Reversible Reactions

One-way (Irreversible) Reactions

  • Most reactions you’ve met in GCSE go one way only.

    • Example: Magnesium + Oxygen → Magnesium Oxide

    • Arrow only points right (products do not change back).

Reversible Reactions

  • Reactions where the products can react to reform the reactants.

  • Shown with a double arrow (⇌).

  • Example:

    • Ammonium chloride ⇌ Ammonia + Hydrogen chloride

    • Forward: Heating (decomposition).

    • Reverse: Cooling (re-forms ammonium chloride).

  • Changing conditions can change the direction.

Energy Changes in Reversible Reactions

  • Example: Hydrated copper sulfate (blue) ⇌ Anhydrous copper sulfate (white) + Water

    • Forward reaction (heating): Endothermic (energy in).

    • Reverse reaction (adding water): Exothermic (energy out).

✅ Key Fact:

  • If a reversible reaction is exothermic in one direction, it is endothermic in the other.

  • The same amount of energy is transferred in each case.

Equilibrium

  • In a sealed container, reactants and products cannot escape.

  • Eventually, the forward and reverse reactions happen at the same rate.

  • At this point the system is in equilibrium.

Definition:

Equilibrium is when the forward and reverse reactions occur at the same rate in a closed system.

✅ Exam Tip:
You need to know the copper sulfate reversible reaction example — it is often tested.