one heading. They are not the same thing twice. Nitrogen is slow because starting costs too much. Oxygen is slow because the start is disallowed.

Oxygen travels with two of its electrons unpaired — which is why liquid oxygen clings between the poles of a magnet, and why the structure a first course draws for it gets an observable property flatly wrong. Where that diagram fails, and what replaces it, is set out on oxygen's own page. What this comparison needs is the consequence: ordinary combustible matter has every one of its electrons paired up, and putting an unpaired molecule against a paired one asks for a transition the rules do not allow directly. So oxygen meets wood, sugar or living tissue with an obstacle in the way that has nothing to do with the energy on offer.

The difference in kind shows in what it takes to defeat each barrier. Nitrogen's is a toll: pay the 945 kilojoules a mole, by heat or catalyst or enzyme, and it lets you through — and you pay again for the next mole. Oxygen's is a locked side door that one radical opens, and a fire then manufactures its own radicals and keeps it open. That is the whole practical asymmetry. A spark is enough to start a fire in air and nothing remotely similar makes a nitrogen atmosphere self-sustaining. Nitrogen is safe because it is expensive to begin; oxygen is dangerous because it is nearly free to continue.

Breaking the triple bond is the most expensive thing agriculture does

Plants cannot use atmospheric nitrogen. Getting it into a form they can use — fixing it — means breaking that 945 kilojoule bond, and doing it at industrial scale is what the Haber-Bosch process is for: nitrogen and hydrogen over a promoted iron catalyst at roughly 400 to 500 degrees and a couple of hundred atmospheres.

The energy bill for that is on the order of one to two per cent of all the energy humanity uses, and a substantial slice of world natural gas consumption goes into it, mostly to make the hydrogen. In exchange, a large fraction of the nitrogen atoms in the protein of everyone alive has passed through a Haber-Bosch reactor. Few industrial processes have a claim on the human population that direct.

Biology does the same job at ambient temperature and pressure, which remains slightly embarrassing for chemistry. Nitrogenase, the enzyme in root-nodule bacteria, splits N₂ using an iron-molybdenum cofactor and a great deal of ATP. It is slow, but it does not need a pressure vessel.

Oxygen needs no equivalent effort. Photosynthesis produces it as a waste product, and it has been accumulating since the Great Oxidation Event around 2.4 billion years ago, when cyanobacteria first outpaced the planet's capacity to absorb it.

They come out of the same column

Both gases are produced by the same operation: air is compressed, cooled until it liquefies, and distilled. Nitrogen boils at 77 K and oxygen at 90 K, so nitrogen leaves the top of the column and oxygen collects at the bottom. Argon comes off a side draw between them.

That shared origin has a commercial consequence. A plant built to make oxygen makes nitrogen whether it wants to or not, and since the feed is 78% nitrogen there is far more of it. Bulk nitrogen is correspondingly cheap, and where absolute purity is not required it can be made without cryogenics at all, by pressure-swing adsorption or by membranes that let oxygen through faster.

Where each one actually goes

Industrial oxygen is dominated by one customer: steel. The basic oxygen furnace blows high-purity oxygen through molten pig iron to burn the carbon out, and steelmaking accounts for around half of all industrial oxygen consumption. Chemical synthesis, medical supply, cutting and welding, and wastewater aeration take most of the rest.

Nitrogen's uses are almost all about exclusion. Blanketing fuel tanks and reactors so no flammable mixture can form; packaging food so fats do not go rancid; purging semiconductor and fibre-optic processes; freezing food fast in liquid form. The point is never what nitrogen does — it is what it prevents.

Put them together hot and you get the pollutant

The two gases coexist peacefully in air because neither can be bothered with the other. Raise the temperature past roughly 1,300 degrees, as happens inside an engine cylinder or a gas turbine, and atmospheric nitrogen and oxygen begin to combine directly into nitric oxide, which oxidises further to nitrogen dioxide.

This is thermal NOₓ, and it is a peculiar kind of pollution: it does not come from the fuel. Burn perfectly pure hydrogen in air and you still make it, purely because the air was hot. Every catalytic converter, selective reduction system and low-NOₓ burner ever built exists to undo a reaction between the two most common gases in the atmosphere.

Start a reaction, or stop one

• Anything that must burn hotter, faster or cleaner — oxygen, and steel is the biggest buyer.
• Anything that must not oxidise, ignite or spoil — nitrogen.
• Cheap bulk cryogenic cooling — liquid nitrogen, at 77 K and a fraction of the cost.
• Cooling something that must not touch a fuel or a lubricant — nitrogen, for the same reason.
• Growing a crop — nitrogen, but only after somebody has spent the energy to fix it.
• Filling a tyre — air is already mostly nitrogen; the upgrade buys very little.