Reversible Reactions and Equilibria
Physical Chemistry
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 6% of your exam marks.
Haber process conditions and Le Chatelier's principle regularly examined.
What "position of equilibrium" means
- The position of equilibrium describes how much of each species is present once dynamic equilibrium has been reached
- The position lies "to the right" if products are favoured (more product, less reactant at equilibrium)
- The position lies "to the left" if reactants are favoured (less product, more reactant)
- Changing the conditions of the system shifts where this balance sits
Le Chatelier's principle
- states that if a closed system at equilibrium is disturbed, the equilibrium position shifts so as to oppose the change imposed on it
- Le Chatelier's principle predicts the shift direction caused by:
- Changing the temperature
- Changing the pressure (gases)
- Changing the concentration of a species (not assessed numerically, but the direction is)
Effect of temperature
- Identify whether the forward reaction is exothermic or endothermic; the reverse direction is then the opposite
- Increasing temperature shifts the equilibrium in the direction of the reaction (the system "absorbs" the extra heat to oppose the rise)
- Decreasing temperature shifts the equilibrium in the direction of the reaction (the system releases heat to oppose the fall)
- Worked example. In the Haber process N2(g) + 3 H2(g) ⇌ 2 NH3(g), the forward reaction is exothermic.
- Raising the temperature drives the equilibrium to the left — ammonia decomposes — so the yield of NH3 falls
- Industry compromises by running the reaction at about 450 °C: hot enough for a fast rate, cool enough that the yield is still useful
Predicting the effect of temperature on yield
What comes up: "Explain the effect on the yield of [product] at equilibrium when the temperature is increased/decreased" (2 marks).
Write (two marks): (1) State whether the yield increases or decreases. (2) Explain this by identifying whether the forward or reverse reaction is exothermic or endothermic and stating that the equilibrium shifts in that direction. For example, if the forward reaction is exothermic: raising the temperature shifts the equilibrium towards the reactants side (to the left), so the yield decreases.
Watch out: Mentioning Le Chatelier's principle by name scores no marks — the examiner ignores it. You must state the direction of the shift and link it to the thermochemistry of the reaction.
Effect of pressure (gaseous reactions only)
- Count the number of moles of gas on each side of the equation; only species in the gaseous state count
- Increasing pressure shifts the equilibrium toward the side with fewer moles of gas — that side occupies a smaller volume, opposing the pressure rise
- Decreasing pressure shifts the equilibrium toward the side with more moles of gas
- If the two sides have the same number of moles of gas, changing the pressure has no effect on the position of equilibrium (it does still alter the rate at which equilibrium is reached)
- Worked example. In the Contact process 2 SO2(g) + O2(g) ⇌ 2 SO3(g):
- Left side has 3 mol of gas (2 SO2 + 1 O2); right side has 2 mol of gas (2 SO3)
- Raising the pressure shifts the equilibrium to the right, producing more SO3
- The industrial reaction runs at slightly above atmospheric pressure because the yield is already high; the extra cost of higher-pressure equipment is not worth the small gain
Predicting the effect of pressure on yield
What comes up: "Explain the effect on the yield of [product] at equilibrium when the pressure is increased/decreased" (2 marks).
Write (two marks): (1) State whether the yield increases or decreases. (2) Explain by counting the total moles of gas on each side and stating which side has fewer — the equilibrium shifts towards that side. For example, for a reaction with 4 mol of gas on the left and 2 mol on the right, increasing pressure shifts the equilibrium to the right (the product side), so the yield increases.
Watch out: Mentioning Le Chatelier's principle by name scores no marks — the examiner ignores it. Only molecules or moles in the gaseous state count when comparing the two sides; any aqueous or solid species should be ignored.
Effect of concentration
- Adding more of a reactant shifts the equilibrium toward the products to consume the added material — position shifts to the right
- Adding more of a product shifts the equilibrium toward the reactants — position shifts to the left
- Removing a product as it forms (e.g. condensing it out of a gas mixture) drains material from the right side, so the system makes more product to compensate — equilibrium keeps shifting to the right
- This last trick is heavily used in industry: continuously removing the product is a way to push reactions that would otherwise stop at a moderate yield
Effect of a catalyst
- A speeds up both the forward and the reverse reactions by the same factor
- It lets equilibrium be reached faster, but it does not change the position once equilibrium has been reached
- The amounts of reactant and product at equilibrium are the same with or without the catalyst
- Industrially, catalysts are still used because reaching a useful yield in minutes instead of hours saves enormous amounts of energy
Effect of a catalyst on the position of equilibrium
What comes up: "Give the reason for using a catalyst" or "Explain the effect of adding a catalyst on the yield at equilibrium" (1–2 marks).
Write: A catalyst increases the rate of reaction, allowing the system to reach equilibrium faster. It has no effect on the position of equilibrium once equilibrium is reached, so the yield of product remains unchanged.
Watch out: The mark scheme explicitly rejects the answer "to increase the yield." Only credit is given for the idea of reaching equilibrium more quickly (or speeding up the rate of reaction). Do not confuse faster rate with greater yield.
| Change made | Position of equilibrium shifts | Why |
|---|---|---|
| Increase temperature | Toward the endothermic side | System absorbs the added heat |
| Decrease temperature | Toward the exothermic side | System releases heat to oppose the fall |
| Increase pressure (gases) | Toward the side with fewer moles of gas | Reduces total moles → reduces pressure |
| Decrease pressure (gases) | Toward the side with more moles of gas | Increases total moles → opposes the drop |
| Add more reactant | To the right (products) | Consumes the excess reactant |
| Remove a product | To the right (products) | Replaces the missing product |
| Add a catalyst | No change in position; equilibrium reached faster | Catalyst speeds both rates equally |