Group VII Displacement Reactions
Section: 8. The Periodic Table | Syllabus: Cambridge AS Level Physics 9702
Halogen Displacement Reactions A displacement reaction occurs when a more reactive halogen displaces a less reactive halogen from its compound. These reactions demonstrate the reactivity order of the halogens.
Displacement Reaction A reaction in which a more reactive element takes the place of a less reactive element in a compound. Reactivity Order Decreasing reactivity: Fluorine > Chlorine > Bromine > Iodine More reactive halogens can displace less reactive halogens from solutions of their salts Chlorine can displace bromine and iodine Bromine can displace iodine Iodine cannot displace chlorine or bromine The reaction proves the order of reactivity Why This Happens More reactive halogens have a greater tendency to gain electrons.
When added to a solution containing a less reactive halide ion, the more reactive halogen "steals" the electron, displacing the less reactive halogen. Chlorine Displacement Reactions Chlorine can displace both bromine and iodine from their salt solutions.
Chlorine Displacing Bromine Chlorine + sodium bromide: Cl₂(aq) + 2NaBr(aq) → 2NaCl(aq) + Br₂(aq) Ionic equation: Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq) Observations Before: Colorless sodium bromide solution After adding chlorine water: Solution turns orange-brown Orange-brown color indicates bromine has been produced Chlorine (more reactive) has displaced bromine (less reactive) Chlorine Displacing Iodine Chlorine + potassium iodide: Cl₂(aq) + 2KI(aq) → 2KCl(aq) + I₂(aq) Ionic equation: Cl₂(aq) + 2I⁻(aq) → 2Cl⁻(aq) + I₂(aq) Observations Before: Colorless potassium iodide solution After adding chlorine water: Solution turns brown Brown color indicates iodine has been produced Adding cyclohexane or hexane produces a purple layer (iodine dissolves) Testing with Organic Solvents Adding an organic solvent like cyclohexane helps identify the halogen produced.
The halogen dissolves in the organic layer, creating distinct colors: Br₂ = orange/brown, I₂ = purple/violet. Diagram Test tubes showing: (1) colorless KI solution, (2) brown solution after adding Cl₂, (3) two layers with purple organic layer after adding cyclohexane Bromine Displacement Reactions Bromine can displace iodine but cannot displace chlorine.
Bromine Displacing Iodine Bromine + potassium iodide: Br₂(aq) + 2KI(aq) → 2KBr(aq) + I₂(aq) Ionic equation: Br₂(aq) + 2I⁻(aq) → 2Br⁻(aq) + I₂(aq) Observations Before: Colorless potassium iodide solution After adding bromine water: Solution turns brown/dark brown Brown color from iodine produced Adding cyclohexane produces a purple layer Bromine with Chloride Ions (No Reaction) Bromine + sodium chloride: Br₂(aq) + NaCl(aq) → No reaction Observations Solution remains orange-brown (color of bromine water) No displacement occurs Bromine is less reactive than chlorine Cannot displace chlorine from chlorides Why No Reaction?
Bromine is less reactive than chlorine, so it cannot displace chloride ions from solution. Only more reactive halogens can displace less reactive ones. Iodine Displacement Reactions Iodine is the least reactive halogen (excluding astatine) and cannot displace any other halogens.
Iodine with Bromide Ions (No Reaction) Iodine + potassium bromide: I₂(aq) + KBr(aq) → No reaction Observations Solution remains brown (color of iodine solution) No displacement occurs Iodine cannot displace bromine Iodine with Chloride Ions (No Reaction) Iodine + sodium chloride: I₂(aq) + NaCl(aq) → No reaction Observations Solution remains brown (color of iodine solution) No displacement occurs Iodine is the least reactive halogen Halogen Added Halide Solution Reaction?
Observation Cl₂ Bromide ✓ Yes Orange-brown (Br₂ produced) Cl₂ Iodide ✓ Yes Brown (I₂ produced) Br₂ Chloride ✗ No Remains orange (Br₂ color) Br₂ Iodide ✓ Yes Dark brown (I₂ produced) I₂ Chloride ✗ No Remains brown (I₂ color) I₂ Bromide ✗ No Remains brown (I₂ color) Testing with Organic Solvents Adding an organic solvent helps identify which halogen has been produced in a displacement reaction.
Why Use Organic Solvents? Halogens are more soluble in organic solvents than in water Forms a separate layer on top of the aqueous solution Each halogen has a distinct color in organic solvent Common solvents: cyclohexane, hexane, or tetrachloromethane Halogen Colors in Organic Solvent Halogen Color in Water Color in Organic Solvent Chlorine (Cl₂) Pale green Pale green Bromine (Br₂) Orange-brown Orange-red Iodine (I₂) Brown Purple/violet Procedure Add 2-3 cm³ of cyclohexane to the test tube Shake gently to mix Allow layers to separate Observe the color of the upper organic layer Purple color confirms iodine Orange-red color confirms bromine Safety Note Cyclohexane and other organic solvents are flammable and should be used away from naked flames.
Work in a well-ventilated area. Diagram Test tubes showing two-layer systems: aqueous layer (bottom) and organic layer (top) with different halogen colors - orange for Br₂, purple for I₂ Understanding Displacement Reactions Displacement rea…
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