Chemistry • Year 12 • Module 5 • Lesson 6
Le Chatelier's Principle: Pressure, Volume & Catalysts
Lock in the gas-mole counting rule, the catalyst misconception, and the key distinction between factors that change Keq and those that only shift equilibrium position.
1. Term–definition match
Write the matching term in the right-hand column. Choose from: pressure disturbance, volume disturbance, inert gas addition (constant V), catalyst, activation energy, equilibrium position, Keq, mole ratio of gases, Le Chatelier's Principle, collision frequency. 10 marks
| # | Definition | Matching term |
|---|---|---|
| 1.1 | The principle stating that a system at equilibrium will shift to partially oppose any imposed disturbance. | |
| 1.2 | The numerical ratio of equilibrium concentrations of products to reactants, raised to stoichiometric powers; changes only with temperature. | |
| 1.3 | Increasing the total pressure at constant temperature by reducing the volume of a gas-phase equilibrium system. | |
| 1.4 | Reducing the container size, which simultaneously raises the pressure, used interchangeably with a pressure increase when the cause is compression. | |
| 1.5 | Adding a noble gas (e.g. argon) to a closed equilibrium vessel at constant volume; has no effect on the partial pressures of reacting species. | |
| 1.6 | The relative concentrations of reactants and products in an equilibrium mixture; can change without changing Keq. | |
| 1.7 | The minimum energy required for a collision to result in a chemical reaction. | |
| 1.8 | A substance that lowers the activation energy for both the forward and reverse reactions equally, speeding the approach to equilibrium without shifting the position. | |
| 1.9 | The stoichiometric coefficients of gaseous species on each side of an equation; determines whether a pressure change will shift equilibrium. | |
| 1.10 | The rate at which reacting particles collide per unit volume; increases when pressure increases, driving a temporary change in forward and reverse rates. |
2. True or false — with correction
Circle T or F. If the statement is false, write the corrected version on the line. 10 marks (1 for T/F, 1 for each correction)
2.1 Increasing pressure always shifts a gas-phase equilibrium to the right. T / F
2.2 For the reaction H2(g) + I2(g) ⇌ 2HI(g), increasing the pressure has no effect on the equilibrium position. T / F
2.3 Adding a catalyst to a system at equilibrium increases the value of Keq. T / F
2.4 When counting gas moles for a pressure prediction, solid and aqueous species must be excluded. T / F
2.5 Increasing the pressure on N2(g) + 3H2(g) ⇌ 2NH3(g) shifts equilibrium to the right and increases Keq. T / F
3. Function recall
Answer each in 1–2 precise sentences. 8 marks (2 each)
3.1 What is the function of the gas mole count when predicting the effect of a pressure change on a gas-phase equilibrium?
3.2 What is the function of an iron catalyst in the industrial Haber process (N2(g) + 3H2(g) ⇌ 2NH3(g))?
3.3 What is the function of high pressure (~200 atm) in the Haber process, as used at Incitec Pivot?
3.4 What is the function of temperature as the only factor that changes the value of Keq? Why do pressure and catalyst not change Keq?
4. Cloze — fill the blanks
Use the word bank below to fill in the numbered blanks. Each word is used once. 10 marks
Word bank: fewer • more • temperature • catalyst • faster • position • both • equally • no effect • four
When the pressure on a gas-phase equilibrium is increased, the system shifts toward the side with (1) _____________ moles of gas. For N2(g) + 3H2(g) ⇌ 2NH3(g), the left side has (2) _____________ moles of gas, so increasing pressure shifts equilibrium to the right. For H2(g) + I2(g) ⇌ 2HI(g), pressure changes have (3) _____________ on equilibrium because the mole count is equal on both sides. A (4) _____________ lowers activation energy for (5) _____________ the forward and reverse reactions (6) _____________, so neither direction is favoured over the other. As a result, the equilibrium (7) _____________ does not change, nor does Keq. The system simply reaches equilibrium (8) _____________. The only factor that changes Keq is (9) _____________. Concentration and pressure changes shift the (10) _____________ of equilibrium without changing Keq.
5. Build a concept map
Draw labelled arrows between the six terms below. Each arrow must carry a linking phrase (e.g. “shifts toward”, “has no effect on”, “is the only factor that changes”). Aim for at least 6 labelled arrows. 6 marks
Terms: pressure increase • fewer gas moles side • catalyst • equilibrium position • Keq • temperature change
Q1 — Term–definition matches
1.1 Le Chatelier's Principle • 1.2 Keq • 1.3 pressure disturbance • 1.4 volume disturbance • 1.5 inert gas addition (constant V) • 1.6 equilibrium position • 1.7 activation energy • 1.8 catalyst • 1.9 mole ratio of gases • 1.10 collision frequency
Q2 — True / false with correction
2.1 False. Increasing pressure shifts equilibrium toward the side with fewer moles of gas — if fewer gas moles are on the right, it shifts right; if fewer are on the left, it shifts left; and if equal moles are on both sides, there is no shift.
2.2 True. Both sides have 2 moles of gas (1 H2 + 1 I2 left; 2 HI right), so a pressure change has no effect.
2.3 False. A catalyst does not change Keq. It lowers activation energy equally for both directions, so both rates increase equally — no shift, no change in Keq. Only temperature changes Keq.
2.4 True. Only gaseous species contribute to pressure. Solids and aqueous species are excluded from the gas mole count.
2.5 False. Increasing pressure shifts equilibrium to the right (4 mol gas left → 2 mol gas right), but Keq does not change. Keq changes only when temperature changes.
Q3 — Function recall
3.1 The gas mole count tells you which side of the equilibrium has fewer gas molecules. When pressure is increased, the system shifts toward that side, partially reducing the pressure and partially counteracting the disturbance. Without counting gas moles on both sides, you cannot predict the direction of shift.
3.2 The iron catalyst provides an alternative reaction pathway with lower activation energy for both the forward and reverse reactions. It allows the Haber process to proceed at a profitable rate at 400–500°C. Without it, the rate at this temperature would be too slow for commercial production. The catalyst does not change the yield (Keq) — only pressure and temperature affect yield.
3.3 High pressure (~200 atm) shifts the equilibrium toward the side with fewer moles of gas. N2 + 3H2 ⇌ 2NH3 has 4 mol gas left and 2 mol gas right, so high pressure shifts equilibrium right, increasing NH3 yield.
3.4 Temperature changes Keq because it changes the energy distribution in the system, directly affecting the Boltzmann energy of molecules and therefore the ratio of equilibrium concentrations at the new temperature. Pressure changes and catalysts change rates but not the fundamental thermodynamic balance between products and reactants — so Q is driven back to the same Keq value.
Q4 — Cloze
(1) fewer • (2) four • (3) no effect • (4) catalyst • (5) both • (6) equally • (7) position • (8) faster • (9) temperature • (10) position
Q5 — Sample concept map
Accept any chemically valid set of arrows. A strong map should include:
- pressure increase — shifts toward → fewer gas moles side
- pressure increase — changes → equilibrium position
- pressure increase — has no effect on → Keq
- catalyst — has no effect on → equilibrium position
- catalyst — has no effect on → Keq
- temperature change — is the only factor that changes → Keq
Award 1 mark per correctly labelled directional arrow (max 6).