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Uses and Production of Ammonia by the Haber Process

Key Concepts

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Equilibrium Considerations

The reaction between nitrogen gas and hydrogen gas to produce ammonia gas is an exothermic equilibrium reaction, releasing 92.4 kJ mol-1 of energy at 298 K (25oC).

+ 3H2(g)
heat, pressure, catalyst
ΔH = -92.4 kJ mol-1

OR, including the enthalpy change as a product of the reaction:

+ 3H2(g)
heat, pressure, catalyst
+ 92.4 kJ mol-1

By Le Chetalier's Principle, increasing the pressure on the reaction mixture favours the formation of ammonia gas:

Increasing the pressure causes the equilibrium position to move to the right resulting in a higher yeild of ammonia since there are more gas molecules on the left hand side of the equation (4 in total) than there are on the right hand side of the equation (2).
Increasing the pressure means the system adjusts to reduce the effect of the change, that is, to reduce the pressure by having fewer gas molecules.

By Le Chetalier's Principle, decreasing the temperature of the reaction mixture favours the formation of ammonia gas:

Decreasing the temperature causes the equilibrium position to move to the right resulting in a higher yield of ammonia since the reaction is exothermic (releases heat).
Reducing the temperature means the system will adjust to minimise the effect of the change, that is, it will produce more heat since energy is a product of the reaction, and will therefore produce more ammonia gas as well.

However, the rate of the reaction at lower temperatures is extremely slow, so a higher temperature must be used to speed up the reaction which results in a lower yield of ammonia.

The equilibrium expression for this reaction is:

Kc =     [NH3(g)]2    

As the temperature increases, the value of the equilibrium constant decreases as the yield of ammonia decreases.

Kc trend
25 6.4 x 102 higher Kc
200 4.4 x 10-1
300 4.3 x 10-3
400 1.6 x 10-4
500 1.5 x 10-5 lower Kc

In summary, higher pressure favours formation of ammonia gas, lower temperature favours formation of ammonia gas BUT the reaction rate is slow at lower temperatures.

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Reaction Rate (Kinetic) Considerations

Refer to the graph on the right which records the yield of ammonia as a percentage at different pressures:

At 200oC and pressures above 750 atm there is an almost 100% conversion of reactants to the ammonia product.

Since there are difficulties associated with containing larger amounts of materials at this high pressure, lower pressures of around 200 atm are used industrially.

By using a pressure of around 200 atm and a temperature of about 500oC, the yield of ammonia is 10-20%, while costs and safety concerns in the building and during operation of the plant are minimised

During industrial production of ammonia, the reaction never reaches equilibrium as the gas mixture leaving the reactor is cooled to liquefy and remove the ammonia.
The remaining mixture of reactant gases are recycled through the reactor.
The heat released by the reaction is removed and used to heat the incoming gas mixture.

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Uses of Ammonia

Industry Use
Fertilser production of:
  • ammonium sulfate, (NH4)2SO4
  • ammonium phosphate, (NH4)3PO4
  • ammonium nitrate, NH4NO3
  • urea, (NH2)2CO, used in the production of barbiturates (sedatives), is made by the reaction of ammonia with carbon dioxide as shown below:
carbon dioxide
+ 2NH3
ammonium carbonate

Chemicals synthesis of:

  • nitric acid, HNO3, which is used in making explosives such as TNT (2,4,6-trinitrotoluene), nitroglycerine which is also used as a vasodilator (a substance that dilates blood vessels) and PETN (pentaerythritol nitrate).
  • sodium hydrogen carbonate (sodium bicarbonate), NaHCO3
  • sodium carbonate, Na2CO3
    (Solvay Process)
  • hydrogen cyanide (hydrocyanic acid), HCN
  • hydrazine, N2H4 (used in rocket propulsion systems)

Explosives ammonium nitrate, NH4NO3

Fibres and Plastics nylon, -[(CH2)4-CO-NH-(CH2)6-NH-CO]-,and other polyamides

Refrigeration used for making ice, large scale refrigeration plants, air-conditioning units in buildings and plants

Pharmaceuticals used in the manufacture of drugs such as sulfonamide which inhibit the growth and multiplication of bacteria that require p-aminobenzoic acid (PABA) for the biosynthesis of folic acids, anti-malarials and vitamins such as the B vitamins nicotinamide (niacinamide) and thiamine.

Pulp and Paper ammonium hydrogen sulfite, NH4HSO3, enables some hardwoods to be used

Mining and Metallurgy used in nitriding (bright annealing) steel,
used in zinc and nickel extraction

Cleaning ammonia in solution is used as a cleaning agent such as in 'cloudy ammonia'

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A Brief History of Ammonia Production

At the beginning of the 20th century there was a shortage of naturally occurring, nitrogen-rich fertilisers, such as Chile saltpetre, which prompted the German Chemist Fritz Haber, and others, to look for ways of combining the nitrogen in the air with hydrogen to form ammonia, which is a convenient starting point in the manufacture of fertilisers.
This process was also of interest to the German chemical industry as Germany was preparing for World War I and nitrogen compounds were needed for explosives.

The hydrogen for the ammonia synthesis was made by the water-gas process (a Carl Bosch invention) which involves blowing steam through a bed of red hot coke resulting in the separation of hydrogen from oxygen.
The nitrogen was obtained by distillation of liquid air, then by cooling and compressing air.

These days, the hydrogen is produced by reforming light petroleum fractions or natural gas (methane, CH4) by adding steam:

CH4(g) + H2O(g) Ni catalyst

CO(g) + 3H2(g)

Enough steam is used to react with about 45% of the methane (CH4), the rest of the methane is reacted with air:

2CH4(g) + O2(g) + 4N2(g)
Ni catalyst

2CO(g) + 4H2(g) + 4N2(g)

All the carbon monoxide (CO) in the mixture is oxidised to CO2 using steam and an iron oxide catalyst:

CO(g) + H2O(g) iron oxide catalyst

H2(g) + CO2(g)

The carbon dioxide (CO2) is removed using a suitable base so that only the nitrogen gas (N2) and hydrogen gas (H2) remain and are used in the production of ammonia (NH3).

In ammonia production the hydrogen and nitrogen are mixed together in a ratio of 3:1 by volume and compressed to around 200 times atmospheric pressure.

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