Reversible Reactions

Not all chemical reactions go to completion. Some reactions can proceed in both directions — the products can react to reform the reactants. These are called reversible reactions, and they are an important part of the AQA GCSE Chemistry specification. This lesson explains what reversible reactions are, how to recognise them, and the key energy changes involved.

What Is a Reversible Reaction?

A reversible reaction is a reaction that can proceed in both the forward and backward (reverse) directions.

In a reversible reaction:

The forward reaction converts reactants into products
The backward (reverse) reaction converts products back into reactants

The symbol for a reversible reaction is a double-headed arrow: A + B <=> C + D

This replaces the single arrow (-->) used for irreversible reactions.

Type of Reaction	Symbol	Description
Irreversible	-->	Reaction goes to completion; reactants fully converted to products
Reversible	<=>	Reaction can go in both directions; does not go to completion

Exam Tip: In AQA GCSE Chemistry, the reversible reaction symbol is two half-arrows pointing in opposite directions (one above the other). Make sure you draw this correctly in exams — do NOT draw a double-headed arrow (<-->) as this means something different in other contexts. Use the equilibrium symbol.

Examples of Reversible Reactions

Example 1: Heating Ammonium Chloride

When ammonium chloride (NH4Cl) is heated, it decomposes into ammonia gas (NH3) and hydrogen chloride gas (HCl):

NH4Cl(s) <=> NH3(g) + HCl(g)

Forward reaction (heating): The white solid breaks down into two colourless gases
Reverse reaction (cooling): When the gases cool, they recombine to form the white solid again

You can observe this in a test tube: when heated, the white solid at the bottom disappears and reforms as a white solid higher up the tube where the gases have cooled.

Example 2: Hydrated and Anhydrous Copper Sulfate

Hydrated copper sulfate (CuSO4.5H2O) is a blue crystalline solid. When heated, it loses its water of crystallisation and becomes anhydrous copper sulfate (CuSO4), a white powder:

CuSO4.5H2O(s) <=> CuSO4(s) + 5H2O(l)

Forward reaction (heating): Blue crystals turn to white powder as water is driven off
Reverse reaction (adding water): Adding water to the white powder turns it blue again and releases heat

graph LR
    A[Hydrated copper sulfate - BLUE] -->|Heat - endothermic| B[Anhydrous copper sulfate - WHITE + Water]
    B -->|Add water - exothermic| A

This reaction is used as a test for water: if anhydrous copper sulfate turns from white to blue, water is present.

Exam Tip: The copper sulfate example is one of the most commonly examined reversible reactions in AQA GCSE Chemistry. You must know the colour change (blue to white and back to blue) and that adding water to anhydrous copper sulfate is exothermic while heating hydrated copper sulfate is endothermic.

Energy Changes in Reversible Reactions

A crucial principle for reversible reactions is:

If the forward reaction is exothermic, the reverse reaction is endothermic — and by the same amount of energy.

If the forward reaction is endothermic, the reverse reaction is exothermic — and by the same amount of energy.

The energy transferred in the forward direction is exactly equal to the energy transferred in the reverse direction.

Forward Reaction	Reverse Reaction	Energy Relationship
Exothermic (releases energy)	Endothermic (absorbs energy)	Same amount of energy
Endothermic (absorbs energy)	Exothermic (releases energy)	Same amount of energy

Example: Copper Sulfate

Direction	Process	Energy Change
Forward	CuSO4.5H2O --> CuSO4 + 5H2O	Endothermic (energy is taken in to break bonds)
Reverse	CuSO4 + 5H2O --> CuSO4.5H2O	Exothermic (energy is released as bonds form)

The energy taken in during the forward reaction equals the energy released during the reverse reaction.

graph TD
    A[Reversible Reaction Energy Rule] --> B[Forward reaction: Exothermic]
    A --> C[Forward reaction: Endothermic]
    B --> D[Reverse reaction: Endothermic by same amount]
    C --> E[Reverse reaction: Exothermic by same amount]

Why Do Reversible Reactions Not Go to Completion?

In an irreversible reaction, the reactants are completely converted to products. The reaction stops when one of the reactants runs out.

In a reversible reaction, as products form, they immediately begin to react to reform the reactants. This means:

The reactants are never fully converted to products
There is always a mixture of reactants and products present
The reaction does not "stop" — both the forward and reverse reactions continue to occur

This leads to the concept of equilibrium, which you will study in the next lesson.

Identifying Reversible Reactions

How can you tell if a reaction is reversible?

Clue	Example
The question uses the equilibrium symbol (<=> or reversible arrows)	N2 + 3H2 <=> 2NH3
The question mentions "reversible" or "equilibrium"	"In a reversible reaction..."
The question describes forward and backward reactions	"When heated... when cooled..."
Products can be changed back to reactants by changing conditions	Hydrated to anhydrous copper sulfate and back

Exam Tip: Not all reactions are reversible. Combustion reactions, for example, are irreversible — you cannot easily convert carbon dioxide and water back into a hydrocarbon and oxygen. If a question uses a single arrow (-->), the reaction is irreversible. If it uses the equilibrium symbol, it is reversible.