Reversible Reactions (CIE IGCSE Chemistry)

• 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 forming the products
  • The reverse reaction forming the reactants
• If the forward reaction is exothermic, then the reverse reaction will be endothermic 
  • The same amount of heat is transferred in both directions

Chemical equations for reversible reactions

• When writing chemical equations for reversible reactions, the following symbol is used:

 

• An example is, the reaction for the Haber process which produces ammonia from nitrogen and hydrogen 

N₂ + 3H₂ ⇌ 2NH₃ 

• • The forward reaction, producing ammonia, is endothermic
  • So, the reverse reaction is exothermic

Hydrated and anhydrous salts

• Hydrated salts are salts that contain water of crystallisation which affects their molecular shape and colour
  • Water of crystallisation is the water that is stoichiometrically included in the structure of some salts during the crystallisation process
• One example is copper(II) sulfate:

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

CuSO₄•5H₂O ⇌ CuSO₄ + 5H₂O

• • The hydrated salt is copper(II) sulfate pentahydrate, CuSO₄•5H₂O
    • These are usually seen as blue crystals
    • The hydrated calt can be heated / dehydrated to form anhydrous copper(II) sulfate, CuSO₄ 
    • This reaction is endothermic as energy is taken in to remove the water
  • The anhydrous salt is copper(II) sulfate
    • This is usually seen as white crystals / powder
    • Adding water to the anhydrous salt forms the hydrated copper(II) sulfate pentahydrate, CuSO₄•5H₂O
    • This reaction is highly exothermic

The forward reaction is exothermic and the reverse reaction is endothermic

• Another example is cobalt(II) chloride:

hydrated cobalt(II) chloride ⇌ anhydrous cobalt(II) chloride + water

CoCl₂•6H₂O ⇌ CoCl₂ + 6H₂O

• • The hydrated salt is cobalt(II) chloride hexahydrate, CoCl₂•6H₂O
    • These are usually seen as pink crystals
    • The hydrated salt can be heated / dehydrated to form anhydrous cobalt(II) chloride, CoCl₂ 
    • This reaction is endothermic as energy is taken in to remove the water
  • The anhydrous salt is cobalt(II) chloride, CoCl₂
    
    • This is usually seen as blue crystals 
    • Adding water to the anhydrous salt forms the hydrated cobalt(II) chloride hexahydrate, CoCl₂•6H₂O
    • This reaction is highly exothermic

The forward reaction is exothermic and the reverse reaction is endothermic