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The equilibrium that exists between a liquid and its vapour depends on the nature of the forces between the particles in the liquid. |
Interparticular forces
Forces that hold the particles together in the body of a liquid must be overcome for the liquid to vaporise. As the forces get stronger, the particles in the liquid have more difficulty overcoming these forces and the vapor pressure reflects this.
For example, the vapour pressure of water is a lot less than that of ethoxyethane at a similar temperatures.
|
Temperature
|
vapour pressure/atmos
|
|
|---|---|---|
|
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water
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ethoxyethane
|
|
20ºC
|
0.023
|
0.5
|
This is due to the different types of intermolecular forces in these two compounds. Water has a high degree of hydrogen bonding, whereas ethoxyethane only has dispersion forces between its molecules. dispersion forces are much weaker than those resulting from hydrogen bonding for similar sized molecules.
Enthalpy of vaporisation
The enthalpy of vaporisation is the energy required to convert 1 mole of a liquid into gaseous particles at infinite separation.
H2O(l) 
H2O(g) ΔH (vap) = 40.8 kJ mol-1
The enthalpy of vaporisation gives a measure of the strength of interparticular bonding and is consequently directly related to the vapour pressure of a liquid. Liquids with high values for the enthalpy of vaporisation have low values for vapour pressure.
|
enthalpy of vaporisation/kJ mol-1
|
|
|---|---|
|
water
|
ethanol
|
|
40.8
|
38.6
|
This topic is directly linked to the intermolecular forces section of the bonding. Students should be familiar with dispersion forces, permanent dipolodipole interactions and hydrogen bonding.
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Example: Using the data below state and explain the relationship between the enthalpy of vaporisation and intermolecular forces.
From the data given, more energy is needed to vaporise propanoic acid than pentane. The forces that attract the molecules to one another in propanoic acid are stronger than the intermolecular forces in pentane. Pentane is a non-polar molecule whose intermolecular forces are due to dispersion (induced dipole - dipole attractions) only, whereas in propanoic acid there are dispersion and also hydrogen bonding between the hydrogen of the O-H group and oxygen atoms on neighbouring molecules. Hydrogen bonding is considerable stronger than dispersion forces in molecules of similiar size. In this case the relative molecular mass of the two molecules is comparable, pentane = 72, propanoic acid = 74, so the effect of dispersion forces is roughly the same. Pentane, however has no hydrogen bonding, while propanoic acid does. |
Boiling point
The boiling point of a liquid is the temperature at which the vapour pressure of the liquid equals the atmospheric pressure. Generally, boiling points are defined at standard atmospheric pressure, although certain substances may be defined at lower pressures if they have a tendency to decompose at higher temperatures.
As the volatility of a liquid and vapour pressure depend on its intermolecular forces, the boiling point is related to the enthalpy of vaporisation.
|
boiling point/ºC
|
|
|---|---|
|
water
|
ethanol
|
|
100
|
79
|
In the above table the boiling point of water is higher than that of ethanol. The hydrogen bonds in water are more extensive, having two atoms of hydrogen per molecule with which to form the bonds. Water's intermolecular forces are, therefore stronger than those of ethanol.
This information can be extrapolated to give a general pattern for boiling points, enthalpy of vaporisation and intermolecular forces.
|
boiling point
|
Intermolecular forces
|
enthalpy of vaporisation
|
|---|---|---|
|
high
|
strong
|
high
|
|
low
|
weak
|
low
|
Distillation
Distillation is an experimental technique that makes use of the change of state from liquid to a gas at the boiling point in order to separate volatile liquids from other, non-volatile substances.
The procedure involves:
- 1 Heating to boiling and vaporising
- 2 Leading the vapour away from the heated vessel
- 3 Cooling and condensing
Distillation can only be done when the mixed phases do not interfere with one another. If there is appreciable interaction between the mixed phases then fractional distillation must be used.
Fractional distillation
In many cases mixtures of miscible liquids cannot be separated by simple distillation as they form boiling mixtures in which the vapour is also a mixture of the components. In such cases fractional distillation must be used.
The procedure is very similar except that a fractionating column is included above the heated vessel (usually a flask) before the vapours arrive at the junction of the condenser.
Fractional Distillation uses repeated cycles of boiling and condensing within the fractionating column to separate the volatile components of a miscible liquid mixture.
Although it is an efficient procedure there are some miscible liquid mixtures that cannot be purified to 100%, as the components form a constant boiling mixture.
This is a liquid mixture that boils to produce a vapour with the same percentage of the components as the liquid. One example is ethanol and water.
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Example: Ethanol - water distillation. The boiling points of ethanol and water are 79ºC and 100ºC respectively. They form a mixture which is miscible in all proportions. Fractional distillation can be used to purify ethanol (as the more volatile component), but not to 100% pure. When the liquid/vapour reaches a composition of 96% ethanol and 4% water no amount of repeated cycles can increase the percentage of ethanol. This percentage composition represents a constant boiling mixture. i.e. if it is boiled the vapour also has the same composition. |
