Reversible Reactions
Section: 6. Chemical Reactions | Syllabus: Cambridge AS Level Physics 9702
What is a Reversible Reaction? A reversible reaction is a reaction where the products can react together to reform the original reactants. The reaction can proceed in both the forward and backward directions.
Symbol: Reversible reactions are shown with a ⇌ symbol (double arrow) A + B ⇌ C + D Forward reaction: A + B → C + D Backward reaction: C + D → A + B Reversible vs Irreversible Reactions Aspect Irreversible Reaction Reversible Reaction Direction Only goes one way (forward) Goes both ways (forward and backward) Symbol Single arrow → Double arrow ⇌ Products Cannot reform reactants easily Can reform reactants Completion Goes to completion Reaches equilibrium (never completes) Examples Combustion, neutralization Haber process, thermal decomposition of ammonium chloride Examples of Reversible Reactions 1.
Thermal decomposition of ammonium chloride NH₄Cl(s) ⇌ NH₃(g) + HCl(g) • Forward: White solid ammonium chloride decomposes into ammonia and hydrogen chloride gases when heated • Backward: The gases recombine to form white solid when cooled 2.
Hydrated copper(II) sulfate CuSO₄·5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(l) • Forward: Blue hydrated copper sulfate → white anhydrous copper sulfate + water (when heated) • Backward: White anhydrous copper sulfate + water → blue hydrated copper sulfate (exothermic) 3.
The Haber Process (making ammonia) N₂(g) + 3H₂(g) ⇌ 2NH₃(g) • Forward: Nitrogen and hydrogen combine to form ammonia • Backward: Ammonia decomposes back into nitrogen and hydrogen 4. Dissolving carbon dioxide in water CO₂(g) + H₂O(l) ⇌ H₂CO₃(aq) • Forward: Carbon dioxide dissolves to form carbonic acid • Backward: Carbonic acid decomposes releasing CO₂ The Ammonium Chloride Demonstration This is a classic demonstration of a reversible reaction that can be observed in a test tube.
What happens: Heating (forward reaction): White solid ammonium chloride at bottom of tube disappears as it sublimes and decomposes into two colorless gases (NH₃ and HCl) Cooling (backward reaction): The gases cool at the top of the tube and recombine, forming white solid ammonium chloride again as a ring near the top This shows both forward and backward reactions can occur Why it's visible: The solid appears and disappears, making it easy to see the reaction is reversible.
The Hydrated Copper Sulfate Test Another excellent demonstration of a reversible reaction with a clear color change. Forward reaction (heating): Blue hydrated copper(II) sulfate crystals (CuSO₄·5H₂O) When heated, water is driven off Color changes from BLUE → WHITE White anhydrous copper(II) sulfate remains (CuSO₄) This is an endothermic reaction (requires heat) Backward reaction (adding water): Add a few drops of water to white anhydrous copper sulfate Color changes from WHITE → BLUE Heat is released (exothermic - gets hot!) Blue hydrated copper sulfate is reformed Test for water: Anhydrous copper sulfate turning blue is used as a chemical test for the presence of water!
Energy Changes in Reversible Reactions Important principle: If the forward reaction is exothermic, the backward reaction is endothermic by the SAME amount of energy. Reaction Forward Direction Backward Direction Energy change Exothermic (releases heat) Endothermic (absorbs heat) Energy value ΔH = -X kJ/mol ΔH = +X kJ/mol Effect Temperature increases Temperature decreases Example: Hydrated copper sulfate CuSO₄·5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(l) Forward (heating): Endothermic (+78 kJ/mol) - needs heat energy Backward (adding water): Exothermic (-78 kJ/mol) - releases heat energy Conditions Affecting Reversible Reactions Reversible reactions are affected by changes in conditions: Temperature: Increasing temperature favors the endothermic direction Pressure: Increasing pressure favors the direction with fewer gas molecules Concentration: Increasing concentration of reactants favors forward reaction These effects are explained by Le Chatelier's Principle (covered in the Equilibrium topic).
Closed System vs Open System Closed system: A system where no substances can enter or leave. Reversible reactions can reach equilibrium in a closed system. Open system: A system where substances can enter or leave.
Equilibrium cannot be achieved in an open system. Aspect Closed System Open System Substances Cannot enter or leave Can enter or leave (e.g., gases escape) Equilibrium CAN reach equilibrium CANNOT reach equilibrium Example Sealed container, pressure vessel Open beaker, flask without bung What happens Forward and backward rates become equal Products may escape, reaction doesn't balance Why Reversible Reactions Are Important Understanding reversible reactions is crucial for: Industrial processes: Haber process (ammonia), Contact process (sulfuric acid) Optimizing yield: Adjusting conditions to favor product formation Biological systems: Many biological reactions are reversible (e.g., oxygen binding to hemoglobin) Environmental chemistry: CO₂ dissolving in oceans, acid rain formation Identifying Reversible Reactions How to…
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