CIE A Level Chemistry (9701) exams from 2022

Revision Notes

5.5.9 Partition Coefficients

Partition Coefficient & Calculations

  • The partition coefficient (Kpc) is the ratio of the concentrations of a solute in two different immiscible solvents in contact with each other when equilibrium has been established (at a particular temperature)
  • For example, methylamine (CH3NH2) is dissolved in two immiscible solvents:
    • Water
    • An organic solvent
  • A separating funnel is shaken with the organic solvent and aqueous methylamine
  • The methylamine is soluble in both solvents, so when the mixture is left to settle an equilibrium is established
    • The rate of methylamine molecules moving from the organic layer into the aqueous layer is equal to the rate of molecules moving from the aqueous layer to the organic layer

CH3NH2(aq) ⇌ CH3NH2(organic solvent)

  • The value of its equilibrium constant is also called the partition coefficient

Partition Coefficient & Calculations equation

Equilibria - Partition Coefficient, downloadable AS & A Level Chemistry revision notes

The partition coefficient is the ratio of methylamine molecules in the organic and aqueous layer once equilibrium has been established

Calculating Partition Coefficients

  • The partition coefficient (Kpc) for a system in which the solute is in the same physical state in the two solvents can be calculated using the equilibrium expression

Worked example: Calculating the partition coefficient

Equilibria - Worked example - Calculating the partition coefficient, downloadable AS & A Level Chemistry revision notes

Answer

  • Step 1: Write down the equilibrium equation

CH3NH2(aq) ⇌ CH3NH2(organic solvent)

  • Step 2: Write down the equilibrium expression 

Calculating Partition Coefficients worked example equation 1

  • Step 3: Determine how many moles of CH3NH2 has reacted with HCl at the end-point

At the end-point, all CH3NH2 (aq) has been neutralised by HCl (aq)

CH3NH2 (aq) + HCl (aq) → CH3NH3Cl (aq)

CH3NH2 and HCl react in a ratio of 1:1

Mol (HCl) = mol (CH3NH2) = 0.225 x 0.0141

= 3.18 x 10-3 mol

  • Step 4: Determine the number of moles of CH3NH2 present in the aqueous layer

Only 50.0 cm3 of the aqueous layer was used to titrate against HCl

Thus, 3.18 x 10-3 mol of CH3NH2 was present in only 50.0 cm3 of the aqueous layer

The number of moles of CH3NH2 in 100 cm3 aqueous layer is, therefore:

Mol (CH3NH2 aqueous layer) = 3.18 x 10-3 x 2 = 6.34 x 10-3 mol

  • Step 5: Determine the number of moles of CH3NH2 in the organic layer

Mol CH3NH2 (organic layer) = mol CH3NH2 (total) – mol CH3NH2(aqueous layer)

Mol CH3NH2 (total) = 0.100 x 0.150 = 0.015 mol

Mol CH3NH2 (organic layer) = 0.015 – 6.34 x 10-3 = 8.67 x 10-3 mol

  • Step 6: Change the number of moles into concentrations

Calculating Partition Coefficients worked example equation 2

= 0.063 mol dm-3

Calculating Partition Coefficients worked example equation 3

= 0.116 mol dm-3

  • Step 7: Substitute the values into the Kpc expression

Calculating Partition Coefficients worked example equation 4

= 1.83

Since the value of Kpc is larger than 1, methylamine is more soluble in the organic solvent than in water

Factors Affecting the Partition Coefficient

  • The partition coefficient (Kcp) depends on the solubilities of the solute in the two solvents
  • The degree of solubility of a solute is determined by how strong the intermolecular bonds between solute and solvent  are
  • The strength of these intermolecular bonds, in turn, depends on the polarity of the solute and solvent molecules
  • For example, ammonia is more soluble in water than in an organic solvent such as carbon tetrachloride (CCl4)
    • Ammonia and water are both polar molecules that form hydrogen bonds with each other
    • Ammonia forms permanent dipole-induced dipole forces with the non-polar CCl4 molecules
    • Since these forces are much weaker than hydrogen bonding, ammonia is less soluble in CCl4
  • When Kpc is < 1 the solute is more soluble in water than the organic solvent
  • When Kpc is > 1 the solute is more soluble in the organic solvent than the water

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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