Chemistry • Year 12 • Module 5 • Lesson 6
Le Chatelier's Principle: Pressure, Volume & Catalysts
Apply the gas mole counting rule, interpret real equilibrium data, and reason through the NO2/N2O4 two-stage compression experiment.
1. Interpret equilibrium data — Haber process conditions
The table below shows the approximate equilibrium percentage yield of NH3 at various pressures and temperatures for the Haber process: N2(g) + 3H2(g) ⇌ 2NH3(g), ΔH = −92 kJ/mol. 8 marks
| Pressure (atm) | % yield at 300°C | % yield at 400°C | % yield at 500°C |
|---|---|---|---|
| 10 | 51 | 25 | 11 |
| 50 | 74 | 47 | 24 |
| 100 | 82 | 59 | 34 |
| 200 | 89 | 69 | 47 |
| 400 | 95 | 79 | 59 |
Data adapted from Atkins, P. & de Paula, J. (2010). Physical Chemistry, 9th ed. Oxford University Press.
1.1 Describe the trend in % yield as pressure increases at 400°C. 2 marks
1.2 Explain, using the gas mole counting rule and Le Chatelier's Principle, why increasing pressure increases the % yield of NH3. 3 marks
1.3 At 200 atm, the yield decreases from 89% at 300°C to 47% at 500°C. Incitec Pivot operates at ~400–500°C despite this lower yield. Using the data and your knowledge of industrial compromise, explain why 400–500°C is chosen. 3 marks
2. Interpret graph — NO2/N2O4 compression experiment
The graph below models the relative colour intensity of the NO2/N2O4 mixture (2NO2(g) ⇌ N2O4(g), ΔH = −57 kJ/mol) before, immediately after, and after re-equilibration following sudden compression. A higher value means a darker brown colour. 7 marks
2.1 Describe the immediate effect on colour intensity when compression is applied. What causes this? 2 marks
2.2 Describe what happens to colour intensity during re-equilibration. Using Le Chatelier's Principle and the gas mole count, explain why the colour pales. 3 marks
2.3 The new equilibrium colour (after full re-equilibration) is darker than the original baseline. Explain why, even though the equilibrium has shifted toward N2O4. 2 marks
3. Cause-and-effect chain — inert gas addition
Complete the cause-and-effect chain for two scenarios of inert gas (argon) addition to the equilibrium PCl3(g) + Cl2(g) ⇌ PCl5(g). Draw a line from each cause to its effect, or fill in the empty boxes. 6 marks
Scenario A: Argon added at constant volume
| Step | Cause / event | Effect (fill in) |
|---|---|---|
| 1 | Argon added at constant volume | |
| 2 | Partial pressures of PCl3, Cl2, PCl5… | |
| 3 | Equilibrium position… |
Scenario B: Argon added at constant pressure (volume expands)
| Step | Cause / event | Effect (fill in) |
|---|---|---|
| 1 | Argon added; volume increases to keep total pressure constant | |
| 2 | Partial pressures of PCl3, Cl2, PCl5… | |
| 3 | Equilibrium position… |
4. Predict and justify — Contact process industrial scenario
The Contact process at Orica produces sulfuric acid via: 2SO2(g) + O2(g) ⇌ 2SO3(g), ΔH = −196 kJ/mol. The process uses a V2O5 catalyst. 4 marks
4.1 An engineer proposes adding more V2O5 catalyst to increase SO3 yield. Predict whether this would work and justify your answer using the mechanism of catalyst action. 2 marks
4.2 A second engineer proposes increasing the pressure. Predict the direction of shift (if any) and justify your answer by counting gas moles on each side. 2 marks
Q1.1 — Trend at 400°C
As pressure increases from 10 atm to 400 atm, the % yield of NH3 increases consistently, from 25% at 10 atm to 79% at 400 atm. The relationship is non-linear — larger gains at lower pressures, with diminishing returns at higher pressures. 1 mark for direction (increases); 1 mark for supporting figures from the data.
