Reversible reactions

What you'll learn

What a reversible reaction is.
The special symbol used to show a reversible reaction.
How changing conditions (like heating or cooling) can change the direction of the reaction.

The one-way street vs. two-way traffic

Think about frying an egg. Once the egg white turns from clear to white, you can never cool it down to turn it back into a raw, runny egg. Most chemical reactions you have studied so far are like this: they are "one-way streets". The reactants turn into products, and the reaction stops. We write these with a standard single arrow ( $→\rightarrow$ ).

However, some reactions are more like two-way traffic. In these reactions, not only do the reactants turn into products, but the products can also react together to turn back into the original reactants.

Definition

Reversible reaction

A chemical reaction in which the products can react together to produce the original reactants.

The reversible reaction symbol

Because a reversible reaction can go in both directions, a single straight arrow does not tell the whole story. Instead, we use a special double half-arrow symbol: $⇌\rightleftharpoons$ .

If we have two reactants, $A$ and $B$ , reacting to make products $C$ and $D$ , we write it like this:

\rightleftharpoons C + D

This single equation actually describes two separate reactions happening at the same time:

The forward reaction: $\rightarrow C + D$ (reading left to right).
The backward reaction (or reverse reaction): $\rightarrow A + B$ (reading right to left).

Tip

Drawing the symbol

Make sure you use the correct symbol in your exams. It is made of two half-arrows (one pointing right on top, one pointing left on the bottom). Do not draw a standard double-headed arrow ( $↔\leftrightarrow$ ) or an equals sign ( $=$ ).

Changing the direction of a reaction

If a reaction can go both ways, how do we know which way it will actually go? The direction of a reversible reaction can be changed by altering the conditions, such as the temperature.

Key Idea

Controlling the reaction

If the forward reaction is triggered by heating, you can force the backward reaction to happen by cooling. You are in control of the direction by changing the physical conditions.

A classic example: Ammonium chloride

A great example of this is the thermal decomposition of a white solid called ammonium chloride.

If you heat solid ammonium chloride, it breaks down into two colourless gases: ammonia and hydrogen chloride. If you let those two gases cool down, they will immediately react together to re-form the solid white ammonium chloride.

We can write this as a single reversible word equation:

\text{ammonium chloride} \rightleftharpoons \text{ammonia} + \text{hydrogen chloride}

Or, showing the conditions above and below the arrows:

\text{ammonium chloride} \xrightleftharpoons[\text{cool}]{\text{heat}} \text{ammonia} + \text{hydrogen chloride}

You can see this happen in a single test tube. If you place solid ammonium chloride at the bottom of a tube and heat it with a Bunsen burner, the solid breaks down into invisible gases. As those hot gases rise up the tube away from the flame, they cool down. When they hit the cold glass near the top of the tube, they react backwards and re-form as a white crust of solid ammonium chloride.

Test tube showing the reversible reaction of ammonium chloride

Common Mistake

Forgetting state symbols

Even though we didn't use them in the word equation above, exam questions often ask you to spot a reverse reaction based on state changes. For ammonium chloride, the forward reaction turns a solid (s) into gases (g). The backward reaction turns gases (g) back into a solid (s).

Example

Deducing reaction conditions

White anhydrous copper(II) sulfate turns into blue hydrated copper(II) sulfate when water is added. This is a reversible reaction. Deduce how you would turn the blue hydrated crystals back into the white anhydrous powder.

Identify the forward reaction and its condition. The text tells us: $sulfate\text{anhydrous copper(II) sulfate} + \text{water} \rightarrow \text{hydrated copper(II) sulfate}$ . The condition for this forward reaction is adding water.
Identify the reverse reaction. To go backward, we need to turn the blue hydrated copper(II) sulfate into white anhydrous copper(II) sulfate and water.
State the opposite condition needed. Since the forward reaction required adding water to a dry powder at room temperature, the reverse reaction requires removing water. We can drive the water away by heating the blue crystals until the water evaporates.

Exam technique

In the exam

If you are asked to define a reversible reaction, explicitly mention that the products can react to produce the original reactants.
When asked how to reverse a specific reaction, look at the condition given for the forward reaction (e.g. heating) and state the direct opposite (e.g. cooling).
Always check equations for the $⇌\rightleftharpoons$ symbol. It instantly tells you that changing conditions will affect the outcome.

