Contents
1 · Reversible Reactions
A reaction in which the products can react to reform the reactants. Shown by a double arrow (⇌) in the equation. Most reactions in closed systems are reversible to some extent.
In an open system (e.g. products escape), the reaction can go to completion. In a closed system, both forward and reverse reactions occur simultaneously, leading to equilibrium.
2 · Dynamic Equilibrium
A state in a closed system where the rate of the forward reaction equals the rate of the reverse reaction, and the concentrations of reactants and products remain constant (though both reactions are still occurring).
Key points about dynamic equilibrium:
- It can only be established in a closed system.
- Both forward and reverse reactions are still happening — it is not a static state.
- The concentrations are constant, not necessarily equal.
- Macroscopic properties (colour, pressure, concentration) remain constant.
Students say "at equilibrium, the concentrations of reactants and products are equal." This is wrong. They are constant, not equal. The equilibrium position can strongly favour products or reactants.
3 · Le Chatelier's Principle
When a system at equilibrium is subjected to a change in conditions (temperature, pressure, concentration), the equilibrium position shifts in the direction that tends to oppose or reduce the effect of that change.
In other words, the system "fights back" against whatever change is imposed on it. This principle lets you predict which way the equilibrium will shift without knowing the full kinetics.
4 · Effect of Temperature
Temperature changes affect the equilibrium position AND the value of the equilibrium constant.
| Change | Effect on equilibrium position | Reason |
|---|---|---|
| Increase temperature | Shifts toward the endothermic direction | System absorbs the extra heat to oppose the change |
| Decrease temperature | Shifts toward the exothermic direction | System releases heat to oppose the decrease |
For a temperature change question, always identify whether the forward reaction is exothermic or endothermic first (look for ΔH or "heat" in the equation). Then apply Le Chatelier.
5 · Effect of Pressure (for Gases)
Pressure changes affect equilibria involving gases with different numbers of moles on each side.
| Change | Effect on equilibrium position |
|---|---|
| Increase pressure | Shifts toward the side with fewer moles of gas |
| Decrease pressure | Shifts toward the side with more moles of gas |
| Equal moles on both sides | No effect on equilibrium position |
For N₂ + 3H₂ ⇌ 2NH₃: left side has 1+3 = 4 mol gas, right side has 2 mol. Increasing pressure shifts the equilibrium to the right (toward fewer moles), favouring ammonia production.
6 · Effect of a Catalyst
A catalyst increases the rate of both the forward and reverse reactions equally. It does NOT change the equilibrium position or the equilibrium constant — it only helps the system reach equilibrium faster.
Students say "adding a catalyst shifts the equilibrium to the right." A catalyst does NOT shift the equilibrium position — it speeds up the approach to equilibrium without changing where the equilibrium lies.
7 · The Haber Process (Manufacture of Ammonia)
Industrial Conditions and Compromise
| Condition | Value used | Why this value? |
|---|---|---|
| Temperature | ~450 °C | Lower T favours more NH₃ (exothermic), but is too slow. 450°C is a compromise: reasonable yield at acceptable speed. |
| Pressure | ~200 atm | Higher P favours NH₃ (fewer moles). Very high pressure is costly and dangerous; 200 atm is economically viable. |
| Catalyst | Iron (with promoters) | Speeds up the reaction without changing equilibrium position; allows lower temperature to be used. |
| Recycling | Unreacted N₂ and H₂ recycled | Increases overall yield and efficiency; prevents waste. |
Although a lower temperature would increase the equilibrium yield of ammonia (forward reaction is exothermic), the rate of reaction would be too slow to be economically viable. 450°C represents a compromise between an acceptable yield and a sufficiently fast rate of reaction.
8 · The Contact Process (Manufacture of Sulfuric Acid)
Sulfuric acid (H₂SO₄) is one of the most widely produced industrial chemicals. The Contact Process has three key stages:
Students often write that SO₃ is dissolved directly in water. This is wrong and dangerous — the process uses concentrated sulfuric acid to absorb SO₃, producing oleum, which is then diluted.
🧠 Quick Practice Quiz
8 questions · O-Level style · Instant feedback
What happens to the equilibrium position when temperature is increased?
What happens to the equilibrium position when pressure is increased?