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Mahesh

15/10/24 09:55 AM IST

Haber-Bosch process

In News
  • A hundred million tonnes of nitrogen are now removed from the atmosphere and converted into fertilizer via the Haber-Bosch process.
Nitrogen molecule
  • Nitrates are molecules of oxygen and nitrogen, abundant in the earth’s atmosphere.
  • Nearly eight metric tonnes of nitrogen lie on every square metre of the earth’s surface, yet it can’t feed a single blade of grass.
  • Nitrogen in the air is mostly in the form of N2.
  • When two nitrogen atoms join together, they share three pairs of electrons to form a triple bond, rendering the molecule nearly unbreakable.
  • The energy required to break the nitrogen triple bond is so high (946 kJ/mol) that molecular nitrogen is nearly inert.
  • But if the bond is broken, atomic nitrogen can form ionic nitrides such as ammonia (NH3), ammonium (NH4+), or nitrates (NO3–).
  • Plants need these types of nitrogen, called reactive nitrogen, to synthesise enzymes, proteins, and amino acids.
  • Healthy plants often contain 3-4% nitrogen in their above-ground tissues, significantly more than other nutrients.
Nitrogen in nature
  • In a lightning bolt, nitrogen in the air combines with oxygen to generate nitrogen oxides such as NO and NO2.
  • They can then combine with water vapour to create nitric and nitrous acids (HNO3 and HNO2, respectively).
  • Reactive nitrogen-rich droplets fertilize farmlands, woods, and grasslands when it rains.
  • This pathway is estimated to replenish soil by around 10 kg of nitrogen per acre per year.
  • Apart from lightning, a gentle metabolic process carried out by Azotobacter bacteria can also create reactive nitrogen.
  • Some microorganisms such as Rhizobia have developed symbiotic relationships with legume plants (clover, peas, beans, alfalfa, and acacia) to provide reactive nitrogen in exchange for nutrition.
  • Azolla, a species of aquatic fern with a symbiotic association with the cyanobacterium Anabaena azollae, can absorb and convert nitrogen from the air to reactive nitrogen, so dried and decaying Azolla is an effective fertilizer for farmland.
Nitrogen cycle
  • Plants usually get their reactive nitrogen from the soil, where they absorb minerals dissolved in water such as ammonium (NH4+) and nitrate (NO3-).
  • Humans and animals need nine pre-made nitrogen-rich amino acids from plants. Nitrogen makes up approximately 2.6% of the human body.
  • The nitrogen ingested by plants and animals returns to the soil through excreta and the decomposition of dead bodies.
  • But the cycle is incomplete: some nitrogen is released back into the environment in molecular form. Nitrogen from human waste is also rarely returned to the fields.
  • Although legumes can produce nitrogen independently, important food crops such as rice, wheat, corn, and potatoes and less well-known crops like cassava, bananas, and common fruits and vegetables draw nitrogen from the soil.
  • As the human population multiplies, nitrogen in agricultural soil depletes faster, needing fertilizers to compensate.
  • Farmers understood this early. They cultivated legumes or fertilized their crops with ammonia to increase output where possible.
  • They also used ammonium-bearing minerals from volcanic eruptions and naturally occurring nitrates found in caves, walls, and rocks as fertilizer.
Origin of Ammonia
  • Ammonia (NH4) is made of nitrogen and hydrogen, both of which exist naturally as two-atom molecules.
  • Under extreme heat, the molecules separate and form a compound, but it is short-lived because of the heat.
  • The reversible reaction N2 + 3H2 = 2NH3 (the ‘=’ sign has been used here as a stand-in for bidirectional arrows) must be maintained in specific conditions to harvest considerable amounts of ammonia.
  • The German chemist Fritz Haber heated the N2-H2 combination to various temperatures in a platinum cylinder and calculated the amount of ammonia created.
  • He also used hot ammonia to decompose into nitrogen and hydrogen, attempting to approach the equilibrium point from the opposite direction.
  • At 1,000 degrees Celsius, Haber found that harvestable ammonia made up just one-hundredth of 1% of the mixture — too little for commercial production.
Haber bosch process
  • The heated hydrogen and nitrogen combination would circulate in a steel chamber at a pressure of 200 atm.
  • The chamber had a valve that could withstand the high pressure while allowing the N2-H2 mixture to pass through.
  • Haber also built a contraption to ensure the hot gases departing from the reaction chamber passed their heat to the cooler incoming gases.
  • Thus the departing combination would rapidly cool while the ingested gas would be heated, achieving two objectives at once. 
  • When Haber inserted an osmium sheet into the pressure chamber, filled it with a N2-H2 mixture, and heated them, the nitrogen triple bond cracked away, leaving reactive nitrogen to fuse with hydrogen and produce a very large amount of ammonia.
  • German propagandists soon hailed the feat as “brot aus luft!” — producing bread out of air, as Jesus is fabled to have done.
  • The Haber-Bosch method allowed industries to develop cheap synthetic fertilizers, which was a critical component in the sevenfold rise in the world’s food supply during the 20th century.
  • According to one estimate, one-third of the world’s population —around two billion people — would be without food if the Haber process for nitrogen fixation did not exist.
  • An average adult carries 1-2 kg of nitrogen in human tissues, but in many countries, the yearly fertilizer application currently exceeds 50 kg of nitrogen per capita. The global average is around 13 kg.
  • The ‘extra’ nitrogen is drawn up by plants along with a greater quantity of other minerals. It also over-nourishes bacteria and accelerates biochemical processes that release reactive nitrogen into the atmosphere, where it acidifies rain, corroding and destroying land.
  • Nitrogen fertilizers are also part of the surface runoff into fresh and coastal water, accidentally fertilizing and deoxygenating it. Even a small amount of moisture here causes weeds to flourish.
  • In sum, Haber’s work was essential but insufficient to feed humanity. Starvation, hunger, and malnutrition coexist with godowns bursting with grain surpluses, sometimes even within the same country.
  • The saga of nitrogen fixation teaches us a more significant lesson: technological fixes alone can’t solve people’s problems. We also need political action and social mobilisation. 
Source- The Hindu

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