The partition law, or also commonly known as the distribution law in chemistry (not to be confused with legal terms) refers to the principle where proportionate distribution of a solute occurs when it is added to a two-phase system (at a given temperature). The proportion is based on the solubility level of the solute in each of the two phases (indivudually). Now that you have the layman’s definition of the partition law, you will be more than ready to read and memorise the more scientific definition of the partition law in Berry Berry Easy’s short notes – Part 8 of STPM Form 6 Chemistry notes on Phase Equilibrium. Please also try out the example given in the post as the minimum requirement in understanding this topic is solving the calculations for it. The concept is important but the calculations are what award you marks in exams, at least for this topic.
[Tips: While most students can solve calculation questions involving the partition/distribution law, they neglect to know the actual implication of it in physical terms. Please remember that the Henry's law is a particular case of the distribution law. Do read up on it to gain a better understanding of the application of the distribution law.]
STPM Chemistry Form 6 Notes – Phase Equilibrium (Part 8 )
Distribution of a Solute (X) between Two Solvent (Solvent A and Solvent B)
Distribution law = partition law – A solute distributes itself between two immiscible liquids so that the ratio of its concentration in each solvent is constant at constant temperature
Partition coefficient / distribution coefficient, K (a constant)
= (Concentration of X in solvent A / Concentration of X in solvent B)
The partition law follows the condition:
- The temperature is constant
- Two solutions are dilute
- Solute does not react with the solvents
- Solute exists in the same molecular state in the two solvents (does not dissociate or associate)
A solution contains 2.0 g of iodine dissolved in 20 cm3 of potassium iodide solution. If this solution is shaken with 20 cm3 of tetrachloromethane, how much iodine will be extracted by into the tetrachloromethane?
(partition coefficient of iodine between tetrachloromethane and water = 85)
Let the mass of iodine extracted be x
85 = (Concentration of iodine in tetrachloromethane / Concentration of iodine in water)
85 = (x / 12) / [(2 – x) / 20]
x = 1.98 g
- Separating funnels are used for solvent extraction
- Organic compounds can be extracted from aqueous solution by using the solvent
- Example: ether extraction
So this is the end of Part 8. Next up in Part 9 of Berry Berry Easy’s summarised notes on Phase Equilibrium for STPM Form 6 Chemistry students will be on ideal solution (miscible liquids) and non-ideal solution (miscible liquids) and immiscible liquids.