Reversible Reactions (AQA GCSE Combined Science: Trilogy): Revision Note

Reversible reactions

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

A + B → C + D

• 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)

 A + B → C + D

• The reverse direction (which forms the reactants)

C + D → A + B

• 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: ⇌

A + B  C + D

• The direction a reversible reaction takes can be changed by changing the temperature

Reversible reaction example

• Heating ammonium chloride produces ammonia and hydrogen chloride gases

  • This is an endothermic process as energy is provided for the reaction

NH₄Cl (s) → NH₃ (g) + HCl (g) 

• As the hot gases cool down they recombine to form solid ammonium chloride

  • This is an exothermic process

NH₃ (g) + HCl (g) → NH₄Cl (s)  

• So, the reversible reaction is represented like this:

NH₄Cl (s) ⇌ NH₃ (g) + HCl (g)

Energy changes & reversible reactions

• Energy changes also accompany chemical changes and energy can be given out (exothermic) or taken in (endothermic)

• The majority of chemical reactions are exothermic with only a small number being endothermic

• For a reversible reaction, if it is exothermic in one direction then it must be endothermic in the opposite direction

• The amount of energy transferred in either direction is the same

• Reversible reactions can be seen in some hydrated salts

• These are salts that contain water of crystallisation which affects their shape and colour

  • Water of crystallisation is the water that is included in the structure of some salts during the crystallisation process

  • Water of crystallisation is indicated with a dot written in between the salt 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

Reversible reaction example

• A common example is copper(II) sulfate which crystallises forming the salt copper(II) sulfate pentahydrate, CuSO₄.5H₂O

• If hydrated copper(II) sulfate crystals are heated, the blue crystals turn into a white powder and a clear, colourless liquid (water) collects at the top of the test tube 

  • This forward reaction is endothermic

• When anhydrous copper(II) sulfate crystals are then added to water, the crystals turns blue and heat is given off 

  • This backward / reverse reaction is exothermic 

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