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K324 / 2027
Acid-Base Chemistry

Lesson 7 of 7

Ammonia: choose useful industrial conditions

Reversible reactions require a balance between yield, speed and cost.

O-Level 6092 (2026) / SEC G3 K324 (2027)

Pure only
  • I can identify ammonia feedstocks.Syllabus

    Syllabus K324, 5.3(a). Identify ammonia feedstocks

  • I can recognize reversibility.Syllabus

    Syllabus K324, 5.3(b). Recognize reversibility

  • I can interpret industrial process data.Syllabus

    Syllabus K324, 5.3(c). Interpret industrial process data

Ammonia: choose useful industrial conditions

Reversible reactions require a balance between yield, speed and cost.

Make a guess. You are not marked.

In the Haber process, N2(g) + 3H2(g) <=> 2NH3(g). What does raising the pressure do to the yield of ammonia?

  1. The yield increases, as fewer gas moles form
  2. The yield decreases, as the gases are squeezed
  3. Pressure changes the rate but not the yield
Show the answer

The yield increases, as fewer gas moles form

The right side has 2 moles of gas and the left side has 4. Higher pressure favours the side with fewer gas molecules, which is ammonia.

In the Haber process, nitrogen from air reacts with hydrogen to form ammonia: N2(g) + 3H2(g) reversibly forms 2NH3(g). The syllabus connects hydrogen supply to cracking crude-oil hydrocarbons. The reverse reaction breaks ammonia down, so conversion is not necessarily complete.

Industrial plants use elevated pressure, a moderately high temperature and an iron catalyst. Ammonia is cooled and removed; unreacted nitrogen and hydrogen are recycled. A catalyst speeds attainment of the final composition without improving the equilibrium yield. This chapter uses supplied trends, not Le Chatelier's principle.

Illustrative process data at fixed pressure
TemperatureAmmonia yieldTime to approach final composition
LowerHigherLong
IntermediateIntermediatePractical
HigherLowerShort

Worked exampleInterpret a compromise

Given these trends, why might a manufacturer avoid the lowest temperature?

Try it first. Then show the worked stepsHide the worked steps
  1. A high eventual yield is not useful if formation is too slow.
  2. An intermediate temperature can give more ammonia per hour despite a lower final percentage.
  3. Higher pressure may improve yield in supplied data but adds compression costs and demands stronger equipment.
Answer

Use yield, production rate and cost together. Do not infer that the highest yield automatically gives the most economical operating conditions.

Check your understanding

In the Haber process, why are the gases left after ammonia is removed recycled?

  1. They are ammonia that failed to condense, which is cooled again to collect it.
  2. They are unreacted nitrogen and hydrogen, which get another chance to react.
  3. They are waste gases from the iron catalyst, which are cleaned before release.
Show the answer

They are unreacted nitrogen and hydrogen, which get another chance to react.

The reaction is reversible, so each pass converts only part of the gases. Recycling them raises the overall conversion and wastes less raw material.