Q1.2 — Why pressure increases yield
N2(g) + 3H2(g) ⇌ 2NH3(g): left side has 1 + 3 = 4 mol gas; right side has 2 mol gas. By Le Chatelier's Principle, increasing pressure shifts equilibrium toward the side with fewer moles of gas — in this case, the right side (products). More NH3 is produced at new equilibrium, so the % yield increases. 1 mark for mole count (4 left, 2 right); 1 mark for applying LCP (shifts toward fewer moles = right); 1 mark for linking to increased NH3 yield.
Q1.3 — Industrial temperature compromise
Lower temperatures give higher equilibrium yields (e.g. 89% at 300°C, 47% at 500°C at 200 atm) because the forward reaction is exothermic — lower T favours products. However, at 300°C the reaction rate is extremely slow even with a catalyst, making the process economically unviable. At 400–500°C, the rate is fast enough for profitable NH3 production, even though the yield per pass is lower. This is the classic rate–yield compromise. 1 mark for identifying lower T gives higher yield; 1 mark for explaining the rate is too slow at 300°C (even with catalyst); 1 mark for stating 400–500°C is a commercial rate–yield compromise.
Q2.1 — Immediate colour change
Colour intensity increases sharply (mixture darkens) immediately after compression. This is because the volume is suddenly halved, so the concentration of all species — including the brown NO2 — doubles instantly. This is a simple concentration/density effect; no equilibrium shift has occurred yet. 1 mark for ‘darkens/increases’; 1 mark for explaining it is a concentration increase with no equilibrium shift yet.
Q2.2 — Re-equilibration colour change
Colour intensity decreases (pales) as the system re-equilibrates. By Le Chatelier's Principle, increasing pressure shifts equilibrium toward the side with fewer moles of gas. Left side: 2 mol NO2; right side: 1 mol N2O4. Equilibrium shifts right — NO2 (brown) is converted to N2O4 (colourless) — so the brown colour fades. 1 mark for ‘pales’; 1 mark for LCP + mole count (2 left, 1 right, shifts right); 1 mark for linking NO2 consumption to colour fading.
Q2.3 — New equilibrium darker than original
At new equilibrium the total volume is smaller than original, so even though the proportion of NO2 has decreased (equilibrium shifted right), the concentration of NO2 per unit volume is still higher than in the original larger-volume vessel. Colour intensity depends on concentration, not proportion — higher [NO2] in the compressed vessel produces a darker colour than the pre-compression mixture despite the rightward shift. 1 mark for noting [NO2] is still higher due to smaller volume; 1 mark for linking colour intensity to concentration (not proportion).
Q3 — Cause-and-effect chain
Scenario A (constant volume): 1. Partial pressures of PCl3, Cl2, PCl5 are unchanged — argon occupies new space without displacing reacting gases at constant V. 2. Forward and reverse rates remain equal. 3. Equilibrium position does not shift.
Scenario B (constant pressure / volume expands): 1. Volume expands, so all species become more dilute. 2. Partial pressures of PCl3, Cl2, PCl5 decrease (equivalent to lowering total pressure). 3. Equilibrium position shifts toward more moles of gas — left side (1 + 1 = 2 mol gas) vs right (1 mol gas) — shift LEFT, producing more PCl3 and Cl2. Award 1 mark per correct effect (max 3 per scenario).
Q4.1 — Catalyst and yield
Adding more V2O5 catalyst would not increase SO3 yield. A catalyst lowers the activation energy equally for both the forward (SO3 formation) and reverse (SO3 decomposition) reactions. Both rates increase by the same factor, so the system reaches the same equilibrium position faster, but the equilibrium concentrations and Keq are unchanged. Yield depends only on Keq (and therefore temperature), not on catalyst amount. 1 mark for ‘no change in yield’; 1 mark for mechanism (lowers Ea equally, both rates increase equally).
Q4.2 — Pressure shift in Contact process
Left: 2 SO2 + 1 O2 = 3 mol gas. Right: 2 SO3 = 2 mol gas. The right side has fewer gas moles, so increasing pressure shifts equilibrium right, increasing SO3 yield. 1 mark for correct mole count (3 left, 2 right); 1 mark for predicting shift right with justification.