Intermolecular Forces

Vapor Pressure

Suppose we have a closed container into which we pour some water. As soon as we add the water we check a pressure gauge connected to the container. We let the container sit for a while and then we check the pressure again. What might the pressure guage indicate?

As the water evaporates the pressure exerted by the vapor above the liquid increases, until at some point, the pressure reaches a constant value, the vapor pressure of the substance:

The kinetic energy of the molecules at the surface of a liquid varies over a range of values:

• Some of the molecules have enough kinetic energy to overcome the attractive forces between the molecules
• The weaker the attractive forces, the greater the fraction of molecules with enough kinetic energy to escape
• The greater the fraction of molecules which can escape the liquid, the greater the vapor pressure

Not only can water molecules leave the surface, but molecules in the vapor phase can also hit and go into the water

• Initially, there are no molecules in the vapor phase and the number of molecules in the vapor which are rejoining the water is zero
• As time goes on there are more molecules in the vapor phase and the number of a vapor molecule striking the water increases
• At some point in time the number of vapor molecules rejoining the water equals the number leaving to go into the vapor phase
• an equilibrium has been reached, and the pressure has stabilized at the characteristic vapor pressure of the substance

Vapor pressure increases with temperature

• At higher temperature more molecules have the necessary kinetic energy to escape the attractive forces of the liquid phase
• The more molecules in the vapor phase, the higher the vapor pressure

• This interior bubble will rapidly collapse if the external pressure is greater than the vapor pressure
• If the external pressure is equal to, or lower than the vapor pressure, then the bubble will remain or expand and the liquid boils

Vapor pressure increases with increasing temperature

• At 100°C the vapor pressure of water is 760 torr (1 atm) or equal to the atmospheric pressure on the liquid (in an open container)
• At this temperature, interior bubbles will not collapse and the water boils
• At high altitudes (i.e. up in the Mountains) the air pressure is less than at sea level. Thus, water will boil at a lower temperature (the vapor pressure needed to support a bubble is lower at high altitude). Therefore, cooking times are longer for things that need to be boiled (e.g. boiled eggs take longer to cook at high altitudes).

1996 Michael Blaber