Reversible reactions
A reversible reaction can go both forwards and backwards. The products of the forward reaction can react to re-form the reactants. This is shown by the special arrow ⇄ (written as ⇌). A familiar example is heating hydrated copper(II) sulfate: the blue solid turns white as it loses water, and adding water turns it blue again.
Key idea
At dynamic equilibrium in a closed system, the forward and backward reactions occur at the same rate, so the amounts of reactants and products stay constant (but both reactions are still happening).
Le Chatelier’s principle
If a change is made to a system at equilibrium, the position of equilibrium shifts to oppose that change:
- Increase concentration of a reactant → shifts forward, making more product.
- Increase pressure → shifts to the side with fewer moles of gas.
- Increase temperature → shifts in the endothermic direction.
- A catalyst speeds up both directions equally, so it does not change the position of equilibrium — it only helps equilibrium be reached faster.
Worked example
Haber process: N2 + 3H2 ⇌ 2NH3 (forward reaction exothermic). There are 4 moles of gas on the left and 2 on the right. Increasing the pressure shifts equilibrium to the right (fewer moles), increasing the yield of ammonia.
Remember
- Equilibrium is only reached in a closed system.
- ⇌ means the reaction is reversible.
- A catalyst does not move the equilibrium position.