ZIMSEC O Level Combined Science Notes: The Haber process and the Manufacture of Ammonia

  • Sable Chemicals produces ammonia at their Kwekwe plant
  • Ammonia is mainly used to make nitrogen fertilizers such as ammonium nitrate which are used in the agriculture industry
  • Ammonia is made using Hydrogen and Nitrogen
  • Hydrogen is obtained from electrolysis of water and
  • Nitrogen is obtained from the liquification of air
  • The Sable Chemicals plant is located close to these raw materials
  • It is close to the company’s own electrolysis and liquification plant
  • ZISCO which is close to the company also gets its oxygen from Sable Chemicals

The Haber Process

The haber process

The haber process

  • Ammonia is produced using the Haber Process
  • At the plant hydrogen and nitrogen are mixed in the ration 3:1 by volume
  • The gas pressure is raised to 200 atm (atmospheres)
  • The temperature is also raised to 500°C
  • The mixture is then passed over an iron catalyst
  • About 17% of it is then converted to ammonia
  • The ammonia produced here is then separated by condensation in a cooler
  • The unconverted gases are mixed with more nitrogen and oxygen and passed over the catalyst again
  • This process can be shown using the following word equation:
  • \text{nitrogen+hydrogen}\xrightleftharpoons[\text{iron catalyst}]{500 \degree C, 200atm}\text{ammonia+heat}
  • The reaction between nitrogen and hydrogen is reversible and exothermic
  • As much hydrogen and nitrogen must be converted to ammonia to make the process economic
  • This is done by increasing the pressure of the mixture and moderating the temperature
  • As ammonia is produced the pressure of the gases drops
  • If pressure is increased more ammonia is produced
  • A good yield is obtained at lower temperatures since this reaction is exothermic
  • The temperature of the reaction is kept as low as possible (around 450°C)
  • Reducing the temperature below this results in the reaction slowing down and becoming uneconomic
  • To speed the reaction up at these low temperatures an iron catalyst is used
  • The catalyst merely speeds up the rate of the reaction but not the amount of ammonia produced
  • The catalyst itself is not used up in the reaction and is thus used over and over again
  • The exothermic reaction produces heat in the converter which might reach temperatures of about 560°C
  • This heat is used to heat up steam which is used in other parts of the factory

To access more topics go to the Combined Science Notes page.

Quick NetOne, Econet, And Telecel Airtime Recharge