7.4 Haber Process

Introduction

Ammonia (NH₃) is commercially produced by the Haber process. It is one of the most important industrial processes because ammonia is a key ingredient in fertilizers that help increase agricultural yields and feed the growing population.

This section explains what the Haber process is, the sources of its reactants, the optimum conditions for the reaction, and why these conditions are chosen.

Definition

Haber Process: The Haber process is the industrial method for producing ammonia (NH₃) by reacting nitrogen (N₂) from air with hydrogen (H₂) obtained from the decomposition of methane.

In simple words: The Haber process is how ammonia is made in industry — by combining nitrogen from air and hydrogen from natural gas under high pressure and temperature with a catalyst.

Main Idea

• Ammonia (NH₃) is commercially produced by the Haber process.
• Ammonia can be liquefied, and its boiling point is -33.35^° C.
• It can be solidified as white ice crystals.
• Most of the commercial ammonia is prepared by fixing atmospheric nitrogen.
• Sources of reactants:
• Hydrogen is obtained by the decomposition of methane.
• Nitrogen is obtained from air.

Core Concept

Key point: The Haber process uses a reversible, exothermic reaction with a catalyst under high pressure and moderate temperature to produce ammonia.

Detailed Explanation

What is the Haber Process?

Ammonia (NH₃) is commercially produced by the Haber process.

Properties of ammonia:

• Ammonia can be liquefied.
• Its boiling point is -33.35^° C.
• It can be solidified as white ice crystals.

Method:

- Most of the commercial ammonia is prepared by fixing atmospheric nitrogen.

Sources of Reactants

Sources of reactants:

• Hydrogen is obtained by the decomposition of methane.
• Nitrogen is obtained from air.

Optimum Conditions for the Haber Process

The source gives the following optimum conditions:

i) Concentration of Reactant: A mixture of 1 volume of nitrogen and 3 volumes of hydrogen.

ii) Catalytic Chamber: These two gases are compressed to 200 - 500\ atm pressure by special pumps, injected into the heated reaction vessel, and then passed over a catalyst consisting of a mixture of Fe/Mo (Molybdenum) at 450^° C.

iii) Refrigerated Brine: The gases coming out of the catalytic chamber contain 10 - 12\% Ammonia. Ammonia is removed periodically and then passed through refrigerated brine (concentrated solution of NaCl) to liquefy Ammonia. The residual or unreacted gases are recycled.

Excess of nitrogen is used to favour the reaction to the right.

Equation: N_2(g) + 3H_2(g) Fe/Mo→ 2NH_3(g)

Key point: The Haber process is the cheapest method for manufacturing Ammonia.

Why These Conditions Are Chosen

The reaction between nitrogen and hydrogen is an exothermic and reversible process and is accompanied by a decrease in the volume of the product.

Favourable conditions according to Le Chatelier's Principle:

1) High pressure (200 atm) due to a decrease in the volume of the product.

• The reaction produces 2 moles of NH₃ from 4 moles of reactants (1 mole N₂ + 3 moles H₂).
• This means the volume decreases during the reaction.
• According to Le Chatelier's principle, high pressure favours the forward reaction.

2) The temperature must be kept as low as possible. In the presence of a catalyst, the optimum temperature is 450^° C.

• The reaction is exothermic (releases heat).
• According to Le Chatelier's principle, low temperature favours the forward reaction.
• However, at very low temperatures, the reaction rate is too slow.
• Therefore, a moderate temperature of 450^° C is used with a catalyst to ensure a reasonable reaction rate.

Key point: The optimum conditions are a compromise between maximum yield and reasonable reaction rate.

Summary Table

Intermediate Level

Why these conditions are chosen:

• The reaction is exothermic (releases heat).
• The reaction is reversible.
• The reaction involves a decrease in volume (4 moles → 2 moles).

According to Le Chatelier's principle:

• High pressure favours the forward reaction (decrease in volume).
• Low temperature favours the forward reaction (exothermic).
• However, low temperature slows down the reaction.
• Therefore, a moderate temperature (450^° C) is used with a catalyst to ensure a reasonable reaction rate.

