Reversible Reactions and Dynamic Equilibrium (C6.2)

Reversible reactions

Some reactions are reversible — they can proceed in both forward and reverse directions. Indicated by ⇌.

Example: A + B ⇌ C + D
Forward: A + B → C + D
Reverse: C + D → A + B

Example: ammonium chloride decomposition:
NH₄Cl(s) ⇌ NH₃(g) + HCl(g)
Heated → decomposes; cooled → recombines.

Dynamic equilibrium

In a closed system, a reversible reaction reaches dynamic equilibrium when:

• The forward rate equals the reverse rate.
• The concentrations of reactants and products remain constant (but not necessarily equal).
• Both reactions are still occurring simultaneously.

"Dynamic" means both reactions continue — equilibrium is not static.

Le Chatelier's principle

If a system at equilibrium is subjected to a change, it will respond to oppose that change.

The Haber process — industrial application

N₂ + 3H₂ ⇌ 2NH₃ ΔH = −92 kJ/mol (forward reaction exothermic)

Optimal conditions (compromise):

• Temperature: 450°C. Lower temperature → more NH₃ (equilibrium favours forward/exothermic direction) but rate too slow. Higher temperature → more NH₃ decomposed + faster but less yield. 450°C = compromise.
• Pressure: 200 atm (high). 4 mol gas → 2 mol gas → high pressure favours forward reaction (fewer moles of gas). Higher pressure is costly and dangerous.
• Iron catalyst: increases rate without affecting equilibrium position; NH₃ continuously removed → equilibrium shifts forward.
• Recycled unreacted N₂ and H₂: improve atom economy.

Common exam errors

1. Saying a catalyst shifts the equilibrium — it does NOT; it speeds up reaching equilibrium.
2. Saying the concentrations are equal at equilibrium — they are constant, not necessarily equal.
3. Forgetting to specify "closed system" when defining equilibrium.