The Liquid State
Chemistry: Cambridge International A Level, topic 1.4.5
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
- 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
- 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
- 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 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
- 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.
Test yourself on The Liquid State
The presence of which of the following intermolecular forces in a substance would reduce the vapor pressure most?
The boiling point of a liquid is the temperature at which its equilibrium vapor pressure is equal to what?