CIE IGCSE Chemistry

Revision Notes

7.2.1 Reversible Reactions

Reversible Reactions

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)

 

Chemical equations for reversible reactions

  • 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

Example

  • The reaction for the Haber Process which is the production of ammonia from hydrogen and nitrogen:
N2 + 3H2 ⇌ 2NH3

 

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
  • A common example is copper(II) sulfate which crystallises forming the salt copper(II) sulfate pentahydrate, CuSO4.5H2O
  • Water of crystallisation is indicated with a dot written in between the salt molecule and the surrounding water molecules
  • Anhydrous salts are those that have lost their water of crystallisation, usually by heating, in which the salt becomes dehydrated

 

Dehydration of Hydrated Copper (II) Sulfate:

Hydrated Copper (II) Sulfate ⇌ Anhydrous Copper (II) Sulfate     +     Water

 

Dehydration-of-hydrated-Copper-II-Sulfate, IGCSE & GCSE Chemistry revision notesDiagram showing the dehydration of Hydrated Copper (II) Sulfate

 

Explanation:

  • When anhydrous copper (II) sulfate crystals are added to water they turn blue and heat is given off (exothermic); this reaction is reversible
  • When Copper (II) Sulfate crystals are heated in a test tube, the blue crystals turn into a white powder and a clear, colourless liquid (water) collects at the top of the test tube
  • The form of Copper (II) Sulfate in the crystals is known as Hydrated Copper (II) Sulfate because it contains water of crystallisation
  • When Hydrated Copper (II) Sulfate is heated, it loses its water of crystallisation and turns into anhydrous Copper (II) Sulfate:
CuSO4.5H2O (s) ⇌ CuSO4 (s) + 5H2O (l)

 

Dehydration of Hydrated Cobalt (II) Chloride:

Hydrated Cobalt (II) Chloride ⇌ Anhydrous Cobalt (II) Chloride + Water

 

Dehydration-of-Hydrated-Cobalt-II-Chloride, IGCSE & GCSE Chemistry revision notesDiagram showing the dehydration of Hydrated Cobalt (II) Chloride 

 

Hydration of Cobalt(II) Chloride

  • When anhydrous blue cobalt(II) chloride crystals are added to water they turn pink and the reaction is reversible
  • When the cobalt(II) chloride crystals are heated in a test tube, the pink crystals turn back to the blue colour again as the water of crystallisation is lost
  • The form of cobalt(II) chloride in the crystals that are pink is known as hydrated cobalt (II) chloride because it contains water of crystallisation
  • When hydrated cobalt(II) chloride is heated, it loses its water of crystallisation and turns into anhydrous cobalt(II) chloride:
CoCl2.6H2O (s) ⇌ CoCl2 (s) + 6H2O (l)

Exam Tip

Both the hydration of CoCl2 and  CuSO4 are chemical tests which are commonly 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

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.
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