3.3.1 Reversible Reactions | Edexcel IGCSE Chemistry: Double Science Revision Notes 2019

Reversible Reactions

Some reactions go to completion, where the reactants are used up to form the product molecules and the reaction stops when the reactants have been exhausted
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 opposing arrows are used to indicate the forward and reverse reactions occurring at the same time
Each one is drawn with just half an arrowhead – the top one points to the right, and the bottom one points to the left
The direction a reversible reaction takes can be changed by changing the reaction conditions

NH₄Cl (s) → NH₃(g) + HCl (g)

NH₃(g) + HCl (g) → NH₄Cl (s)

NH₄Cl (s) ⇌ NH₃(g) + HCl (g)

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
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 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
When anhydrous copper(II) sulfate is added to water, it turns blue and heat is given off so the reaction is exothermic
When hydrated 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 equation for the reaction is:

CuSO₄.5H₂O (s) ⇌ CuSO₄ (s) + 5H₂O (l)

Energy Changes & Reversible Reactions, downloadable IGCSE & GCSE Chemistry revision notes

The dehydration of hydrated salts is often a reversible reaction

The reverse reaction may also be called the backwards reaction. A generic reversible reaction is shown as

A + B ⇌ C + D