Ch. 09: Liquids & SolidsVapor Pressure

Vapor Pressure• Vapor = the gaseous state of a substance that has escaped

from the liquid phase

e.g., ½-filled cup of coffee:

• vaporization = liquid is converted to its gas form

(in open system, it continues to complete evaporation)

liquid vapor

• dynamic equilibrium: rate vaporization = rate condensation

• vapor (a gas) follows gas laws

• this chapter on closed system: dynamic equilibrium between liquid & vapor.

Vapor Pressure

Vapor Pressure 9.1

• Theory:

• When temperature rises, the vapor pressure of a liquid increases. (molecules have sufficient energy to break away from the surface of the liquid)

• When the vapor pressure = pressure above the liquid, the liquid boils.

• Special case: @1 atm, bp is called the normal boiling Tb

Vapor Pressure 9.1

• Vapor pressure varies exponentially with temperature.

Clausius-Claperon equation:

or:

y = mx + b

Vapor Pressure 9.1

Clausius-Claperon equation:

@ T2: @ T1:

or

R = 8.31 J/mol-K

E.g., can find heat of vaporization; vapor pressures; temperature for specific vapor pressures

(similar equations)

• Clasius-Claperyon:Vapor Pressure

• Arrhenius:Temperature on Reaction Rate

• van’t HoffTemperature on Rate Constant

Vapor Pressure

Vp• = pressure exerted by the vapor over the liquid

• At equilibrium, Vp is independent of volume.

• The larger the volume, the more liquid that vaporizes

• ratio “n/V” remains constant, so:

• P = nRT/V does not change until all of the liquid has vaporized, and Vp will be less that Pequilibrium.

Vapor Pressure

Example 9.1A 2.0-L “cool mist” vaporizer adds moisture to dry air at 25oC. The room is 12’ x 12’ x 8’. VpH2O is 24 mmHg at 25oC. (Assume density of 1.00 g/mL)(a) If the vaporizer runs until it is empty, what is the vapor pressure of the water in the room?