HSC Chemistry — Module 5
Haber Process — Flashcards & Quiz
The Haber process for synthesising ammonia is the canonical industrial application of equilibrium principles in HSC Chemistry Module 5. You need to justify each operating condition — high pressure, the temperature compromise, the iron catalyst, continuous removal of NH₃ — using Le Chatelier's principle. Top responses link the choices to economic and environmental trade-offs, not just yield. Be ready to write the equation, identify it as exothermic, and explain why a catalyst speeds equilibrium without shifting position.
Key Points
- The Haber process synthesises ammonia: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = −92 kJ mol⁻¹ (exothermic).
- Pressure: ~200 atm shifts equilibrium right (4 mol gas → 2 mol). Higher P increases yield but is expensive and dangerous.
- Temperature: 450°C is a compromise — higher T speeds the reaction but shifts equilibrium LEFT (exothermic). 450°C gives acceptable rate and yield.
- Catalyst: iron (Fe) with K₂O/Al₂O₃ promoters speeds both forward and reverse — reaches equilibrium faster without changing position.
- NH₃ is continuously removed by cooling/liquefaction, pulling equilibrium right by Le Chatelier.
- Social/environmental impact: Haber-Bosch produces fertiliser that feeds ~half the world's population, but consumes ~1-2% of global energy.
Common Mistakes to Avoid
- Forgetting to explain the temperature COMPROMISE — lower T gives higher yield but too slow; higher T gives faster reaction but lower yield.
- Claiming high pressure always works — high pressure is expensive, dangerous and imposes engineering limits.
- Not naming the iron catalyst — "a catalyst" loses marks where "iron with K₂O/Al₂O₃ promoters" gets them.
- Listing conditions without Le Chatelier justification — markers want the REASON for each choice.
- Forgetting to remove ammonia continuously — this is what shifts equilibrium right and drives yield.
Exam Strategy
HSC Module 5 Haber process questions ask you to (1) state the conditions and (2) JUSTIFY each using Le Chatelier's principle. Structure the response: for each condition (P, T, catalyst, NH₃ removal) give (a) the value, (b) the effect on yield/rate, (c) the Le Chatelier explanation. Temperature always needs the compromise argument for full marks.
Sample Flashcards
Q1: Describe the conditions used in the Haber process and justify each.
N₂ + 3H₂ ⇌ 2NH₃ (exothermic). Conditions: High pressure (~200 atm) → shifts right (4 mol gas → 2 mol). Moderate temperature (~450°C) → compromise between yield (lower T = higher yield) and rate (higher T = faster). Iron catalyst → faster equilibrium (doesn't change yield). Continuous removal of NH₃ → shifts right.
Q2: Why is ammonia (NH₃) important industrially?
Ammonia is primarily used to produce fertilisers (ammonium nitrate, urea — ~80% of NH₃ production). Also used in explosives, cleaning products, nylon production, nitric acid manufacture (Ostwald process), and refrigeration.
Sample Quiz Questions
Q1: The Haber process uses high pressure to increase the yield of ammonia.
Answer: TRUE
N₂ + 3H₂ ⇌ 2NH₃ has 4 moles of gas on the left and 2 on the right. High pressure (~200 atm) shifts equilibrium right, increasing NH₃ yield.
Q2: A very high temperature is used in the Haber process to maximise ammonia yield.
Answer: FALSE
The forward reaction is exothermic, so high temperature actually DECREASES yield. A moderate temperature (~450°C) is a compromise between yield (favoured at low T) and rate (faster at high T).
Q3: The iron catalyst in the Haber process increases the equilibrium yield of ammonia.
Answer: FALSE
The catalyst does NOT change equilibrium yield or Kc. It only speeds up the rate at which equilibrium is reached.
Revision Tip
The Haber process is a staple exam question — drill a Revizi flashcard for each condition with its value AND its Le Chatelier justification so you can recall them together.
Related Concepts
Last updated: March 2026 · 2 flashcards · 4 quiz questions