Self review

Check yourself

What symbol is used to represent a reversible reaction?
In the ammonium chloride reaction, what conditions favour the formation of the white solid?
If a reaction forms a gas when heated, what condition might force the gas to turn back into the original solid?

Reversible reactions

What you'll learn

What a reversible reaction is.
The special symbol used to show a reversible reaction.
How changing conditions (like heating or cooling) can change the direction of the reaction.

The one-way street vs. two-way traffic

Definition

Reversible reaction

A chemical reaction in which the products can react together to produce the original reactants.

The reversible reaction symbol

Because a reversible reaction can go in both directions, a single straight arrow does not tell the whole story. Instead, we use a special double half-arrow symbol: $⇌\rightleftharpoons$ .

If we have two reactants, $A$ and $B$ , reacting to make products $C$ and $D$ , we write it like this:

\rightleftharpoons C + D

This single equation actually describes two separate reactions happening at the same time:

The forward reaction: $\rightarrow C + D$ (reading left to right).
The backward reaction (or reverse reaction): $\rightarrow A + B$ (reading right to left).

Tip

Drawing the symbol

Changing the direction of a reaction

If a reaction can go both ways, how do we know which way it will actually go? The direction of a reversible reaction can be changed by altering the conditions, such as the temperature.

Key Idea

Controlling the reaction

If the forward reaction is triggered by heating, you can force the backward reaction to happen by cooling. You are in control of the direction by changing the physical conditions.

A classic example: Ammonium chloride

A great example of this is the thermal decomposition of a white solid called ammonium chloride.

We can write this as a single reversible word equation:

\text{ammonium chloride} \rightleftharpoons \text{ammonia} + \text{hydrogen chloride}

Or, showing the conditions above and below the arrows:

\text{ammonium chloride} \xrightleftharpoons[\text{cool}]{\text{heat}} \text{ammonia} + \text{hydrogen chloride}

Test tube showing the reversible reaction of ammonium chloride

Common Mistake

Forgetting state symbols

Example

Deducing reaction conditions

Identify the forward reaction and its condition. The text tells us: $sulfate\text{anhydrous copper(II) sulfate} + \text{water} \rightarrow \text{hydrated copper(II) sulfate}$ . The condition for this forward reaction is adding water.
Identify the reverse reaction. To go backward, we need to turn the blue hydrated copper(II) sulfate into white anhydrous copper(II) sulfate and water.
State the opposite condition needed. Since the forward reaction required adding water to a dry powder at room temperature, the reverse reaction requires removing water. We can drive the water away by heating the blue crystals until the water evaporates.

Exam technique

In the exam

If you are asked to define a reversible reaction, explicitly mention that the products can react to produce the original reactants.
When asked how to reverse a specific reaction, look at the condition given for the forward reaction (e.g. heating) and state the direct opposite (e.g. cooling).
Always check equations for the $⇌\rightleftharpoons$ symbol. It instantly tells you that changing conditions will affect the outcome.

Self review

Check yourself

What symbol is used to represent a reversible reaction?
In the ammonium chloride reaction, what conditions favour the formation of the white solid?
If a reaction forms a gas when heated, what condition might force the gas to turn back into the original solid?

Reversible reactions

What you'll learn

The one-way street vs. two-way traffic

The reversible reaction symbol

Changing the direction of a reaction

A classic example: Ammonium chloride

You've reached the end

Reversible reactions

What you'll learn

The one-way street vs. two-way traffic

The reversible reaction symbol

Changing the direction of a reaction

A classic example: Ammonium chloride

You've reached the end

Reversible reactions

What you'll learn

The one-way street vs. two-way traffic

The reversible reaction symbol

Changing the direction of a reaction

A classic example: Ammonium chloride

You've reached the end

Reversible reactions Revision Guide

Reversible reactions

What you'll learn

The one-way street vs. two-way traffic

The reversible reaction symbol

Changing the direction of a reaction

A classic example: Ammonium chloride

You've reached the end

Reversible reactions Revision Guide