Reversible Reactions & Equilibrium

Reversible reactions

Some chemical reactions can go in both directions. These are shown with a reversible arrow ($\rightleftharpoons$), meaning the forward and reverse reactions can happen at the same time.

If the forward reaction is exothermic, the reverse reaction is endothermic by the same amount of energy.

Examples

• Hydrated copper sulfate $\rightleftharpoons$ anhydrous copper sulfate + water – heating drives the forward reaction (the white powder turns blue again when water is added).
• Ammonium chloride $\rightleftharpoons$ ammonia + hydrogen chloride – heating drives the forward reaction; cooling the products drives the reverse.

Dynamic equilibrium

In a closed system, a reversible reaction can reach dynamic equilibrium:

• The forward and reverse reactions happen at the same rate.
• The concentrations of reactants and products stay constant – but they are not necessarily equal.
• The reaction has not stopped – particles are still reacting in both directions.

A closed system is one where no reactants or products can escape, such as a sealed container.

Le Chatelier's principle

Le Chatelier's principle states that if a system at equilibrium is disturbed, the position of equilibrium shifts to oppose the change.

Example: the Haber process

${\text{N}}_2+3{\text{H}}_2\rightleftharpoons 2{\text{NH}}_3$

The forward reaction is exothermic and produces fewer moles of gas (4 moles of reactants form 2 moles of product).

The key idea

The system always shifts to counteract the change you impose on it. Adding more reactant pushes the equilibrium towards products. Raising the temperature favours the endothermic direction. Increasing pressure favours the side with fewer gas molecules.

Catalysts and equilibrium

A catalyst speeds up both the forward and reverse reactions equally, so equilibrium is reached faster. However, a catalyst does not shift the position of equilibrium – the yield of products at equilibrium stays the same.