CAIE A-Level Chemistry 25.2.3 Polarity and Partition Coefficients

Practise explaining partition preference from solute and solvent polarities and intermolecular attractions.

Exam points

compare the solute’s polar and non-polar regions with each solvent rather than using size alone
predict greater concentration in the solvent offering stronger compatible intermolecular attractions
explain that Kpc is meaningful only after equilibrium and changes when the solute or solvent pair changes

[Maximum number: 2]

Iodine is found naturally in compounds in many different oxidation states.

Iodide ions, $\mathrm{I}^{-}$ , react with acidified $\mathrm{H}_{2} \mathrm{O}_{2}(\mathrm{aq})$ to form iodine, $\mathrm{I}_{2}$ , and water. This reaction mixture is shaken with cyclohexane, $\mathrm{C}_{6} \mathrm{H}_{12}$ , to extract the $\mathrm{I}_{2}$ . Cyclohexane is immiscible with water.

Suggest how the value of $K_{p c}$ of $\mathrm{I}_{2}$ between hexan-2-one, $\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{3} \mathrm{COCH}_{3}$ , and water compares to the value given in (a)(ii). Explain your answer.

$\quad K_{\mathrm{pc}}$ would be lower

- hexan-2-one is more polar (than cyclohexane)

OR hexan-2-one is polar AND cyclohexane is non-polar

- $\quad \mathrm{I}_{2}$ is (therefore) less soluble in hexan-2-one

All three correct for two marks

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25.2.3—Factors affecting partition coefficient question 1