VisualChem The interactive chemistry textbook
中文

Halogen displacement: identifying halogens by their hexane layer

Add chlorine, bromine and iodine water to KCl, KBr and KI solutions, then extract with hexane: the colour of the hexane layer on top tells you which halogen is present. Finally, use the same trick to identify an unknown halide solution.

Difficulty ●●○ ≈ 25 min

Aim Decide from colours which displacements happen, rank Cl₂, Br₂ and I₂ as oxidising agents, and compare what you see with the equilibrium constants computed from standard potentials.
Halogen water + halide solution + hexane: colours computed from absorption spectra

Halogen displacement bench

Halogen water (0.010 M X₂, 2 mL)
Halide solution (0.10 M, 2 mL)

1 Pour in the halide solution → 2 Add the halogen water → 3 Add 2 mL hexane → 4 Stopper and shake

The unknown contains
The extent of reaction is solved exactly from equilibrium constants computed from CRC standard potentials, with K = 698 for I₂ + I⁻ ⇌ I₃⁻; shaking distributes the neutral halogens between the layers by partition coefficients (the hexane/water values are approximate teaching values — anything above ~8 moves almost all the halogen into the hexane). Layer colours come from the species’ absorption bands (teaching fits at literature maxima) through the CIE observer, over the tube’s 1.5 cm inner width. Not included: the hydrolysis of chlorine water, Br₃⁻, interhalogens such as ICl, and the further oxidation of I₂ to colourless IO₃⁻ by excess chlorine water. The unknown is chosen by a deterministic seed.

Procedure

  1. Start with the three known halides and chlorine water: pour 2 mL of 0.10 M halide solution into the tube, add 2 mL of chlorine water and watch the water change colour.
  2. Add 2 mL of hexane (lighter than water, it floats on top), stopper, shake and let the layers separate. Record the colour of the hexane layer and compare it in the results table.
  3. Repeat with bromine water and iodine water until you have all nine combinations (three halogen waters × three halides). In which tubes is the hexane a different colour from the halogen water you added?
  4. Write ionic equations for the reactions that happened and rank Cl₂, Br₂ and I₂ as oxidants. Check your ranking against the E°cell and K the bench reports.
  5. Finally choose the unknown solution. Using only chlorine water, bromine water and hexane, design the fewest tests that decide whether it contains Cl⁻, Br⁻ or I⁻, then submit your answer. Press “New unknown” and try again.

What to notice

  • Chlorine water turns a Br⁻ solution yellow-orange and an I⁻ solution brown; after shaking, the hexane turns orange (Br₂) and violet (I₂) respectively.
  • Bromine water oxidises I⁻ but not Cl⁻; iodine water oxidises neither — the hexane simply shows the colour of the halogen you added.
  • The order Cl₂ > Br₂ > I₂ matches E°(X₂/X⁻) = 1.36 > 1.07 > 0.54 V: every reaction that goes has K above 10¹⁰, every one that does not has K below 10⁻⁹.
  • In water, iodine joins excess I⁻ as brown I₃⁻, which is why iodine solutions look brown rather than violet; only in a non-polar solvent such as hexane does iodine show its true violet colour.

In a real lab

  • Chlorine water and bromine water are toxic and irritating: work in a fume cupboard with goggles and gloves. Liquid bromine causes severe burns — use only dilute bromine water.
  • Hexane is highly flammable — keep it away from flames. Hold the stopper firmly with your thumb while shaking and vent now and then. Pour the organic layers into the halogenated-waste container.
  • Many older texts extract with CCl₄, which is denser than water, so its layer is at the bottom; because it is toxic and ozone-depleting, hexane or cyclohexane (layer on top) is now used instead.
  • Chlorine water goes off on standing (Cl₂ escapes and hydrolyses); excess chlorine water also oxidises I₂ further to colourless IO₃⁻, bleaching the violet — add it drop by drop.
  • The simulated colours are computed from teaching spectra fitted to literature bands, and the hexane/water partition coefficients are approximate; real colours also depend on concentration, lighting and solvent purity. In bright light bromine slowly substitutes into hexane and fades.

Related lessons

All experiments