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Manufacturing Ammonia (Haber Process)

Haber process: feedstocks, reversible equation, ammonia separation, gas recycling and interpretation of supplied industrial data.

  • SEC G3 Pure Chemistry 2027
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Learning objectives

  • describe the use of nitrogen, from air, and hydrogen, from the cracking of crude oil, in the manufacture of ammonia
  • state that some chemical reactions are reversible, e.g. manufacture of ammonia
  • interpret data relating to the conditions used in industry for processes involving reversible reactions, e.g. manufacture of ammonia by the Haber Process (knowledge of Le Chatelier’s Principle is not required).

This lesson follows the materials through the Haber process: where the feed gases come from, how ammonia forms, and how the product is separated. The next lesson develops the separate skill of interpreting reversible-reaction data.

1. Definition

A. Haber process

The Haber process manufactures ammonia by reacting nitrogen with hydrogen in a reversible reaction. Ammonia is separated from the reactor mixture and unreacted gases are recycled.

2. Key Ideas

  • Nitrogen is obtained from air; hydrogen is obtained by cracking hydrocarbons from crude oil.
  • Balanced equation: N₂(g) + 3H₂(g) ⇌ 2NH₃(g).
  • The double arrow shows that both forward and backward reactions occur.
  • An iron catalyst increases reaction rate.
  • Cooling condenses ammonia; unreacted nitrogen and hydrogen are recycled.

3. Detailed Explanations

Quick Recall
  • Feed gases: nitrogen and hydrogen in a 1:3 mole ratio.
  • Product: ammonia.
  • Reaction: reversible.
  • Process: react → cool → separate ammonia → recycle unreacted gases.

A. Raw materials and equation

  • Nitrogen, N₂(g), is obtained from air.
  • Hydrogen, H₂(g), is obtained when hydrocarbons from crude oil are cracked.

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

The coefficients give the reacting mole ratio. They do not mean that every pass converts all the feed gases into ammonia.

B. Reactor, catalyst and separation

The gases pass through a reactor containing an iron catalyst. The catalyst provides an alternative pathway with lower activation energy, increasing the rate of reaction without being used up.

The outlet mixture is cooled. Ammonia condenses and is removed, while unreacted nitrogen and hydrogen remain gaseous and are returned to the reactor.

Haber process flow with ammonia separation and gas recyclingNitrogen and hydrogen in a one to three mole ratio enter a reactor. The reversible reaction forms ammonia. The mixture is cooled so ammonia condenses and is removed, while unreacted nitrogen and hydrogen return to the reactor through a recycle loop.Continuous manufacture of ammoniaFeed gasesN₂(g) : H₂(g)1 : 3 mole ratioReactorN₂ + 3H₂ ⇌ 2NH₃high pressure · moderate temperatureiron catalyst increases ratereaction is incomplete in one passCool and separateNH₃ condensesN₂ and H₂ remain gasesLiquid ammoniaproduct removedunreacted N₂ and H₂ recycledHow to read the processMacroscopic: liquid ammonia is separated after cooling.Particle level: unreacted gas particles are not discarded; they return to the reactor.
Haber process flow: nitrogen and hydrogen pass through the reactor, ammonia is removed by cooling and condensation, and unreacted gases are recycled.

C. Why recycling matters

Recycling reduces waste and gives unreacted feed gases another opportunity to react. It improves the use of raw materials but does not mean that a single pass has 100% conversion.

4. Common Mistakes

  • Writing an irreversible arrow instead of ⇌.
  • Using the wrong ratio; the equation shows 1N₂:3H₂:2NH₃.
  • Saying the catalyst is consumed or that it creates extra ammonia by itself.
  • Saying recycling changes the balanced equation or guarantees complete conversion in one pass.

5. Exam Tips

  • Include state symbols when asked for the full equation.
  • For the process description, include cooling/condensation, ammonia removal and recycling.
  • Continue to Reversible Reactions to practise interpreting supplied rate, yield and cost data.

