Manufacture of Ammonia by Haber’s Process

Ammonia ($\mathrm{NH}{}_3$) is manufactured industrially by the Haber’s process, also known as the Haber–Bosch process. This process involves the direct combination of nitrogen ($\mathrm{N}{}_2$) and hydrogen ($\mathrm{H}{}_2$) gases under high pressure and temperature in the presence of a catalyst.

Reaction and Conditions

The chemical reaction for the synthesis of ammonia is: $$\mathrm{N}{}_2(\mathrm{g})+3\mathrm{H}{}_2(\mathrm{g})\rightleftharpoons 2\mathrm{NH}{}_3(\mathrm{g})\mathrm{\Delta H}=-92.4\text{kJ/mol}$$ This reaction is exothermic and reversible, meaning it proceeds in both forward and backward directions. The equilibrium yield of ammonia depends on the following factors:

1. Pressure

• The reaction involves 4 moles of gaseous reactants producing 2 moles of gaseous products.
• According to Le Chatelier’s principle, increasing the pressure shifts the equilibrium towards the side with fewer moles of gas (i.e., the product side).
• Optimum pressure: 200–400 atm (industrially, ~200 atm is used for economic reasons).

2. Temperature

• The reaction is exothermic, so lowering the temperature favors the forward reaction (more ammonia production).
• However, a very low temperature slows down the reaction rate.
• Optimum temperature: 400–500°C (a compromise between yield and reaction speed).

3. Catalyst

• A promoted iron catalyst (iron oxide with promoters like $\mathrm{Al}{}_2\mathrm{O}{}_3$ and $\mathrm{K}{}_2\mathrm{O}$) is used to speed up the reaction.
• The catalyst does not affect the equilibrium position but increases the rate of reaction.

4. Removal of Ammonia

• Ammonia is continuously removed from the reaction mixture to shift the equilibrium forward (Le Chatelier’s principle).
• This is done by liquefying ammonia under high pressure (boiling point of $\mathrm{NH}{}_3$ is −33.3°C at 1 atm, but it liquefies at higher pressures).

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Industrial Process Overview

1. Raw Materials:
   • Nitrogen ($\mathrm{N}{}_2$): Obtained from fractional distillation of liquid air (air is ~78% $\mathrm{N}{}_2$).
   • Hydrogen ($\mathrm{H}{}_2$): Obtained from:
     • Steam reforming of methane ($\mathrm{CH}{}_4$): $$\mathrm{CH}{}_4(\mathrm{g})+\mathrm{H}{}_2\mathrm{O}(\mathrm{g})\to \mathrm{CO}(\mathrm{g})+3\mathrm{H}{}_2(\mathrm{g})$$
     • Water-gas shift reaction (to remove CO and produce more $\mathrm{H}{}_2$): $$\mathrm{CO}(\mathrm{g})+\mathrm{H}{}_2\mathrm{O}(\mathrm{g})\to \mathrm{CO}{}_2(\mathrm{g})+\mathrm{H}{}_2(\mathrm{g})$$

N₂+3H₂Fe catalyst, 400–500°C, 200 atm22NH₃

2. Purification:

   • The $\mathrm{H}{}_2$ and $\mathrm{N}{}_2$ mixture is purified to remove impurities like $\mathrm{CO}$, $\mathrm{CO}{}_2$, and $\mathrm{O}{}_2$, which poison the catalyst.

3. Compression:

   • The gas mixture is compressed to ~200 atm using compressors.

4. Catalytic Conversion:

   • The compressed gas is passed over a promoted iron catalyst at 400–500°C.
   • About 15–20% of ammonia is produced in a single pass.

5. Separation and Recycling:

   • Ammonia is liquefied under high pressure and removed.
   • Unreacted $\mathrm{N}{}_2$ and $\mathrm{H}{}_2$ are recycled back into the reactor.

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Reactions When Ammonia is Passed Through

NH₃+CuSO₄ (aq)[Cu(NH₃)₄]²⁺+H₂O

2NH₃+3CuO (heated)Δ (heat)3Cu+N₂+3H₂O

(i) Copper Sulphate Solution

When ammonia gas is passed through a copper sulphate solution, it reacts to form a deep blue complex ion, tetraamminecopper(II) sulphate.

