GCSE · Chemistry · AQA · Spec 8462

Reversible reactions

An ordinary arrow runs one way. The ⇌ sign says more: the products can turn round and react to give back what you started with.

One reaction, two directions

Which way will A + B ⇌ C + D go?

Walk one branch to its end. Then use ‹ Back one branch and walk the other. Watch what reacts, and what you finish with, each time.

The conditions → What reacts → What forms

The ⇌ sign means this reaction can go in either direction. Pick a branch to send it one way.

2 directions this one reaction can go.

On A + B ⇌ C + D. 2 branches to choose from.

Reading the equation

Four parts of A + B ⇌ C + D

Showing 1 layer: Equation

Explore

Select a part of the equation: the left side, the right side, or one of the two half-arrows.

What does ‘reversible’ really claim?

Which idea is yours?

You have sent A + B ⇌ C + D both ways. Now pin down exactly what the word ‘reversible’ is saying.

Which is closest to what you think right now?

Every chemical reaction can go backwards. ‘Reversible’ is just a name for all of them.In reverse, C and D react to make new, different substances.The equation fixes the direction. Once it is written down, nothing can change which way the reaction goes.In some reactions the products can react to give back the original reactants, and changing the conditions can change which way the reaction goes.

How sure are you?

I'm sureI think soI'm guessing

Every chemical reaction can go backwards. ‘Reversible’ is just a name for all of them.

This is a common misconception

Not every reaction. Reversible reactions are the ones in which the products can react to produce the original reactants, and that is true of some chemical reactions, not all of them. The ⇌ sign is what marks a reaction out as reversible.

In reverse, C and D react to make new, different substances.

This is a common misconception

The reverse direction gives back A and B, the original reactants, not something new. It is the same four substances in the same equation, just going the other way: C and D react to re-form A and B.

The equation fixes the direction. Once it is written down, nothing can change which way the reaction goes.

This is a common misconception

The ⇌ does not choose a direction. It says the reaction can go either way. What can change the direction of a reversible reaction is a change in the conditions, not the way the equation is written.

In some reactions the products can react to give back the original reactants, and changing the conditions can change which way the reaction goes.

This matches the evidence

Exactly right. Only some reactions do this, the reverse direction re-forms the original reactants, and changing the conditions can change the direction. That is the full meaning of ⇌.

Reversible means: in some reactions the products can react to re-form the original reactants, and changing the conditions can change the direction.

Now write it yourself

Say what ⇌ means, in full

A reaction is represented by the equation A + B ⇌ C + D. Explain what the ⇌ symbol tells you about this reaction, and how the direction of the reaction can be changed. [3 marks]

Your answer

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The mark scheme

It is a reversible reaction: it can go in both directions (forwards and in reverse). (1 mark)
The products (C and D) can react to produce the original reactants (A and B). (1 mark)
The direction can be changed by changing the conditions. (1 mark)

Model answer

The ⇌ symbol shows that this is a reversible reaction, so it can go in both directions. In the forward direction A and B react to form C and D, and the products C and D can react to produce the original reactants A and B. The direction of the reaction can be changed by changing the conditions.

Three points, three marks: both directions, the products re-form the original reactants, and changing the conditions can change the direction.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

In some reactions the products are not the end of the story: they can react to give back the substances the reaction started from.

What you need to know

In some chemical reactions, the products can react to produce the original reactants. These are called reversible reactions.
A reversible reaction is represented as A + B ⇌ C + D: A and B are the reactants on the left, and C and D are the products on the right.
The ⇌ symbol shows the reaction can go in both directions: forwards (A and B react to form C and D) and in reverse (C and D react to re-form A and B).
The direction of a reversible reaction can be changed by changing the conditions.

The big picture

In some chemical reactions, the products of the reaction can react to produce the original reactants. These are called reversible reactions, and they are represented as A + B ⇌ C + D. In the forward direction A and B react to form C and D. In the reverse direction C and D react to re-form A and B. The direction of a reversible reaction can be changed by changing the conditions.

Key points

1Reversible: the products can react to give back the ORIGINAL reactants.

2Not every reaction is reversible. The ⇌ sign marks the ones that are.

3Forward: A + B → C + D. Reverse: C + D → A + B.

4Change the conditions and you can change the direction.

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Worked example

Problem

Substances E and F react to form G and H. When the conditions are changed, G and H react to form E and F again. (a) Write the equation for this reaction using the correct symbol. (b) Name the products of the forward reaction. (c) What can be done to make G and H react?

⚠ Watch out

Writing that in reverse the products ‘make something else’, or that the reaction just ‘goes back’ without saying what reacts. Be precise: in the reverse direction the products (C and D) react to re-form the original reactants (A and B).

🧠

Memory hook

⇌ is a two-way road, and the conditions are the signpost: the same A, B, C and D, sent one way or the other.

✓

Check yourself

Without looking back: write the general equation for a reversible reaction. Under it, say what reacts and what forms in each direction, and what can switch the reaction from one direction to the other.

Flashcards

(7)

What is a reversible reaction?

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

Which symbol shows that a reaction is reversible?

⇌: two half-arrows pointing in opposite directions.

In A + B ⇌ C + D, which substances are the reactants and which are the products?

Reactants: A and B (left side). Products: C and D (right side).

What happens in the forward direction of A + B ⇌ C + D?

A and B react to form C and D.

What happens in the reverse direction of A + B ⇌ C + D?

C and D react to re-form A and B, the original reactants.

How can the direction of a reversible reaction be changed?

By changing the conditions.

Are all chemical reactions reversible?

No. Only some reactions are: those whose products can react to produce the original reactants.

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Which way will A + B ⇌ C + D go?

Which idea is yours?

Say what ⇌ means, in full

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