Tuesday, 4 March 2014


Ammonia (Manumia you can never have enough Wings) is, by some accounts, the foundation of every human thing on the planet.  At least as important as concrete, steel or styrofoam hamburger boxes.  80% of the atmosphere is Nitrogen N2, which is remarkably unreactive, because the triple N≡N bond between the two N atoms is very strong.  But nitrogen is required in large quantities to make proteins and the living world needs proteins (as enzymes, receptors) to carry out all their metabolic activities, including making more proteins. I've mentioned this before in the context of where Ammonia got its name.  There are two ways we can convert N2 into a bio-useful form.  Using a convenient enzyme in some living organism OR a brute force chemical approach.

Just a few days ago, I was talking about how the elegant biochemical option was used to generate large quantities of acetone during WWI.  Chaim Weizmann mobilised an enzyme from Clostridium acetobutylicum to convert abundant starch into scarce acetone, and was rewarded with his own country. Acetone is a key precursor for cordite which is used to propel bullets and a lot of those were used up in Flanders between 1914 and 1918. The British government, for whom Weizmann worked, also required a lot of nitrate to make TNT (three molecules of nitrate for every molecule of Tri-Nitro-Toluene) and other more powerful explosives.  But they didn't need to make any nitrate (elegantly or expensively) because they could go and dig as much as they needed out of the ground in places such as Chile where generations of birds shitting on the rocks of the dry coast extending south from the Atacama Desert had created layers of 'guano' many meters thick and easily shovelable. Nitrates are highly soluble and elsewhere birdshit gets dissolved and washed away by the least shower of rain.

The Germans, boxed into continental Europe and with the Royal Navy interdicting any external trade, needed explosives as much as the Brits but had to make their own nitrate.  Fritz Haber, their wunderkind chemist, developed a brute force (no surprise here?), high energy solution to this problem of reactive nitrogen insufficiency.  The formula is represented thus.
N2 + 3 H2 → 2 NH3   (ΔH = −92.4 kJ·mol−1)
 No I don't really know what it means either.  But two things are important: 1) on the left side of the equation there are 4 molecules and only 2 on the right; 2) the negative number indicates that the reaction is exothermic - it generates heat.  But more important, and what doesn't appear in the equation, is that there is an enormous 'energy of activation' for this reaction - basically to break open the strong triple bond.  This activation energy needs to be brought down as far as possible to minimise the amount of heat (expensive) required.  You can do this by using an enzyme like nitrogenase which is found in N-fixing bacteria in the roots of leguminous (peas, beans, clover, vetch)  plants.  Or you can trick about with various chemical catalysts that metaphorically hold the N≡N bond steady while you beat it with a big stick.  Developing economic catalysts was one of Haber's contributory break-throughs.  Another brilliant idea was to apply pressure to the system to encourage the four initial molecules to rank-up and convert themselves into two and drive the reaction right.  A gram-molecular weight (mole) of any gas fills the same volume: 22 litres, regardless of the complexity of the molecule, so the right side naturally takes up half as much space.

It was for developing this process that Haber won his Nobel Prize for chemistry in 1918.  The Nobel committee weren't particularly interested in the war-time applications but it was obvious to everyone that nitrogen 'fixed' by the Haber-Bosch process could be used to supplement and replace the bio-generated fixed nitrogen that was at the heart of rotating crops: Wheat - barley - potato - fallow or Maize - soya - maize - soya.  Fallow means leaving the field to grow grass and vitally clover (Trifolium spp.) and grazing animals on the pasture; the bacteria (nitrogenase) in the clover and the animal manure both contributing to the fertility of the field in the following year.  Soya or other beans also fix nitrogen. There are other reasons for rotating crops and not having a mono-culture which hinge on keeping pests and parasites at bay, but that's another day's story.

We demonize Haber nowadays because of his enthusiasm for chlorine and other chemical warfare agents and for being a man-of-his-generation (i.e. bit of a shit) to his long-suffering and brilliant wife, but I put it to you that we have far more reason to curse him for developing a cheap way of making ammonia.  My argument is much the same as that for which we have cause to curse Norman Borlaug lynch-pin of the Green Revolution.  Borlaug is frequently hailed as "The Man Who Saved A Billion Lives", but that's nonsense.  He changed the economic, ecological and agricultural conditions in a way that encouraged a billion subsistence farmers in the third world to try for another baby so he is more realistically named "The Man Who Created A Billion Lives", like some super-Shockley sperm-donor. The half-a-billion tons of nitrogen fixed by the Haber process every year has been another essential leg to the rocketing agricultural productivity we call the Green Revolution. The number of humans of the planet has increased from a, perhaps sustainable, 1.5 bn in 1900 to more than 7 bn now.  Borlaug and Haber have fed them (they say that 80% of the nitrogen in our own proteins has been fixed by the Haber-Bosch process) but their needs for mobile phones, dining tables, plastic washing-up bowls, designer footware and electric light have denuded the planet of trees, depleted mineral resources that have taken a billion years to accumulate and sent it all up in a plume of CO2 which will melt the ice-caps so that the sun can sauna us all to death. 

V e r y    s l o w    h a n d    c l a p.

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