Reversible reactions and their energy changes
Most reactions can in principle go in both directions. A reversible reaction is one in which products can react back to form reactants under suitable conditions.
The reversible arrow is ⇌, replacing the one-way →.
Energy in reversible reactions
If the forward reaction is exothermic, the reverse is endothermic by the same amount.
Example: ammonium chloride.
- Forward (heat applied): NH₄Cl(s) → NH₃(g) + HCl(g) endothermic — ammonium chloride decomposes when heated.
- Reverse (cooling): NH₃(g) + HCl(g) → NH₄Cl(s) exothermic — white smoke forms when cooler.
Common GCSE example: hydrated ↔ anhydrous CuSO₄
CuSO₄·5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(l)
- Heating blue crystals: drives off water → white anhydrous CuSO₄. Endothermic forward (water lost).
- Adding water back: turns white powder blue, releases heat. Exothermic reverse.
This is a classic test for water: anhydrous CuSO₄ goes blue with water; anhydrous cobalt(II) chloride goes from blue to pink.
Closed systems and dynamic equilibrium
In a closed system (no substance enters or leaves) a reversible reaction reaches dynamic equilibrium: forward and reverse rates are equal. Concentrations of reactants and products remain constant — but reactions are still happening at equal rates.
(See C6.5 for changes to equilibrium position.)
✦Worked example— Examples in real life
- Self-indicating silica gel (cobalt-doped) changes blue → pink with humidity.
- Industrial ammonia synthesis (Haber process — C10.10).
- Acid-base chemistry in the body (carbon dioxide + water ⇌ carbonic acid).
Reaction profile
For a reversible reaction:
- Forward Ea = energy from reactants to top of hump.
- Reverse Ea = energy from products to top of hump.
- The forward exothermic (or endothermic) value plus the reverse endothermic (or exothermic) sum to zero.
⚠Common mistakes
- "⇌ means equal forward and reverse rates always." Only at equilibrium.
- Saying reverse reaction has different ΔE — same magnitude, opposite sign.
- Treating closed and open systems the same. A reversible reaction in an open system can lose products, preventing equilibrium.
- Forgetting that anhydrous is white for CuSO₄ (most students remember "blue" but blue is the hydrated form).
Links
Builds on C5.1. Sets up C6.5 (Le Chatelier HT), C10.10 (Haber HT).
AI-generated · claude-opus-4-7 · v3-deep-chemistry