Revision notes for AQA GCSE Chemistry Energy changes and reversible reactions. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.
Energy changes and reversible reactions
What you'll learn
- What a reversible reaction is, and how to recognise the
⇌symbol. - How exothermic and endothermic energy changes compare.
- Why the reverse direction of a reversible reaction has the opposite energy change.
- How to apply this to the copper sulfate example from the GCSE specification.
Starting point: reactions can transfer energy
Chemical reactions usually involve an energy transfer between the chemicals and their surroundings.
The surroundings means everything outside the reacting chemicals — for example, the test tube, thermometer, air, or your hand holding the beaker.
If a reaction warms the surroundings, energy has been transferred to the surroundings. If a reaction cools the surroundings, energy has been transferred from the surroundings into the chemicals.
Definition
Exothermic and endothermic
An exothermic reaction transfers energy to the surroundings, usually causing a temperature increase. An endothermic reaction takes in energy from the surroundings, usually causing a temperature decrease.
For GCSE, you do not need to explain this using entropy or advanced thermodynamics. Keep the idea simple: energy out = exothermic; energy in = endothermic.
Reversible reactions
Some reactions can go both ways. The reactants can form products, and the products can react to reform the original reactants.
Definition
Reversible reaction
A reversible reaction is a reaction where the products can react to form the original reactants again. It is shown using the reversible reaction symbol ⇌.
For example, instead of writing:
reactants → products
we write:
reactants ⇌ products
The forward direction is usually read from left to right. The reverse direction is read from right to left.
Tip
Reading the reversible arrow
In A ⇌ B, the forward reaction is A → B, and the reverse reaction is B → A. The same reaction is being described, just in opposite directions.
The key energy rule for reversible reactions
Here is the main idea in this specification point.
Key Idea
Reverse the reaction, reverse the energy change
If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction. The same amount of energy is transferred, but in the opposite direction.
So if the forward reaction releases energy, the reverse reaction must take in that same amount of energy.
Likewise, if the forward reaction takes in energy, the reverse reaction must release that same amount of energy.
This makes sense because the reverse reaction is undoing the chemical change. The energy change is also “undone”.
Thinking about the same amount of energy
Imagine this reversible reaction:
A ⇌ B
If changing A into B releases 40 kJ of energy, then changing B back into A requires 40 kJ of energy to be taken in.
The amount is the same: 40 kJ.
The direction is different:
- A → B: energy released, so exothermic
- B → A: energy taken in, so endothermic
Common Mistake
Calling both directions exothermic
Do not say that both directions are exothermic because “energy is involved”. In a reversible reaction, the two directions have opposite energy changes: one direction is exothermic and the other is endothermic.
Example
Finding the energy change for the reverse reaction
A reversible reaction releases 25 kJ of energy in the forward direction:
A → B
What is the energy change when B reacts to form A?
-
The forward reaction releases energy, so A → B is exothermic.
-
The reverse reaction must have the opposite type of energy change, so B → A is endothermic.
-
The amount of energy transferred is the same, so the reverse reaction takes in 25 kJ of energy.
Copper sulfate: the GCSE example
The specification uses hydrated copper sulfate as the key example.
Hydrated copper sulfate is blue. When it is heated, it loses water and becomes anhydrous copper sulfate, which is white.
Definition
Hydrated and anhydrous
A hydrated substance contains water chemically joined within its crystal structure. An anhydrous substance has had that water removed.
The reaction can be represented as:
CuSO₄·5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(l)
In words:
hydrated copper sulfate ⇌ anhydrous copper sulfate + water
The colour change is very useful:
- hydrated copper sulfate: blue
- anhydrous copper sulfate: white
Heating hydrated copper sulfate
When blue hydrated copper sulfate is heated, energy is supplied to the substance. The water is driven off, leaving white anhydrous copper sulfate.
So this direction is endothermic.
hydrated copper sulfate → anhydrous copper sulfate + water
CuSO₄·5H₂O(s) → CuSO₄(s) + 5H₂O(l)
The key observation is the colour change from blue to white.
Tip
Link the condition to the energy change
If the reaction needs continuous heating to happen, that is a strong clue that the direction is endothermic.
Adding water to anhydrous copper sulfate
If water is added back to white anhydrous copper sulfate, it becomes blue hydrated copper sulfate again.
This reverse direction is exothermic.
anhydrous copper sulfate + water → hydrated copper sulfate
CuSO₄(s) + 5H₂O(l) → CuSO₄·5H₂O(s)
Energy is released to the surroundings. In a practical, the solid may feel warm, because the surroundings gain energy.
Example
Identifying the energy change in copper sulfate
Blue hydrated copper sulfate is heated and turns white. Later, water is added to the white solid and it turns blue again. Identify which direction is endothermic and which is exothermic.
-
Heating blue hydrated copper sulfate makes it lose water, so the direction hydrated copper sulfate → anhydrous copper sulfate + water needs energy supplied.
-
A reaction that takes in energy from heating is endothermic, so the blue-to-white change is endothermic.
-
The reverse reaction forms hydrated copper sulfate again, so anhydrous copper sulfate + water → hydrated copper sulfate must be exothermic.
Why the energy amount is the same
The forward and reverse reactions involve the same substances, just swapped around.
If one direction needs energy to separate water from the hydrated crystal, the reverse direction releases energy when that water becomes part of the crystal again.
The energy transfer is equal in size because the reverse reaction undoes the forward reaction.
For example, if removing the water from one mole of hydrated copper sulfate required 80 kJ, then adding the water back to form one mole of hydrated copper sulfate would release 80 kJ.
Example
Using the same energy transfer in the opposite direction
For a reversible reaction, the forward reaction is endothermic and takes in 92 kJ of energy. What happens in the reverse reaction?
-
The forward direction is endothermic, so energy is taken in by the reacting chemicals.
-
The reverse direction must have the opposite energy change, so it is exothermic.
-
The same amount of energy is transferred, so the reverse reaction releases 92 kJ of energy.
Linking this to equilibrium
Later in this topic, you will meet dynamic equilibrium. You do not need the full details for this section, but the energy rule still matters.
Definition
Dynamic equilibrium
In a closed system, dynamic equilibrium happens when the forward and reverse reactions are taking place at the same rate, so the amounts of reactants and products stay constant.
At equilibrium, both directions are still happening. If one direction is exothermic, the other is endothermic. This becomes important when thinking about how changing temperature affects reversible reactions.
Exam wording to watch for
Questions may describe the reaction in words rather than giving you a full equation.
They might say:
- “hydrated copper sulfate is heated”
- “water is added to anhydrous copper sulfate”
- “the forward reaction is exothermic”
- “the reverse reaction requires energy”
Your job is to connect the direction of the reaction to the energy change.
Common Mistake
Forgetting the direction
“Hydrated copper sulfate is endothermic” is not precise enough. Say the reaction from hydrated copper sulfate to anhydrous copper sulfate and water is endothermic.
Exam technique
In the exam
-
Identify the direction being described: forward or reverse.
-
Decide whether that direction transfers energy to the surroundings (exothermic) or takes energy from the surroundings (endothermic).
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For the opposite direction, keep the energy amount the same but swap the type of energy change.
Self review
Check yourself
-
If a reversible reaction is exothermic from left to right, what is it from right to left?
-
What colour change happens when hydrated copper sulfate is heated?
-
Why is the energy transferred in the reverse reaction the same amount as in the forward reaction?
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