ALPHY6: VAPOURS

If the temperature of a liquid is fixed and evaporation takes place in a closed system, and there is an excess of liquid, then a point will come at which there will be no net evaporation.

Liquid molecules will leave the liquid, as vapour but they will also return to the liquid at the same rate overall. The vapour is saturated.

As the liquid heats up, the rate of evaporation of the liquid increases. At some point the rate at which molecules return to the liquid will again equal the rate at which they leave it, but the equilibrium will be at a higher saturated vapour pressure.

We may also explain the mechanism in this way. Consider a bubble in the liquid. The pressure within the bubble is the saturated vapour pressure at the temperature of the liquid, neglecting the pressure due to the water above the bubble.

As the liquid heats up, more bubbles form in the liquid and will tend to rise because they are less dense than liquid, so the rate of evaporation increases with temperature.

The last explanation provides insight into the boiling process. When the liquid boils, bubbles are formed within the liquid at a rapid rate because the liquid molecules have enough energy to break the transitory bonds between the liquid molecules.

The bubbles rises to the surface and escape so the vapour pressure inside the bubble must be equal to the pressure above the liquid.

We can say therefore: a liquid boils when it’s saturated vapour pressure is equal to the external pressure. The variation of saturated vapour pressure for a typical liquid is shown below for a liquid from it’s melting point at the origin up to it’s boiling point.

Vapor PressureTHE MICROSCOPIC VIEW

The vapor pressure of a liquid is the equilibrium pressure of a vapor above its liquid (or solid); that is, the pressure of the vapor resulting from evaporation of a liquid (or solid) above a sample of the liquid (or solid) in a closed container. Examples:

 

substance	vapor pressure at 25oC
diethyl ether	0.7 atm
bromine	0.3 atm
ethyl alcohol	0.08 atm
water	0.03 atm

The vapor pressure of a liquid varies with its temperature, as the following graph shows for water. The line on the graph shows the boiling temperature for water.

As the temperature of a liquid or solid increases its vapor pressure also increases. Conversely, vapor pressure decreases as the temperature decreases.

The vapor pressure of a liquid can be measured in a variety of ways. A simple measurement involves injecting a little of the liquid into a closed flask connected to a manometer. Click here for an illustration

THE MICROSCOPIC VIEW

• When a solid or a liquid evaporates to a gas in a closed container, the molecules cannot escape.
• Some of the gas molecules will eventually strike the condensed phase and condense back into it.
• When the rate of condensation of the gas becomes equal to the rate of evaporation of the liquid or solid, the amount of gas, liquid and/or solid no longer changes.
• The gas in the container is in equilibrium with the liquid or solid.

 

Microscopic equilibrium between gas and liquid. Note that the rate of evaporation of the liquid is equal to the rate of condensation of the gas.	Microscopic equilibrium between gas and solid. Note that the rate of evaporation of the solid is equal to the rate of condensation of the gas.

 

• The pressure exerted by the gas in equilibrium with a solid or liquid in a closed container at a given temperature is called the vapor pressure.

FACTORS THAT AFFECT SURFACE AREA

• • Surface Area: the surface area of the solid or liquid in contact with the gas has no effect on the vapor pressure.

• • Types of Molecules: the types of molecules that make up a solid or liquid determine its vapor pressure. If the intermolecular forces between molecules are:
    • relatively strong, the vapor pressure will be relatively low.
    • relatively weak, the vapor pressure will be relatively high.

 

ethyl ether (C4H10O)	ethyl alcohol (C2H6O)
Pvapor (25oC) = 520 torr The relatively weak dipole-dipole forces and London dispersion forces between molecules results in a much higher vapor pressure compared to ethyl alcohol.	Pvapor (25oC) = 75 torr Although dipole-dipole forces and London dispersion forces also exist between ethyl alcohol molecules, the strong hydrogen bonding interactions are responsible for the much lower vapor pressure compared to ethyl ether.

 

• Temperature: at a higher temperature, more molecules have enough energy to escape from the liquid or solid. At a lower temperature, fewer molecules have sufficient energy to escape from the liquid or solid.

 

Microscopic equilibrium between gas and liquid at low temperature. Note the small number of particles in the gas.	Microscopic equilibrium between gas and liquid at high temperature. Note the large number of particles in the gas.