STPM Chemistry Form 6 Notes – Phase Equilibrium (Part 5)

by BerryBerryTeacher

in Berry Reference (Notes)

The differences between ideal solution and non-ideal solution are something that students usually find it easy to memorise but hard to really understand. Most students tend to focus on the obvious differences like the intermolecular forces of attraction, volume change and enthalpy change. It is more important to say that ideal solutions are solutions which obey Raoult’s law at all temperatures and concentrations. From it, you can derive the other system properties. Nonetheless, this post is Berry Berry Easy short study notes or summary on STPM Chemistry Form 6 notes on Phase Equilibrium Part 5. Do read on:

[Tips: Sometimes it is important to remember ideal solutions in pairs of liquids because after all it is about ideal mixtures. So among liquid pairs that are said to be ideal solution includes benzene/toulene, n-heptane/n-hexane and chlorobenzene/bromo benzene. Another critical error (rare) made by students include mixing up ideal solutions with ideal gases. They are not the same.]

STPM Chemistry Form 6 Notes – Phase Equilibrium (Part 5)

Fractional Distillation

Fractional Distillation

Ideal solution

  • Miscible liquids (A & B)
  • The intermolecular forces of attraction between A … A, B … B and A … B (after mixing) are equal
  • No change in volume after mix A & B
  • No enthalpy change on mixing

Non-ideal solutions

  • Miscible liquids (A & B)
  • The intermolecular forces of attraction in liquid mixture A … B are stronger or weaker than the intermolecular forces of attraction between A … A in pure liquid A and between B … B in pure liquid B.
  • There are changes in volume after mix A and B
  • There are enthalpy change on mixing
  • Negative deviation and positive deviation from Raoult’s law
Negative deviation from Raoult’s law Positive deviation from Raoult’s law
Actual vapour pressure is lower than the predicted by Raoult’s Law Actual vapour pressure is higher than the predicted by Raoult’s Law
The formation (after mixing A & B) of stronger intermolecular forces of attraction The formation (after mixing A & B) of weaker intermolecular forces of attraction
Decreases in volume after mixing Increases in volume after mixing
Decreases the tendency of molecules to escape from the liquid mixture Increases the tendency of molecules to escape from the liquid mixture
Temperature increases after mixing Temperature decreases after mixing
Heat is liberated (the heat energy released to form new bonds is greater than the heat energy absorbed to break the bonds in pure liquids) Heat is absorbed (the heat energy released to form new bonds is less than the heat energy absorbed to break the bonds in pure liquids)
Minimum vapour pressure Maximum vapour pressure
Azeotropic mixtures with maximum boiling point Azeotropic mixtures with minimum boiling point
Negative azeotrope

  • Formic acid and water
  • Hydrochloric acid and water
Positive azeotrope

  • Chloroform and methanol
  • Ethanol and water
  • Ethanol and benzene

The next post, Part 6 of Berry Berry Easy‘s series of notes on STPM Chemistry Form 6 notes on Phase Equilibrium will be focused on immiscible liquid.

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January 17, 2012
January 17, 2012

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