Energy Changes & Reversible Reactions (AQA GCSE Chemistry): Revision Notes
📚 Revision Notes
6.2.2 Energy Changes & Reversible Reactions
Energy Changes in Reversible Reactions:
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In reversible reactions, the energy changes in the forwards and backwards reactions are opposing. This means that if the forwards reaction is exothermic, the backwards reaction will be endothermic, and vice versa.
Exothermic vs. Endothermic Reactions:
- Exothermic Reaction: A reaction that transfers energy to its surroundings, usually in the form of heat. This occurs when more energy is released during the formation of bonds in the products than is absorbed to break the bonds in the reactants.
- Endothermic Reaction: A reaction that absorbs energy from its surroundings. This occurs when more energy is required to break the bonds in the reactants than is released during the formation of bonds in the products.
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Example: Formation of Hydrogen Iodide (HI):
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Forward Reaction: The reaction of hydrogen gas () and iodine vapour () to form hydrogen iodide () is exothermic. Reason: More energy is released in forming the H–I bonds than is absorbed in breaking the H–H and I–I bonds.
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Backward Reaction: The reverse reaction, where HI decomposes back into and , is endothermic. Reason: More energy is needed to break the H–I bonds in HI than is released in forming H–H and I–I bonds.
General Principle:
- The energy change of the reverse reaction will always be the opposite of that in the forwards reaction:
- If the forwards reaction is endothermic, the backwards reaction will be exothermic.
- If the forwards reaction is exothermic, the backwards reaction will be endothermic.