Dynamic equilibrium and Le Chatelier's principle

Dynamic equilibrium

In a closed system a reversible reaction reaches dynamic equilibrium: the forward and backward reactions continue at the same rate, so the concentrations of reactants and products stay constant (they are not usually equal).

Le Chatelier's principle

If a system at equilibrium is disturbed, the position of equilibrium shifts to oppose the change.

• Concentration: adding a reactant shifts it right, to use it up.
• Pressure (gases): higher pressure shifts it to the side with fewer gas molecules.
• Temperature: higher temperature shifts it in the endothermic direction.
• A catalyst does not change the position; it speeds up both reactions equally, so equilibrium is reached sooner.

Industrial compromise

Haber process: N2(g) + 3H2(g) ⇌ 2NH3(g), ΔH = -92 kJ/mol.

• Low temperature gives a higher yield (exothermic forward reaction) but a slow rate, so a compromise of about 400 to 450 °C is used with an iron catalyst.
• High pressure gives a higher yield (4 gas molecules to 2) and faster rate, but costs more and is more dangerous, so about 200 atm is used.

Worked example

For 2SO2(g) + O2(g) ⇌ 2SO3(g), ΔH = -196 kJ/mol, predict the effect of raising the pressure and raising the temperature.

1. Pressure: 3 gas molecules on the left, 2 on the right; shifts right.
2. Temperature: the forward reaction is exothermic, so higher temperature favours the backward, endothermic direction.

Answer: Higher pressure increases the yield of SO3; higher temperature decreases it.

Check yourself

1. What is true at dynamic equilibrium?
   • The reactions have stopped
   • Concentrations of reactants and products are equal
   • The forward and backward rates are equal
   • Only the forward reaction occurs
   Show answer

   The forward and backward rates are equal. Both reactions continue at equal rates, so concentrations stay constant.

2. For N2(g) + 3H2(g) ⇌ 2NH3(g), what does increasing the pressure do?
   • Shifts equilibrium to the right
   • Shifts equilibrium to the left
   • Has no effect
   • Lowers the rate
   Show answer

   Shifts equilibrium to the right. 4 gas molecules on the left, 2 on the right; high pressure favours fewer molecules.

3. For an equilibrium with an endothermic forward reaction, raising the temperature
   • shifts equilibrium to the left
   • has no effect
   • shifts equilibrium to the right
   • stops the reaction
   Show answer

   shifts equilibrium to the right. The system absorbs the extra heat by favouring the endothermic direction.

4. What effect does a catalyst have on the position of equilibrium?
   • Shifts it to the right
   • None
   • Shifts it to the left
   • It depends on the temperature
   Show answer

   None. It increases both rates equally, so equilibrium is reached faster but not moved.

5. For H2(g) + I2(g) ⇌ 2HI(g), changing the pressure
   • shifts it to the right
   • shifts it to the left
   • doubles the yield
   • does not change the position of equilibrium
   Show answer

   does not change the position of equilibrium. There are 2 gas molecules on each side.

6. Why is the Haber process not run at a very low temperature, even though that gives a higher yield?
   • The catalyst would melt
   • The rate would be too slow
   • The equilibrium would shift left
   • Ammonia would decompose
   Show answer

   The rate would be too slow. A compromise temperature gives an acceptable yield at an acceptable rate.

7. Why must the system be closed for equilibrium to be reached?
   • So that no catalyst is needed
   • So that the temperature rises
   • So that the reaction is exothermic
   • So that no reactants or products escape
   Show answer

   So that no reactants or products escape. If a product escaped, the backward reaction could not keep up.

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Sources

• Pearson Edexcel IAL Chemistry (2018) scheme of work
• Pearson Edexcel International Advanced Level Chemistry (2018) qualification page

Last updated 7 November 2026. Spotted a mistake? Tell us.