Reversible reactions and equilibrium · Year 11
Some reactions are reversible: the products can react to form the reactants again. The symbol ⇌ shows this. In a closed system a reversible reaction reaches equilibrium, where the forward and backward reactions go at the same rate, so the amounts of everything stay the same. If the conditions change, the position of equilibrium moves to oppose the change (Le Chatelier's principle).
Key facts
- A reversible reaction can go both ways: A + B ⇌ C + D.
- If a reversible reaction is exothermic one way, it is endothermic the other way, and the same amount of energy is transferred.
- Dynamic equilibrium: in a closed system the forward and backward reactions happen at the same rate, so the concentrations stay constant.
- Le Chatelier's principle: if a condition is changed, the position of equilibrium shifts to oppose the change.
- Raising the temperature favours the endothermic direction; lowering it favours the exothermic direction.
- Raising the pressure favours the side with fewer gas molecules. Adding a reactant or removing a product shifts the equilibrium to the right.
- A catalyst does not change the position of equilibrium; it only helps equilibrium be reached sooner.