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Ammonia : Manufacture by Haber’s Process, Properties, Uses And Structure.

Chemist

Manufacture of ammonia by Haber’s process:

When a mixture of nitrogen and hydrogen gas in the ratio 1:3 by volume is heated at a temperature of 450-5000C and 200-900 atmospheric pressure in the presence of iron as catalyst and molybdenum as promoter, ammonia gas is produced. Conditions for optimum yield of ammonia : The reaction is exothermic, reversible and occurs with a decrease in volume. So, Le- Chatelier’s principle can be applied here to get higher yield.
  1. Low temperature : Since the reaction is exothermic, low temperature is favourable for the formation of ammonia. If the temperature is below 4500C, N2 and H2 react too slowly to attain equilibrium state. Thus an optimum temperature of about 450-5000C is maintained.
  2. High pressure : The total volume of product is lower than total volume of reactant so, equilibrium shifts in forward direction with the increase in pressure. In practice, 200 to 900 atmospheric pressure is applied.
  3. High concentration of reactants : Since reaction proceeds in forward direction with decrease in volume, so either one or both reactants( H2 and N2) should be used in excess amount for high yield of NH3.
  4. Catalyst : Presence of the catalyst speeds up the rate of reaction. Finely divided Iron catalyst and Molybdenum promoter is used.
  5. Purity of hydrogen and nitrogen : The nitrogen and hydrogen gas should be very pure, otherwise the catalyst is poisoned and decreases catalytic activity.
Procedure : Nitrogen and hydrogen gas in the ratio 1 : 3 by volume is mixed by passing through the compressor where 200 to 900 atm pressure is maintained Then this mixture is passed in a catalyst chamber containing finely divided iron with little amount of molybdenum. The chamber is heated electrically to about 450-5000C to initiate the reaction. Under these conditions of temperature and pressure, only about 15% reactant gases are converted into ammonia. The gas obtained from the catalyst chamber contains ammonia along with unreacted hydrogen and nitrogen. The gases mixture is then passed through a condenser where ammonia gets condensed and collected in a receiver while unreacted H2 and N2 gas is recirculated by recirculation pump and reprocessed to get more ammonia.

Physical Properties of Ammonia :

  1. It is a colourless and pungent smelling gas.
  2. It is lighter than air.
  3. It is easily liquefiable gas. It can also be solidified. Melting point of solid ammonia is -780C and boiling point of liquid ammonia is -33.40C.
  4. It is neither combustible nor a supporter of combustion.
  5. It is highly soluble in water as it forms intermolecular hydrogen bond with water.

Chemical Properties (reactions) of Ammonia :

  1. Basic nature:
     2. Reducing property : It acts as reducing agent. But when a mixture of NH3 and O2 is heated at about 8500C in presence of platinum then nitric oxide is formed.    3.   Action with metals: a. When ammonia reacts with molten sodium or potassium gives amide and hydrogen.         b) Lithium and magnesium give nitride instead of amide with ammonia.            4.   Dissociation (decomposition) of ammonia : When electric spark(current) is passed through ammonia or when ammonia is passed through a red hot tube at about 5000C then ammonia decomposes to nitrogen and hydrogen. 5. Action with halogens : Depending upon the amount of halogen and ammonia and reaction conditions, ammonia gives different products.       6. Action with carbon dioxide (CO2) : Ammonia reacts with carbon dioxide at 130-1500C under pressure 220 atm to give urea.                7. Precipitation reaction : Ammonia gas dissolves in water to form  ammonium hydroxide. When ammonium hydroxide solution is added into some salt solution  such as FeCl3, AlCl3, CrCl3, ZnCl2,etc. then ppt. of corresponding hydroxides are formed. When ammonia gas is passed into copper sulphate solution, at first a bluish white precipitate of Cu(OH)2 is formed which dissolves in exces of ammonia forming a deep blue coloured tetrammine copper(II) sulphate.     8. Action with mercurous nitrate paper : When mercurous nitrate  paper is placed over a jar containing ammonia gas then paper changes into black. This is due to the formation of mercuric amino nitrate.     9. Action with mercuric chloride : Mercuric chloride reacts with ammonia to give white ppt of mercuric amino chloride.     10. Reaction with Nessler’s reagent : Alkaline solution of potassium mercuric iodide (K2HgI4) is called Nessler’s reagent. When ammonia gas is passed to Nessler’s reagent, a brown ppt. of Millon’s base is formed.

Uses of ammonia :

  1. It is used for the manufacture of urea.
  2. It is used as cooling agent in refrigerator.
  3. It is used for reducing metal oxide.
  4. It is used to manufacture nitric acid by Ostwald’s process

Structure of ammonia :

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