Reversible reactions and equilibrium · GCSE Chemistry

Reversible reactions

In some reactions, the products can react to produce the original reactants. These are reversible reactions, shown by ⇌.
Example: ammonium chloride ⇌ ammonia + hydrogen chloride. Heating breaks it down; cooling makes it reform.
If a reversible reaction is exothermic in one direction, it is endothermic in the other, and the same amount of energy is transferred each way.
Example: hydrated copper sulfate (blue) ⇌ anhydrous copper sulfate (white) + water. Heating is endothermic; adding water is exothermic and turns the white solid blue.

When a reversible reaction happens in a closed system, where nothing can escape, equilibrium is reached.
At equilibrium, the forward and reverse reactions happen at exactly the same rate, so the amounts of reactants and products stay constant.
It is called dynamic because both reactions are still happening.

If a system at equilibrium is subjected to a change in conditions, the system responds to counteract the change.
Concentration: increasing the concentration of a reactant makes more product form, until equilibrium is reached again.
Temperature: increasing the temperature favours the endothermic direction; decreasing it favours the exothermic direction.
Pressure (for gases): increasing the pressure favours the side with fewer molecules of gas.

N₂ + 3H₂ ⇌ 2NH₃. The forward reaction is exothermic, and there are 4 molecules of gas on the left and 2 on the right.
High pressure moves the equilibrium to the right, giving more ammonia.
Low temperature would give more ammonia but a slow rate, so a compromise temperature (about 450 °C) is used with an iron catalyst.

Reversible reaction: A reaction in which the products can react to reform the reactants.
Equilibrium: The state in a closed system when the forward and reverse reactions happen at the same rate.
Dynamic: Still happening; at dynamic equilibrium both reactions continue.
Closed system: A system where no reactants or products can escape.
Le Chatelier's principle: A system at equilibrium responds to a change in conditions to counteract the change.
Anhydrous: Without water.
Hydrated: Containing water molecules within its crystals.
Haber process: The industrial process that makes ammonia from nitrogen and hydrogen.