Reversible Reactions (WJEC GCSE Chemistry)

What are reversible reactions?

• Some reactions go to completion, where the reactants are used up to form the product molecules and the reaction stops when all of the reactants are used up
• In reversible reactions, the product molecules can themselves react with each other or decompose and form the reactant molecules again
• It is said that the reaction can occur in both directions: the forward reaction (which forms the products) and the reverse direction (which forms the reactants)
• When writing chemical equations for reversible reactions, two arrows are used to indicate the forward and reverse reactions
• Each one is drawn with just half an arrowhead – the top one points to the right, and the bottom one points to the left: ⇌ 
• An example is, the reaction for the Haber process which produces ammonia from nitrogen and hydrogen 

N₂ + 3H₂ ⇌ 2NH₃

 

• If the forward reaction is exothermic, then the reverse reaction will be endothermic 
  • The same amount of heat is transferred in both directions
• A good example of this is the hydration of anhydrous copper(II) sulfate:
  • When anhydrous copper(II) sulfate crystals are added to water, they turn blue 
    • The forward reaction is exothermic 
  • If the hydrated copper(II) sulfate crystals are then heated, the blue crystals form a white powder 
    • The reverse reaction is endothermic

hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water

Diagram to show the reversible reaction of copper(II) sulfate

The forward reaction is exothermic and the reverse reaction is endothermic