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If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction, transferring the same amount of energy. Free notes.
Energy Changes in Reversible Reactions is part of Reversible reactions and dynamic equilibrium in GCSE Edexcel Higher Chemistry.
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If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction, transferring the same amount of energy. Free notes.
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Use this lesson to stay inside Reversible reactions and dynamic equilibrium while connecting the detail back to the wider Topic 4: Extracting metals and equilibria topic.
Read the core explanations from Energy Changes in Reversible Reactions before testing yourself from memory.
Here's something that catches students off guard: when a reaction can go both ways, what happens to the energy? If breaking bonds requires energy and making bonds releases it, how does this work when the same reaction runs forwards and backwards?
The answer reveals one of chemistry's most elegant principles. Energy changes in reversible reactions follow a strict rule that makes perfect sense once you see it.
Picture energy like money in a bank account. If you deposit £50, you can withdraw exactly £50 later - no more, no less. Reversible reactions work the same way.
This means if the forward reaction releases 100 kJ of energy, the reverse reaction must absorb exactly 100 kJ. Energy cannot appear from nowhere or disappear into nothing.
Let's examine the classic example that appears in every GCSE exam. The formation and decomposition of ammonium chloride shows this principle perfectly.
Worked example: Energy Analysis of Ammonium Chloride Formation
Analyse the energy changes in: NH₃(g) + HCl(g) ⇌ NH₄Cl(s)
Answer: Forward: exothermic. Reverse: endothermic. Same energy magnitude in both directions.
You can observe this in the lab. When you mix ammonia and hydrogen chloride gases, the tube gets warm (exothermic). Heat the solid ammonium chloride and it decomposes, absorbing exactly the same amount of energy (endothermic).
Energy profile diagrams make this relationship crystal clear. They show how energy changes as reactants become products, revealing why the energy changes must be equal and opposite.
Think of climbing a hill. If you start at the bottom and climb 100 metres up, you've gained 100 metres of height. When you walk back down, you lose exactly 100 metres - you end up where you started.
Chemical reactions work the same way. The energy difference between reactants and products is fixed. Whether you go from A to B or B to A, you cross the same energy gap - just in opposite directions.
Let's work through the calculations you'll see in exams. The key is recognising that energy values have signs - positive for endothermic, negative for exothermic.
Worked example: Calculating Energy Changes in Reversible Reactions
The reaction A + B ⇌ C has ΔH = -120 kJ mol⁻¹ for the forward direction. Calculate ΔH for the reverse reaction.
Answer: ΔH for the reverse reaction = +120 kJ mol⁻¹ (endothermic)
Notice the sign change. This is crucial for exam marks. Exothermic reactions have negative ΔH values, endothermic reactions have positive ΔH values.
Examiners test this concept in predictable ways. They'll give you one direction and ask for the other, or show you an energy profile and ask you to identify which direction is exothermic.
The biggest mistake students make is forgetting that energy conservation applies to all reactions, not just the ones that look obvious. Whether it's a simple acid-base reaction or a complex industrial process, the rule never changes.
Exam precision note: In Energy Changes in Reversible Reactions - Energy changes and reversible reactions - Reversible reactions and dynamic equilibrium, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.
AQA commonly uses hydrated and anhydrous copper sulfate as a reversible-reaction example. Hydrated copper sulfate, CuSO4·5H2O, is blue. When it is heated, water is driven off and white anhydrous CuSO4 forms. Adding water again reverses the change. This example helps students connect reversible reactions with observable colour change and with energy changes in opposite directions.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
The formation of ammonium chloride, NH₃(g) + HCl(g) → NH₄Cl(s), is exothermic, so the tube gets warm. The reverse reaction, heating ammonium chloride so it decomposes back into ammonia and hydrogen chloride, is endothermic and absorbs the same amount of energy.
When blue hydrated copper sulfate, CuSO₄·5H₂O, is heated, water is driven off and white anhydrous CuSO₄ forms. Adding water again reverses the change, showing a reversible reaction with an observable colour change and energy changes in opposite directions.
If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction, and the same amount of energy is transferred in each case. So if the forward reaction releases 100 kJ, the reverse reaction absorbs exactly 100 kJ.
Anhydrous copper sulfate is white, while hydrated copper sulfate, CuSO₄·5H₂O, is blue. Heating the blue hydrated form drives off water and leaves white anhydrous CuSO₄.
When solid ammonium chloride is heated, it decomposes into ammonia and hydrogen chloride gases: NH₄Cl(s) → NH₃(g) + HCl(g). This reverse reaction is endothermic, absorbing the same amount of energy that formation released.
ΔH for a reverse reaction is equal and opposite to the forward reaction: ΔH(reverse) = -ΔH(forward). If A + B → C has ΔH = -120 kJ mol⁻¹, then C → A + B has ΔH = +120 kJ mol⁻¹, which is endothermic.
A negative ΔH means the reaction is exothermic and releases energy, while a positive ΔH means it is endothermic and absorbs energy. When a reversible reaction is reversed, the sign of ΔH changes.
Adding water to white anhydrous copper sulfate reverses the change, forming blue hydrated copper sulfate. This reversible reaction links an observable colour change with energy changes in opposite directions.
The tube gets warm because NH₃(g) + HCl(g) → NH₄Cl(s) is exothermic: bonds form between NH₃ and HCl, releasing energy.
An energy profile shows how energy changes as reactants become products. For a reversible reaction, the energy gap between reactants and products is fixed, so going either way crosses the same gap in opposite directions, like walking up and down the same hill.