Reversible Reactions

In many reactions you meet at school, the change feels one-way:

Reactants → Products (and that’s it)

But some reactions can go in both directions. These are called reversible reactions.

Definition

A reversible reaction is one where the substances produced can change back into the original substances.

We show this using a double arrow:

That symbol (⇌) tells you that:

• The forward reaction (left → right) and

• The reverse reaction (right → left)

can both happen.

How Reversible Reactions Are Written

We normally write two separate equations to show the two directions:

• Forward reaction:

  Reactants → Products

• Reverse reaction:

  Products → Reactants

Then we combine them into one line with a double arrow:

This reminds you that neither direction is “more real” than the other — they’re both valid changes.

Closed Systems

A closed system is essential for studying reversible reactions properly.

A closed system is a situation where:

• No chemicals can enter or leave the container

• Nothing is added or removed once the reaction starts

In practice, this usually means:

• A sealed tube

• A stoppered flask

• A reaction vessel that doesn’t let gas escape

If particles can escape, the reverse reaction might not be able to happen fully, and the reaction will behave more like a one-way change.

Why reversible reactions need a closed system

In a closed system:

• Reactants and products stay together

• Both the forward and reverse processes can keep happening

• The system can reach a state where both directions occur at the same rate

  (this idea leads into dynamic equilibrium, which you usually study next)

If the system is not closed:

• A gas might escape

• A solid might be removed

• One of the substances might be lost to the surroundings

Then the change becomes effectively irreversible, because the particles needed to go backwards are no longer all there.

A Classic Example: NO₂ ⇌ N₂O₄

One of the best reversible reactions to visualise is the equilibrium between nitrogen dioxide and dinitrogen tetroxide.

• NO₂(g) is brown

• N₂O₄(g) is colourless or very pale

Because these gases have different colours, you can see the effect of the reaction shifting one way or the other.

This diagram illustrates the experimental setup for paper chromatography, showing how a pencil start line and specific ink spots are positioned in a beaker of solvent to separate different pigments based on their solubility.

The forward reaction (to the right)

• Two NO₂ molecules join together

• They form one N₂O₄ molecule

• The gas becomes less brown (more colourless)

• This direction is exothermic (releases energy to the surroundings)

The reverse reaction (to the left)

• One N₂O₄ molecule splits into two NO₂ molecules

• The gas becomes more brown

• This direction is endothermic (absorbs energy from the surroundings)

Hydrated Copper(II) Sulfate

Another reversible reaction we should be familiar with involves hydrated copper sulfate and anhydrous copper sulfate.

• CuSO₄ 5H₂O(s) = blue hydrated copper(II) sulfate

• CuSO₄(s) = white anhydrous copper(II) sulfate

Forward direction (heating)

• Heat drives out water

• Blue crystals turn white

• This is usually treated as endothermic

Reverse direction (adding water)

• Adding water reforms the blue crystals

• The change releases energy (feels warm)

• This is exothermic

Recall

Application

Challenge (HT)