### 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

• 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.

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