The Haber Process
Fixing Nitrogen from Air to Make Ammonia for Fertilizers
The history of the Haber Process which saved the world from starving and may have prolonged World War I
At the beginning of the twentieth century, concern was being expressed that food production was not keeping up with world population growth. A German scientist named Fritz Haber put his mind to the problem and came up with a process by which nitrogen from the air could be turned into ammonia, which could then be used to make fertilizers and, less productively, explosives.
Elements for Plant Growth
Plants need three inorganic elements to grow: potassium (K), phosphorous (P), and nitrogen (N). The first two can be produced without much difficulty from abundant rock deposits throughout the world. Nitrogen, on the other hand, was only found in large quantities in Chile in the form of saltpetre or sodium nitrate. These deposits were not going to last forever and population growth demanded a better source of nitrogen.
Nitrogen from Air
The other place that nitrogen was found in large quantities was the air which is 80% nitrogen gas. This gas is very unreactive, however, and no way was known to fix this nitrogen so that it could be added to soil to fertilize plants.
Thermodynamic Compromise
Haber knew that it was possible to make ammonia gas by reacting nitrogen from the air with hydrogen gas. The problem was that it required a lot of energy to make very little product. He used a thermodynamic compromise to find conditions that produced a good yield of ammonia in a short time.
Nitrogen and Hydrogen
The reaction between nitrogen and hydrogen to produce ammonia is actually a reversible reaction, it will go both ways. This means that while nitrogen and hydrogen are joining together to form ammonia, the ammonia is decomposing to form nitrogen and hydrogen. Haber's success was finding conditions in which more ammonia was formed.
Equilibrium
The reaction equation demonstrates that one molecule of nitrogen reacts with three molecules of hydrogen to give two molecules of ammonia. The reaction gives out heat, it is exothermic. Thermodynamics tells us that more ammonia will be formed at high pressure and low temperature. The problem is that low temperatures cause the reaction to go very slowly and high pressures cost a lot to maintain. Haber found a compromise in which the gases were reacted at 450 degrees Celsius and 250 atmospheres of pressure. He also used a catalyst: iron, osmium and uranium all produced good yields of ammonia.
First World War
This process was perfected just before the First World War. In the long term it revolutionised the agricultural production of the world. In the short term, however, it gave the Germans a source of nitrate to use in explosives which may have prolonged the war by up to two years.
