Reversible Reactions (WJEC GCSE Chemistry)
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
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 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: ⇌
An example is, the reaction for the Haber process which produces ammonia from nitrogen and hydrogen
N2 + 3H2 ⇌ 2NH3
If the forward reaction is exothermic, then the reverse reaction will be endothermic
The same amount of heat is transferred in both directions
A good example of this is the hydration of anhydrous copper(II) sulfate:
When anhydrous copper(II) sulfate crystals are added to water, they turn blue
The forward reaction is exothermic
If the hydrated copper(II) sulfate crystals are then heated, the blue crystals form a white powder
The reverse reaction is endothermic
hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water
Diagram to show the reversible reaction of copper(II) sulfate
The forward reaction is exothermic and the reverse reaction is endothermic
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