Knowledge Organiser: Reversible Reactions
Key Terms
- Reversible reaction — uses ⇌ symbol
- Forward reaction — left to right
- Backward reaction — right to left
- Closed system — nothing enters or leaves
Energy Rule
- Forward exo = Backward endo
- Forward endo = Backward exo
- Energy values are equal in magnitude
Copper Sulfate Test
- Blue (hydrated) ⇌ White (anhydrous)
- Heating → endothermic → white
- Add water → exothermic → blue
- Detects water presence
Common Errors
- Using → not ⇌ in reversible equations
- Saying forward AND backward are exothermic
- Thinking products reach 100%
Key Equations
- CuSO₄·5H₂O ⇌ CuSO₄ + 5H₂O (copper sulfate dehydration)
- If forward reaction is exothermic (−x kJ), reverse is endothermic (+x kJ)
- NH₄Cl ⇌ NH₃ + HCl (thermal decomposition — reversible on cooling)
Memory Aid
- → = one-way street (irreversible reaction)
- ⇌ = two-way road (reversible reaction, traffic flows both ways)
Common Mistakes
Misconception 1: "Reversible means the reaction goes backwards" A reversible reaction does not simply "go backwards" — both directions happen simultaneously. Even as products are forming, some are reacting together to reform reactants. The ⇌ symbol explicitly shows that both the forward and backward reactions are occurring at the same time. The net direction just depends on which reaction is currently faster. Misconception 2: "Reversible reactions reach 100% products eventually" In a closed system, a reversible reaction never reaches 100% conversion. The system reaches a state of dynamic equilibrium where both forward and backward rates are equal. This is why industrial processes like the Haber process must recycle unreacted gases — you can never convert all the reactants in one pass. Misconception 3: "The energy values are different for forward and backward reactions" The energy released in the exothermic direction always equals the energy absorbed in the endothermic direction. If the forward reaction releases 100 kJ/mol, the backward reaction absorbs exactly 100 kJ/mol. This follows from conservation of energy.