Raoult’s Law is one of the more difficult concepts in STPM Chemistry’s Phase Equilibrium. This is due to the different ways of stating the Raoult’s Law appearing in the various texts that students might encounter. The discrepancies in the ways to state Raoult’s Law stems from the different situation used when expressing the law. Worry not, as the typical definition used for STPM Chemistry (in most cases) is given in this Part 3 of
Berry Berry Easy‘s short notes on Phase Equilibrium for STPM Form 6 Chemistry. Besides that, you’ll also learn about the total vapour pressure concept to complement Raoult’s Law.
The key to understanding Raoult’s Law is using the visualisation method (although in the future, you would be required to use entropy to describe Raoult’s Law). But for now, you may stick to the notes given below.
[Tips: Berry Readers might attempt to apply Raoult's Law to all situations. However, this is wrong as Raoult's Law is only valid when the chemical interactions between two liquids is the same as the bonding within the liquids. In other words, this is only valid in an ideal solution. Guess what, in the real world, there is virtually no such thing as an ideal solution.]
STPM Chemistry Form 6 Notes – Phase Equilibrium (Part 3)
- Definition: The partial vapour pressure of a component (A) in a solution at a given temperature is equal to the vapour pressure of pure liquid (A) at the same temperature, multiplied by the mole fraction of the component (A) in the solution.
- The vapour pressure depends on the mole fraction and the pure vapour pressure of the components.
- The understanding of this theory is that the mole fraction of a compound corresponds to the percentage of the surface of the liquids mixture due to that compound.
- Any component (A) makes up only half of the surface, then only as much as what normally evaporates from the pure liquid (A), vaporises from the mixture.
- PA = P˚A XA
- Mole fraction of a component in Raoult’s equation is in solution and not in the vapour state.
- Mole fraction of a compound in solution decreases, this results, the vapour pressure of that component above the solution decreases.
0.10 moles of AsH3, NH3 and PH3 are placed into a beaker at -100˚C, where all three exist in liquid phase. What can be said about their relative vapor pressures?
A PAsH3 < PNH3 < PPH3
B PAsH3 < PPH3 < PNH3
C PNH3 < PAsH3 < PPH3
D PPH3 < PNH3 < PAsH3
In Raoult’s law, the vapour pressure depends on the mole fraction and the pure vapour pressure of the component. In the question, the solution contains equal molar quantities of the three components. Therefore, the highest vapour pressure results from the compound with the lowest boiling point (highest pure vapour pressure).
- NH3 has the highest boiling point because it can form hydrogen bonds so it has the lowest vapour pressure.
- AsH3 and PH3 are polar compound so the most significant factor in determining the boiling point is molecular mass.
- Lighter molecules are easier to vaporize = greater vapour pressur for the compound.
- PH3 is lighter than AsH3
Total Vapour Pressure
- A mixture of two or more liquids is the sum of the individual vapour pressures of each component liquid.
- Ptotal = PA + PB + PC + …
- At boiling point of the solution, the total vapour pressure of the components equals the atmospheric pressure.
- Solution obeys Raoult’s law is an ideal solution. However, in real situation, there are other factors that interferes example intermolecular forces between the liquids. The molecules interact in both liquid and gas phases, the vapour pressure relationship is not as linear as Raoult’s law equation.
- Overall increase in attractive forces in solution (components are mixed) –> vapour pressure decreases = negative deviation.
- Overall decrease in attractive forces in solution (components are mixed) –> vapour pressure increases = positive deviation.
This is the end of Part 3. The next part, Part 3 of Berry Berry Easy‘s short notes on Phase Equilibrium for STPM Form 6 Chemistry will be focused on the distillation process, in the form of fractional distillation and vacuum distillation. This series of notes is relatively long but it is worth studying as STPM exams heavily features questions from Phase equilibrium.