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The Liquid State

Intermolecular forces are responsible for determining many of the properties of liquids. The intermolecular forces present in a liquid affect the melting point, vaporisation, and vapour pressure.

Vapour pressure

Vapour pressure

  • When a liquid vaporises in a closed container, gas molecules cannot escape.
  • As these gas-phase molecules move randomly about, they will occasionally collide with the surface of the condensed phase.
    • In some cases, these collisions will result in the molecules re-entering the condensed phase.
  • The change from the gas phase to the liquid is called condensation.
  • When the rate of condensation becomes equal to the rate of vaporisation, neither the amount of the liquid nor the amount of the vapour in the container changes.
Vapor pressure - 2

Vapor pressure - 2

  • The vapour in the container is then said to be in equilibrium with the liquid.
  • Such is an example of dynamic equilibrium:
  • The status of a system in which reciprocal processes (for example, vaporisation and condensation) occur at equal rates.
    • The pressure exerted by the vapour in equilibrium with a liquid in a closed container at a given temperature is called the liquid’s vapour pressure.
Boiling points

Boiling points

  • When the vapour pressure increases enough to equal the external atmospheric pressure, the liquid reaches its boiling point.
    • The boiling point of a liquid is the temperature at which its equilibrium vapour pressure is equal to the pressure exerted on the liquid by its gaseous surroundings.
  • The normal boiling point of a liquid is defined as its boiling point when surrounding pressure is equal to 1 atm (101 kPa).
  • Boiling is also called vaporisation.
Intermolecular forces

Intermolecular forces

  • Intermolecular forces are broken during vaporisation.
  • So, the vapour pressure and boiling point of a substance depend directly on the type and strength of intermolecular forces present within it.
    • Strong intermolecular attractive forces will serve to impede vaporisation, resulting in relatively low vapour pressure.
    • Weak intermolecular attractions present less of a barrier to vaporisation and a reduced likelihood of gas recapture.
      • This results in relatively high vapour pressures.
Melting point

Melting point

  • Intermolecular forces also impact the melting point of a solid.
  • During melting, intermolecular force interactions are just rearranged and not broken.
    • This means the impact of intermolecular forces on melting points can be more subtle.
Jump to other topics
1

Physical Chemistry

1.1

Atoms, Molecules & Stoichiometry

1.2

Atomic Structure

1.3

Chemical Bonding

1.4

States of Matter

1.5

Chemical Energetics

1.6

Electrochemistry

1.7

Equilibria

1.8

Partition Coefficient

1.9

Reaction Kinetics

2

Inorganic Chemistry

3

Organic Chemistry & Analysis

3.1

Introduction to Organic Chemistry

3.2

Hydrocarbons

3.3

Halogen Derivatives

3.4

Hydroxy Compounds

3.5

Carbonyl Compounds

3.6

Carboxylic Acids & Derivatives

3.7

Nitrogen Compounds

3.8

Polymerisation

3.9

Analytical Techniques

3.10

Organic Synthesis

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