6. Worked Examples

Modelled example 1

Balance the reversible equation

Core

Problem

Write the balanced equation for manufacturing ammonia in the Haber process.
Study the worked solution

  1. Balance nitrogen atoms

    Method

    Place 2 before ammonia.

    Reason

    One N₂ molecule contains two nitrogen atoms.

    Working

    N₂ + H₂ ⇌ 2NH₃.

  2. Balance hydrogen atoms

    Method

    Place 3 before hydrogen.

    Reason

    Two ammonia molecules contain six hydrogen atoms.

    Working

    N₂ + 3H₂ ⇌ 2NH₃.

  3. Add states and reversible arrow

    Method

    Show all gases and use ⇌.

    Reason

    The industrial reaction is reversible and reaches equilibrium.

    Working

    N₂(g) + 3H₂(g) ⇌ 2NH₃(g).

Guided practice 2

Read a yield trend

About 5 min

Problem

At one temperature, supplied data show 18% ammonia at 100 atm and 31% at 200 atm. Describe the relationship quantitatively.

State direction and size of change

Direction

Hints

Hint 1: compare in order
Follow pressure from 100 atm to 200 atm and compare the corresponding yields.
Hint 2: percentage points
Subtract the stated percentages: 31-18.
View solution step by step

  1. State the trend

    Method

    Describe yield as increasing with the supplied pressure change.

    Reason

    The value rises from 18% to 31% at the same temperature.

    Working

    100 atm: 18%; 200 atm: 31%.

  2. Quantify the increase

    Method

    Subtract the two percentages.

    Reason

    The absolute difference is expressed in percentage points.

    Working

    31-18 = 13 percentage points.

Common misconception 3

State the catalyst’s role

Find and correct the mistake

Learner claim

A student says iron is used up to make more ammonia at equilibrium. Correct the claim and state iron’s role.

Separate rate from equilibrium yield

Role
Fate of iron

View solution step by step

  1. Correct the rate claim

    Method

    State that iron increases reaction rate.

    Reason

    It provides an alternative pathway with lower activation energy.

    Working

    Both forward and reverse reactions reach equilibrium faster.

  2. Correct yield and consumption

    Method

    State that iron is not used up and does not change equilibrium yield.

    Reason

    A catalyst changes kinetics, not the equilibrium position.

    Working

    Iron is a reusable catalyst, not a reactant.

Challenge 4

Separate and recycle

Minimal support

Process-flow transfer

Explain what happens after the Haber reactor mixture is cooled and why the remaining gases are recycled.

Track each component

Ammonia
Unreacted N2 and H2

Hints

Hint 1: different physical behaviour
On cooling, ammonia is separated by condensation while nitrogen and hydrogen remain gases.
Hint 2: avoid wasting feed
Send unreacted gases through the reactor again.
View solution step by step

  1. Separate ammonia

    Method

    Cool the mixture so ammonia condenses and remove it.

    Reason

    Ammonia is separated from nitrogen and hydrogen by its different condensation behaviour.

    Working

    Liquid ammonia leaves the gas stream.

  2. Recycle feed gases

    Method

    Return unreacted N₂ and H₂ to the reactor.

    Reason

    Repeated passes improve overall raw-material use without claiming complete conversion per pass.

    Working

    Unreacted gases are recycled.

7. Mind Stretchers

Mind stretcher 1: Single pass versus overall processExtension

Question: Why can recycling improve overall raw-material use even when the conversion per pass is unchanged?

Show Answer

Unreacted gases pass through the reactor again instead of being discarded, so more of the original feed can react over repeated passes.

8. Quiz

Quiz Time!

Test feedstocks, the reversible equation, separation and recycling.

Practice next

Lesson quizCheck this lesson quickly with targeted MCQs.Structured practiceSwitch to exam-style written responses after the quiz.