Reversible reactions Revision Notes for AQA GCSE Chemistry Combined Science

Reversible reactions

What are reversible reactions?

Some chemical reactions can go both ways. This means that the products can react together to make the original reactants again. We call these reversible reactions.

Most reactions only go one way - they are irreversible. The products cannot easily change back into the reactants.

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The key difference between reversible and irreversible reactions lies in whether the products can easily reform the original reactants. In everyday life, most reactions we observe (like burning paper or cooking an egg) are irreversible.

How to write reversible reaction equations

We use different arrows to show different types of reactions:

Normal reactions use a single arrow: $A + B \rightarrow C + D$
Reversible reactions use a split arrow: $A + B \rightleftharpoons C + D$

The split arrow (⇌) shows that the reaction can go in both directions.

Example of a reversible reaction

Ammonium chloride gives us a great example of a reversible reaction.

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Worked Example: Ammonium Chloride Decomposition

When you heat ammonium chloride:

It breaks down into ammonia gas and hydrogen chloride gas
The equation is: $NH_4Cl \rightarrow NH_3 + HCl$

When the gases cool down:

They react together to form ammonium chloride again
The equation is: $NH_3 + HCl \rightarrow NH_4Cl$

Overall reaction: $NH_4Cl \rightleftharpoons NH_3 + HCl$

You can see this happen using mineral wool in a test tube. When you heat one end, the ammonium chloride disappears. When it cools, white solid forms again where the tube is coolest.

Energy changes in reversible reactions

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Critical Energy Rule for Reversible Reactions:

If the forward reaction releases energy (exothermic), then the backward reaction absorbs energy (endothermic)
If the forward reaction absorbs energy (endothermic), then the backward reaction releases energy (exothermic)

The same amount of energy is involved in both directions - just in opposite ways.

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Worked Example: Copper Sulphate Changes

Blue hydrated copper sulphate ⇌ white anhydrous copper sulphate + water
Removing water (forwards) = endothermic (absorbs energy)
Adding water back (backwards) = exothermic (releases energy)

Temperature effects on reactions

Temperature affects which direction a reversible reaction prefers by influencing the equilibrium position.

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Worked Example: Making Ammonia

$N_2 + 3H_2 \rightleftharpoons 2NH_3$

Lower temperatures = more ammonia produced
Higher temperatures = less ammonia produced

This happens because the forwards reaction (making ammonia) releases heat. At high temperatures, the reaction shifts backwards to absorb some of that heat.

Real-world applications

The copper sulphate test for water uses a reversible reaction and demonstrates these principles in practice.

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The Copper Sulphate Water Test:

Blue copper sulphate contains water
When heated, it turns white (water removed)
When water is added back, it turns blue again
This proves water was present in the original sample

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Key Points to Remember:

Reversible reactions can go both ways - shown with a split arrow (⇌)
Products can change back into the original reactants
Energy changes are opposite in each direction - same amount involved
Temperature affects which direction the reaction prefers
The ammonium chloride and copper sulphate examples show reversible reactions in action

Reversible reactions (AQA GCSE Chemistry Combined Science): Revision Notes