Key point: The optimum conditions are a compromise between maximum yield and reasonable reaction rate.

Important Points

• - Ammonia (NH₃) is commercially produced by the Haber process.
• - Properties of ammonia:
• - Boiling point: -33.35^° C
• - Can be liquefied
• - Can be solidified as white ice crystals
• - Sources of reactants:
• - Hydrogen: Decomposition of methane
• - Nitrogen: From air
• - Optimum conditions:
• - Concentration: 1 volume N₂ : 3 volumes H₂
• - Pressure: 200 - 500\ atm
• - Temperature: 450^° C
• - Catalyst: Fe/Mo
• - Equation:
• N_2(g) + 3H_2(g) Fe/Mo→ 2NH_3(g)
• - Separation: Ammonia is removed by refrigerated brine.
• - Recycling: Unreacted gases are recycled.
• - Excess nitrogen: Used to favour the reaction to the right.
• - Cheapest method for manufacturing ammonia.

Remember

• - Ammonia produced by Haber process.
• - Sources: H₂ from methane, N₂ from air.
• - Optimum conditions:
• - Pressure: 200 - 500\ atm
• - Temperature: 450^° C
• - Catalyst: Fe/Mo
• - Equation: N₂ + 3H₂ Fe/Mo→ 2NH₃.
• - Ammonia removed by refrigerated brine.
• - Unreacted gases recycled.
• - Excess nitrogen favours reaction to the right.
• - Reaction is exothermic and reversible.
• - Decrease in volume favours high pressure.
• - Cheapest method for ammonia.

Common Mistakes

Quick Revision

Haber Process:

Ammonia (NH₃) is commercially produced by the Haber process. Ammonia can be liquefied and its boiling point is -33.35^° C. It can be solidified as white ice crystals. Most of the commercial ammonia is prepared by fixing atmospheric nitrogen.

• Sources of reactants:
• Hydrogen is obtained by the decomposition of methane and nitrogen is obtained from air.

Optimum Conditions for Haber process:

i) Concentration of Reactant: A mixture of 1 volume of nitrogen and 3 volume of hydrogen.

ii) Catalytic Chamber: These two gases are compressed to 200 - 500\ atm pressure by special pumps injected into the heated reaction vessel and then passed over a catalyst consisting of a mixture of Fe/Mo (Molybdenum) at 450^° C.

iii) Refrigerated Brine: The gases coming out of the catalytic chamber contain 10 - 12\% Ammonia is removed periodically and then passed through refrigerated brine (concentrated solution of NaCl) to liquefy Ammonia. The residual or unreacted gases are recycled. Excess of nitrogen is used to favour the reaction to the right.

• N_2(g) + 3H_2(g) Fe/Mo→ 2NH_3(g)

Haber process is the cheapest method for manufacturing Ammonia. The reaction between nitrogen and hydrogen is an exothermic and reversible process and is accompanied by a decrease in the volume of the product.

Favourable conditions according to the Le-Chatelier's Principal:

• High pressure (200 atm) due to a decrease in the volume of the product.
• The temperature must be kept as low as possible. In the presence of a catalyst, the optimum temperature is 450^° C.

60-Second Revision

• Ammonia produced by Haber process.
• Sources: H₂ from methane, N₂ from air.
• Optimum conditions:
• Pressure: 200 - 500\ atm
• Temperature: 450^° C
• Catalyst: Fe/Mo
• Equation: N₂ + 3H₂ Fe/Mo→ 2NH₃.
• Ammonia removed by refrigerated brine.
• Unreacted gases recycled.
• Excess nitrogen favours reaction to the right.
• Reaction is exothermic and reversible.
• Decrease in volume favours high pressure.
• Cheapest method for ammonia.
• Boiling point of NH₃: -33.35^° C.
• ⚠️ Warning: the temperature is kept at 450^° C (not lower) because a compromise is needed between yield and reaction rate.