AS Chemistry 9701: Chlorine Water Displacement and Cyclohexane Extraction (Group 17)

A student adds a small volume of chlorine water to separate aqueous solutions of potassium bromide and potassium iodide, in two test-tubes labelled P and Q respectively.

(a) State the colour change observed in the aqueous layer of tube P, and write an ionic equation, including state symbols, for the reaction that occurs. [3]

(b) State the colour change observed in the aqueous layer of tube Q, and write an ionic equation, including state symbols, for the reaction that occurs. [3]

(c) The student then adds a few cm $^3$ of cyclohexane to each tube, shakes both gently, and allows the layers to separate. Describe the colour of the upper cyclohexane layer in tube P and in tube Q, and explain why the halogen formed moves into this layer rather than remaining in the aqueous layer. [3]

Part (a): Tube P. Chlorine water added to potassium bromide

Chlorine lies above bromine in Group 17, so it is a stronger oxidising agent than bromine and readily displaces (oxidises) bromide ions to bromine, while itself being reduced to chloride:

$\text{Cl}_2\text{(aq)} + 2\text{Br}^-\text{(aq)} \rightarrow 2\text{Cl}^-\text{(aq)} + \text{Br}_2\text{(aq)}$

Observation: the aqueous layer, initially colourless (potassium bromide solution) with a faint pale-green tinge from the added chlorine water, turns orange as bromine is formed in solution.

Part (b): Tube Q. Chlorine water added to potassium iodide

Chlorine also lies above iodine in the group, and is an even stronger oxidising agent relative to iodine than it is to bromine, so it readily displaces iodide ions to iodine:

$\text{Cl}_2\text{(aq)} + 2\text{I}^-\text{(aq)} \rightarrow 2\text{Cl}^-\text{(aq)} + \text{I}_2\text{(aq)}$

Observation: the aqueous layer, initially colourless (potassium iodide solution), turns brown as iodine is formed in solution.

Part (c): Extraction into cyclohexane

Cyclohexane is a non-polar organic solvent that is immiscible with water and less dense than water, so it forms a separate upper layer once the mixtures are shaken and allowed to settle.

In tube P, the dissolved bromine transfers from the aqueous layer into the cyclohexane layer, which turns orange.
In tube Q, the dissolved iodine transfers from the aqueous layer into the cyclohexane layer, which turns purple/violet, a markedly different colour from the brown iodine solution seen in the aqueous layer in part (b).

Explanation: both $\text{Br}_2$ and $\text{I}_2$ are simple non-polar covalent molecules. Non-polar molecules are more soluble in non-polar solvents than in polar solvents such as water (“like dissolves like”), so the halogens preferentially dissolve in the cyclohexane rather than remaining in the aqueous layer. As a result, the aqueous layer becomes paler (almost colourless) once extraction is complete, while the halogen’s characteristic colour becomes concentrated in the cyclohexane layer above it, a colour that differs from the aqueous colour because the halogen molecule interacts differently with a non-polar solvent than with polar water.

Final answers

(a) Aqueous layer turns orange; $\text{Cl}_2\text{(aq)} + 2\text{Br}^-\text{(aq)} \rightarrow 2\text{Cl}^-\text{(aq)} + \text{Br}_2\text{(aq)}$
(b) Aqueous layer turns brown; $\text{Cl}_2\text{(aq)} + 2\text{I}^-\text{(aq)} \rightarrow 2\text{Cl}^-\text{(aq)} + \text{I}_2\text{(aq)}$
(c) Cyclohexane layer: orange in tube P (bromine), purple/violet in tube Q (iodine), the non-polar halogens dissolve preferentially in the non-polar solvent, leaving the aqueous layer paler.