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.

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