Most chemical reactions go one way, but some can go in both directions. These are called reversible reactions. The products can react to form the original reactants again.

What is a reversible reaction?

• A reversible reaction is one where the products can turn back into the reactants
• It is shown with a double arrow:
  A + B ⇌ C + D
• The forward reaction goes from left to right
• The backward reaction goes from right to left

Examples of reversible reactions

1. Hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water

• Heating blue hydrated copper sulfate removes water and turns it white
• Adding water reverses the reaction and turns it blue again

2. Ammonium chloride ⇌ ammonia + hydrogen chloride

• On heating, ammonium chloride breaks into gases
• When cooled, the gases recombine

Energy in reversible reactions

• If the forward reaction is exothermic, the reverse is endothermic, and vice versa
• The amount of energy transferred is the same in both directions

💡 Exothermic one way = endothermic the other

Closed system

• A closed system means no reactants or products can escape
• Reversible reactions in a closed system can reach equilibrium

Equilibrium is covered in the next lesson.