STPM Chemistry Form 6 Notes – Phase Equilibrium (Part 1)

by BerryBerryTeacher

in Berry Reference (Notes)

Phase equilibrium are relationships of equilibra between the various phases of a chemical compound or mixture when taken under different conditions of composition, temperature and pressure. The phases can be vapour, liquid, solid or others. This Part 1 notes for Berry Berry Easy‘s short notes on STPM Chemistry Form 6 Notes – Phase Equilibrium focuses on the vapour pressure of a liquid. The definition and brief explanations involving it will be given.

[Tips: While often disregarded by students, but it is often the first step to see if the system is an open or closed system. So do try to understand the differences between a closed and open system before answering any questions. Additionally, make sure to imagine how a molecule within a space looks like as the placement of molecules is crucial in certain phenomena such as evaporation.]

STPM Chemistry Form 6 Notes – Phase Equilibrium (Part 1)

Fractional Distillation

Fractional Distillation


Vapour pressure above a liquid – force per unit area above the surface of a liquid exerted by molecules formed upon evaporation of the liquid.

Vapour pressure of a liquid

  • It is the partial pressure exerted by the gas molecules formed by evaporation from the surface of the liquid, when it is in equilibrium with the gas molecules condensing back into the liquid.
  • It is depends on the temperature (energy of the molecules) of the liquid and ΔHvaporisation (the energy necessary to overcome intermolecular forces) of the liquid.
  • The surface of a liquid is the most favorable points for the molecules to evaporate.
  • Molecules can evaporate only from the surface, and tend to evaporate from the corners.
  • Corners molecules have the fewest intermolecular forces and because of the minimal forces, molecules evaporate most readily.
  • It can be measured in either an open or closed system
  • In a closed system, a partial pressure of vapour exists because the rates of vaporisation equal the rate of condensation.
  • In an open system, the vapour escapes, therefore it does not reach a state of equilibrium between the rate of vaporisation and the rate of condensation. It is difficult to measure, so generally in theoretical perspective, the values that have been determined in a closed system at known temperature are applied in an open system.
  • It is independent of the shape and volume of a container.
  • The atmospheric pressure does not significantly affect the vapour pressure (Boiling point depends on the atmospheric pressure).
  • Pvapour ↑ as temperature ↑; Pvapour ↓ as ΔHvaporisation

The next part, Part 2 of Berry Berry Easy‘s short notes on Phase Equilibrium for STPM Form 6 Chemistry will be focused on the relationship between vapour pressure and temperature which leads to an everyday situation of the boiling point. So stay tuned.

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