⚙️ Reaction: \$\ce{N2(g) + 3H2(g) ⇌ 2NH3(g)}\$
🔬 Rate of Reaction: The forward rate (formation of NH₃) increases with pressure because the reaction consumes 4 g of gas → 2 g of gas. By increasing pressure, we push the equilibrium toward the product side (Le Chatelier’s principle). The catalyst lowers the activation energy, so the reaction can proceed at a practical temperature.
⏱️ Position of Equilibrium: Higher pressure shifts equilibrium to the left (NH₃ side). Temperature is a trade‑off: higher T increases rate but shifts equilibrium back to reactants (endothermic). 400–500 °C gives a good balance.
⚠️ Safety: High pressure vessels require robust design; any failure can release hot gases. Hydrogen is flammable; careful handling and leak detection are essential.
💰 Economics: The cost of compressing gases to 200 bar is high, but the high yield of NH₃ (a key fertilizer) justifies it. Energy input is mainly for compression and heating; efficient heat recovery systems reduce operating costs.
Exam Tip: When asked why the Haber process uses high pressure, remember: “The reaction consumes gas molecules, so pressure pushes equilibrium toward products.” Also note the role of the catalyst in increasing the rate without changing equilibrium position.
⚙️ Reaction: \$\ce{2SO2(g) + O2(g) ⇌ 2SO3(g)}\$
) on alumina
🔬 Rate of Reaction: The catalyst dramatically increases the rate of SO₂ oxidation to SO₃. The reaction is exothermic, so higher temperatures decrease equilibrium yield. However, a moderate temperature (≈ 450 °C) keeps the rate high while still allowing a reasonable equilibrium conversion.
⏱️ Position of Equilibrium: The reaction produces fewer gas molecules (3 → 2), so pressure has a smaller effect. Operating near atmospheric pressure keeps equipment simpler and safer.
⚠️ Safety: SO₂ is toxic; SO₃ reacts violently with water to form sulfuric acid. Proper scrubbing and ventilation are mandatory. Catalyst poisoning (by impurities) can reduce efficiency.
💰 Economics: Low pressure reduces capital costs. Energy is mainly used for heating; heat exchangers recover exothermic heat from the second stage to pre‑heat reactants, improving overall efficiency.
Exam Tip: For the Contact process, remember: “High temperature increases rate but lowers equilibrium yield; low pressure is fine because fewer gas molecules are produced.” Also note the role of the vanadium catalyst in speeding up the reaction.
| Parameter | Haber Process | Contact Process |
|---|---|---|
| Pressure | 150–250 bar | 1–2 bar |
| Temperature (°C) | 400–500 (first stage) | 400–450 (first stage), 200–250 (second stage) |
| Catalyst | Fe + K₂O, CaO | V₂O?? on alumina |
| Effect on Equilibrium | High pressure → shifts to NH₃ | Low pressure → negligible effect |
| Key Safety Issue | High‑pressure vessels & flammable H₂ | Toxic gases & exothermic SO₃ formation |
📝 Tip: Practice writing short, clear answers that link conditions to equilibrium shifts and rate improvements.