A REVERSIBLE REACTION is one where the products can react to form the original reactants again.
Symbol: ⇌ (double-headed arrow indicating the reaction goes in both directions).
Examples:
Ammonium chloride thermal decomposition:
NH₄Cl(s) ⇌ NH₃(g) + HCl(g)
When heated: NH₄Cl decomposes to NH₃ and HCl (forward reaction).
When cooled: NH₃ and HCl recombine to form NH₄Cl (reverse reaction).
Hydrated copper sulfate:
CuSO₄·5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(l)
Heating blue crystals → white anhydrous powder + water (forward reaction).
Adding water to white powder → blue crystals reform (reverse reaction).
This is used as a TEST FOR WATER — white powder turns blue in the presence of water.
The DIRECTION of a reversible reaction depends on the conditions (temperature, pressure, concentration).
Dynamic Equilibrium
When a reversible reaction is carried out in a CLOSED SYSTEM (nothing enters or leaves), it reaches EQUILIBRIUM.
At equilibrium:
The FORWARD and REVERSE reactions are still occurring — the reaction doesn't stop.
The RATES of the forward and reverse reactions are EQUAL.
The CONCENTRATIONS of reactants and products remain CONSTANT (but not necessarily equal).
This is called DYNAMIC EQUILIBRIUM — 'dynamic' because both reactions are still happening (not static); 'equilibrium' because the concentrations are balanced (not changing).
Important: at equilibrium the reaction is STILL HAPPENING — it just appears to have stopped because concentrations are constant.
CLOSED SYSTEM requirement:
If products are removed, the reaction shifts to produce more products (restoring equilibrium).
If reactants are removed, the reaction shifts to produce more reactants.
Open systems (like burning fuel in open air) cannot reach equilibrium — products escape.
Energy Changes in Reversible Reactions
In a reversible reaction:
If the FORWARD reaction is EXOTHERMIC → the REVERSE reaction is ENDOTHERMIC.
The SAME amount of energy is released in the forward direction as is absorbed in the reverse direction.
Example — hydrated copper sulfate:
CuSO₄·5H₂O → CuSO₄ + 5H₂O is ENDOTHERMIC (absorbs heat — heat applied to dehydrate).
CuSO₄ + 5H₂O → CuSO₄·5H₂O is EXOTHERMIC (releases heat — hand gets warm when water added).
Example — Haber process:
N₂ + 3H₂ → 2NH₃ is EXOTHERMIC.
2NH₃ → N₂ + 3H₂ is ENDOTHERMIC.
Same energy value — opposite signs.
This principle is important for understanding how changing conditions affects equilibrium position.
⚠️ Common Mistake
At equilibrium, the reaction has NOT stopped — it is DYNAMIC. Both forward and reverse reactions continue at equal rates. Concentrations are CONSTANT but not necessarily EQUAL. Students often say 'equilibrium means equal concentrations' — this is wrong. It means equal RATES.
Reversible reaction: ⇌ symbol. Products can reform reactants. Closed system → dynamic equilibrium: forward and reverse rates equal, concentrations constant (not equal). If forward is exothermic → reverse is endothermic (same energy magnitude). NH₄Cl decomposition and CuSO₄ hydration are key examples.