If the forward reaction is EXOTHERMIC... then the backward reaction is ENDOTHERMIC (and vice versa). The energy released going forward equals the energy absorbed going backward.
Example: Hydration of copper sulfate
CuSO₄·5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(g)
| ← Add water | Heat → |
|---|---|
| Exothermic — mixture gets hot; white powder turns blue | Endothermic — needs heat; blue crystals turn white |
Quick Check: If the forward reaction of a reversible reaction releases 180 kJ/mol, how much energy does the backward reaction absorb?
Exactly 180 kJ/mol. In a reversible reaction, the energy change in the forward direction is exactly equal and opposite to the energy change in the backward direction. If the forward reaction is exothermic (releases 180 kJ/mol), the backward reaction is endothermic and absorbs exactly 180 kJ/mol. This follows from conservation of energy.
This key facts covers Key Facts: Energy in Reversible Reactions within Reversible Reactions for GCSE Chemistry. Revise Reversible Reactions in Rates of Reaction for GCSE Chemistry with 20 exam-style questions and 12 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 5 of 12 in this topic. Use this key facts to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Reversible Reactions
What does the symbol ⇌ mean when used in a chemical equation?
Explain the relationship between the energy changes in the forward and reverse reactions of a reversible reaction.