Reaction: $$\mathrm{CuSO}{}_4(\mathrm{aq})+4\mathrm{NH}{}_3(\mathrm{aq})\to [\mathrm{Cu}(\mathrm{NH}{}_3){}_4]\mathrm{SO}{}_4(\mathrm{aq})$$

• Observation: The pale blue color of $\mathrm{CuSO}{}_4$ solution changes to a deep blue due to the formation of the complex ion $[\mathrm{Cu}(\mathrm{NH}{}_3){}_4]{}^{2+}$.
• Explanation: Ammonia acts as a Lewis base (electron pair donor) and forms a coordination complex with $\mathrm{Cu}{}^{2+}$.

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(ii) Heated Copper Oxide

When ammonia gas is passed over heated copper oxide, it gets oxidized to nitrogen gas and water, while copper oxide is reduced to copper metal.

Reaction: $$3\mathrm{CuO}(\mathrm{s})+2\mathrm{NH}{}_3(\mathrm{g})\to 3\mathrm{Cu}(\mathrm{s})+\mathrm{N}{}_2(\mathrm{g})+3\mathrm{H}{}_2\mathrm{O}(\mathrm{g})$$

• Observation:
  • The black copper oxide (CuO) turns into a brownish-red copper (Cu).
  • Colorless and odorless nitrogen gas is evolved.
• Explanation:
  • Ammonia acts as a reducing agent (loses hydrogen to reduce $\mathrm{Cu}{}^{2+}$ to $\mathrm{Cu}$).
  • This reaction is used in qualitative analysis to detect ammonia.

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Conversion of Ammonia to Nitric Acid

Ammonia is oxidized to nitric acid ($\mathrm{HNO}{}_3$) through the Ostwald process, which involves the following steps:

Nitric Acid (HNO₃)Nitric Acid (HNO₃)

Step 1: Catalytic Oxidation of Ammonia

Ammonia is oxidized to nitric oxide (NO) using a platinum-rhodium gauge catalyst at 800–900°C.

Reaction: $$4\mathrm{NH}{}_3(\mathrm{g})+5\mathrm{O}{}_2(\mathrm{g})\to 4\mathrm{NO}(\mathrm{g})+6\mathrm{H}{}_2\mathrm{O}(\mathrm{g})$$

• Conditions:
  • Temperature: 800–900°C
  • Catalyst: Platinum-rhodium (90% Pt, 10% Rh)
  • Pressure: Atmospheric pressure

Step 2: Oxidation of Nitric Oxide to Nitrogen Dioxide

Nitric oxide (NO) is further oxidized to nitrogen dioxide ($\mathrm{NO}{}_2$) by excess oxygen.

Reaction: $$2\mathrm{NO}(\mathrm{g})+\mathrm{O}{}_2(\mathrm{g})\to 2\mathrm{NO}{}_2(\mathrm{g})$$

• Observation: $\mathrm{NO}{}_2$ is a brown gas that dissolves in water to form nitric acid.

Step 3: Absorption in Water to Form Nitric Acid

Nitrogen dioxide ($\mathrm{NO}{}_2$) is absorbed in water to form nitric acid ($\mathrm{HNO}{}_3$) and nitrous acid ($\mathrm{HNO}{}_2$), which further decomposes to nitric acid and nitric oxide.

Reactions: $$3\mathrm{NO}{}_2(\mathrm{g})+\mathrm{H}{}_2\mathrm{O}(\mathrm{l})\to 2\mathrm{HNO}{}_3(\mathrm{aq})+\mathrm{NO}(\mathrm{g})$$ $$2\mathrm{NO}(\mathrm{g})+\mathrm{O}{}_2(\mathrm{g})\to 2\mathrm{NO}{}_2(\mathrm{g})\text{(recycled)}$$

• Overall Reaction: $$4\mathrm{NH}{}_3(\mathrm{g})+5\mathrm{O}{}_2(\mathrm{g})\to 4\mathrm{HNO}{}_3(\mathrm{aq})$$
• Industrial Consideration:
  • The nitric oxide (NO) produced is recycled back to $\mathrm{NO}{}_2$ to maximize yield.
  • The process is continuous to ensure efficient conversion.

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Flowchart of Ostwald Process