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

  • for example, 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 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 of hydrated salts 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, CuSO45H2O

  • These are usually seen as blue crystals
  • The hydrated salt 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, CuSO45H2O
  • This reaction is highly exothermic
• The forward reaction is exothermic and the reverse reaction is endothermic

  A) CuSO4.5H2O

  B) hydrated copper sulfate

  C) endothermic

  D) CuSO4

  E) anhydrous copper (II) sulfate

  F) 5H2O

  G) water

  H) exothermic

  
• 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, CoCl26H2O

  • These are usually seen as pink crystalsThe hydrated calt 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, CoCl26H2O
  • This reaction is highly exothermic
• The forward reaction is exothermic and the reverse reaction is endothermic

  A) CoCl2.6H2O

  B) Hydrated Cobalt (II) Chloride

  C) endothermic

  D) CoCl2

  E) anhydrous cobalt chloride

  F) 6H2O

  G) water

  H) exothermic

  
• The hydration of CoCl2 and  CuSO4 are chemical tests used to detect the presence of water.

  • You should remember the equations and colour changes:
  • CoCl2 + 6H2O ⇌ CoCl2.6H2O - Blue to pink
  • CuSO4 + 5H2O ⇌ CuSO4.5H2O - White to blue