Energy Changes in Reversible Reactions
Practise Energy Changes in Reversible Reactions for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. If a reversible reaction is exothermic in one direction, it is endothermic by exactly the same amount in the other direction.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
In a reversible reaction, if the forward reaction is exothermic then the reverse reaction is endothermic, and vice versa. The same amount of energy is transferred in each case.
The standard example is hydrated copper sulfate. Heating blue hydrated copper sulfate drives off water to give white anhydrous copper sulfate, and this is endothermic. Adding water to the white solid turns it blue again and releases energy, so the reverse is exothermic.
This provides the chemical test for water: anhydrous copper sulfate turns from white to blue when water is added. The energy transferred in each direction is equal in magnitude but opposite in direction.
Revision notes
The energy relationship
If the forward reaction is exothermic, the reverse reaction is endothermic.
The same amount of energy is transferred in each direction — it is released one way and absorbed the other.
Hydrated copper sulfate
Blue hydrated copper sulfate, heated, gives white anhydrous copper sulfate plus water. This is endothermic.
Adding water to the white anhydrous solid turns it blue again and releases energy, so the reverse reaction is exothermic. The reaction is reversible.
Testing for water
Anhydrous copper sulfate turning from white to blue is the chemical test for water.
Anhydrous cobalt chloride paper turning from blue to pink is an alternative. Both rely on this reversible hydration reaction.
Key points
- If forward is exothermic, reverse is endothermic.
- The same amount of energy is transferred each way.
- Hydrated copper sulfate is blue.
- Anhydrous copper sulfate is white.
- Heating hydrated copper sulfate is endothermic.
- White to blue is the test for water.
Worked examples
Example 1
State the colour change when water is added to anhydrous copper sulfate. [1 mark]
Model answer
It changes from white to blue (1 mark).
Example 2
Explain the energy changes in the reversible reaction of copper sulfate. [3 marks]
Model answer
Heating hydrated copper sulfate to drive off water is endothermic, so energy is taken in (1 mark). Adding water to the anhydrous solid is exothermic, so energy is released (1 mark). The same amount of energy is transferred in each direction (1 mark).
Example 3
Explain why anhydrous copper sulfate can be used to test for water. [2 marks]
Model answer
It reacts with water in a reversible reaction (1 mark), producing a distinctive colour change from white to blue (1 mark).
Common mistakes
Getting the colours the wrong way round.
Hydrated is blue; anhydrous is white. Water turns white to blue.
Saying both directions are exothermic.
If one is exothermic the other must be endothermic.
Saying different amounts of energy are transferred.
The magnitude is the same in each direction.
Confusing hydrated and anhydrous.
Hydrated contains water; anhydrous means without water.
Exam tips
- Learn hydrated as blue and anhydrous as white.
- State that the same amount of energy is transferred each way.
- Give the colour change in the right direction for the water test.
- Use the terms hydrated and anhydrous precisely.
Key terms
- Hydrated
- Containing water of crystallisation, blue for copper sulfate.
- Anhydrous
- Without water, white for copper sulfate.
- Reversible
- Able to proceed in both directions.
- Test for water
- White anhydrous copper sulfate turning blue.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.