STPM Chemistry Form 6 Notes – Phase Equilibrium (Part 2)

by BerryBerryTeacher

in Berry Reference (Notes)

Boiling point or more frequently abbreviated to B.P. or bp, is simply the temperature of an element or substance at which liquid boils. The boiling point is dependent on the relationship between the vapour pressure and temperature of the surroundings. While all might seem unrelated to Berry Readers at the moment, but this Part 2 of Berry Berry Easy‘s short notes on Phase Equilibrium for STPM Form 6 Chemistry will attempt to answer everything. Do read up on the example cases too.

[Tips: Boiling point of a substance is never the same at different environmental conditions. Students typically get confused with boiling point and normal boiling point/atmospheric boiling point. The normal boiling point is a special case in which the vapor pressure of the liquid equals the defined atmospheric pressure at sea level, 1 atmosphere. So do not get confused.]

STPM Chemistry Form 6 Notes – Phase Equilibrium (Part 2)

Fractional Distillation

Fractional Distillation

Relationship between vapour pressure and temperature

  • Vapour pressure increases as the temperature of the solution increases in a linear form
  • Everyday situation: Observe the process of boiling water. This process would take a while for the bubbles to form in the water as it nears its boiling point. Once the bubbles form, it need to take a short period of time before the water actually reach the boiling point.
  • In conclusion, as the temperature of a liquid increases, the pressure of its vapour increases but in an exponential graph.

Boiling point

  • Definition based on phases: Boiling point is the temperature above which a substance may not exist as a liquid
  • Definition based on vapour pressure: Temperature at which the vapour pressure of a liquid is equal to the atmospheric pressure

Case 1: In the mountain

The boiling point of a liquid is lower than normal at reduced atmospheric pressure.

Case 2: In a pressure cooker

The boiling point of a liquid is higher than normal at increased atmospheric pressure

  • Boiling process involves vaporisation of a compound in its liquid phases and boiling point is affected by intermolecular forces (hydrogen bonding, molecular mass and polarity) and atmospheric pressure.

Example:

Which of the following compounds has the highest boiling point?

A H3COH
B H3CNH2
C H3CCH2NH2
D H3CCH2OH

Solution: Highest boiling point = greatest intermolecular forces and greatest molecular mass. Option A, B, C and D can form hydrogen bond, but alcohols (-OH) are more polar and form stronger hydrogen bonds than their amine (-NH2) equivalents. This eliminates option B and C. Option D (ethanol) is heavier than option A (methanol), so the higher boiling point is option D.

Answer: D

The next part, Part 3 of Berry Berry Easy‘s short notes on Phase Equilibrium for STPM Form 6 Chemistry will be focused on Raoult’s Law and within it, the concept of the total vapour pressure. Do visit this site again to get concise notes from our site.

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