25.2.3—Factors affecting partition coefficient
- Syllabus
- 9701–2028–2029
- Objective
- 25.2.3
- Level
- A2
Relative polarity controls which solvent better stabilises the solute
A solute reaches a higher equilibrium concentration in the solvent whose polarity and intermolecular attractions are more compatible with it. Favourable solute–solvent interactions compensate for separating particles of the pure solute and solvent.
| Solute character | Water versus a non-polar organic solvent | Expected Kpc(organic/aqueous) |
|---|---|---|
| non-polar | dispersion interactions are more compatible with the organic phase | relatively large |
| polar and able to interact strongly with water | dipole attractions or hydrogen bonding favour the aqueous phase | relatively small |
Changing either solvent changes the reference pair and therefore the numerical Kpc. A slightly polar organic solvent may stabilise a polar solute better than a hydrocarbon does, so Kpc is a property of the complete solute–two-solvent system at the stated temperature.
‘Like dissolves like’ is a polarity-based prediction, not a universal numerical rule. Also, a larger Kpc has no meaning until the numerator solvent is named.
ConceptA-Level CAIE Chemistry A2