Ammonia for fertilisers and sulfuric acid for almost everything are made by reversible reactions. Engineers choose the temperature, pressure and catalyst as a compromise between a good yield, a fast rate, safety and cost.
Haber process: N2(g) + 3H2(g) ⇌ 2NH3(g). Nitrogen comes from the air and hydrogen from methane. Conditions: 450 °C, 20 000 kPa (200 atm), iron catalyst. Contact process: 2SO2(g) + O2(g) ⇌ 2SO3(g). Sulfur dioxide comes from burning sulfur or roasting sulfide ores; oxygen from the air. Conditions: 450 °C, 200 kPa (2 atm), vanadium(V) oxide catalyst. Extended: both forward reactions are exothermic and reduce the number of gas molecules, so the conditions are a compromise.
Change the temperature and pressure in the simulator and read the equilibrium yield of ammonia. Then look at how long it takes to get there.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What are the conditions used in the Haber process?
Where does the hydrogen for the Haber process come from?
Which catalyst is used in the Contact process?
What is the equation for the reaction in the Contact process?
Follow sulfur from the furnace to sulfuric acid in the simulator.
Extended. Each condition is judged on four things: the position of equilibrium (yield), the rate, safety and cost.
| Condition | Effect on yield | Effect on rate, safety and cost |
|---|---|---|
| temperature 450 °C | lower temperature would give a higher yield, because the forward reaction is exothermic | but a low temperature makes the rate too slow; 450 °C is a compromise |
| Haber: 20 000 kPa | high pressure gives a higher yield: 4 gas molecules on the left, 2 on the right | higher pressures are more expensive and more dangerous; high pressure also increases the rate |
| Contact: 200 kPa | the yield is already very high at low pressure | a higher pressure would add cost for very little extra yield |
| catalyst | no effect on the position of equilibrium | increases the rate, so equilibrium is reached faster at a lower temperature |
The catalyst does not increase the yield. And 450 °C is not chosen for the best yield: the best yield would be at a low temperature. It is a compromise between yield and rate. Name both in your answer.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
(Extended) The forward reaction in the Haber process is exothermic. Why is 450 °C used rather than a lower temperature?
(Extended) Why does a high pressure increase the equilibrium yield of ammonia?
(Extended) Why is a pressure of only 200 kPa used in the Contact process?
(Extended) What is the effect of the iron catalyst in the Haber process?
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