Evaporation and Vapor Evaporation and Vapor PressurePressure

14.214.2

Phase ChangesPhase Changes

        

 

EvaporationEvaporation Evaporation occurs Evaporation occurs when molecules at the when molecules at the

liquids surface are moving fast enough to liquids surface are moving fast enough to escape into the gas phaseescape into the gas phase– Also called Also called vaporizationvaporization

Evaporation Evaporation requires energy to overcome requires energy to overcome intermolecular forcesintermolecular forces between the between the molecules of the liquidmolecules of the liquid

Evaporation – Evaporation – Endothermic or Exothermic?Endothermic or Exothermic?

Is evaporation endothermic or exothermic?Is evaporation endothermic or exothermic?

1.1. Fastest molecules leave from the Fastest molecules leave from the liquid to the gas phaseliquid to the gas phase

2.2. Now there is a decrease in Ave. KENow there is a decrease in Ave. KE

3.3. To equilibrate - heat will flow To equilibrate - heat will flow intointo the system (the system (thusthus endothermic endothermic))

Evaporation in a closed container…Evaporation in a closed container…

Dynamic EquilibriumDynamic Equilibrium Eventually, the same number of Eventually, the same number of

molecules are leaving the liquid as a molecules are leaving the liquid as a returning to itreturning to it– The rate of condensation = the rate of The rate of condensation = the rate of

evaporationevaporation Dynamic because still moving Dynamic because still moving Equilibrium because no net changeEquilibrium because no net change It occurs ONLY in a closed containerIt occurs ONLY in a closed container

Like a bridge in Like a bridge in and out of a cityand out of a city

Vapor Pressure The pressure of the vapor present at

equilibrium in a closed container is called vapor pressure of the liquid

12 molecules

= low pressure

24 molecules = high pressure

Vapor PressureVapor Pressure

The vapor pressure of a liquid The vapor pressure of a liquid depends on intermolecular forcesdepends on intermolecular forces– Liquids with high vapor pressures are Liquids with high vapor pressures are

called called volatilevolatile (evaporate readily) (evaporate readily) Why do Liquids with high Why do Liquids with high

intermolecular forces have a low intermolecular forces have a low vapor pressure?vapor pressure?

Why do Liquids with high intermolecular Why do Liquids with high intermolecular forces have a low vapor pressure?forces have a low vapor pressure?

1.1. High intermolecular force means High intermolecular force means particles are held tightly in the liquid particles are held tightly in the liquid statestate

2.2. The tighter held molecule means it must The tighter held molecule means it must move move FASTFAST to overcome the attraction to overcome the attraction

3.3. Less likely molecules are moving fast Less likely molecules are moving fast enough so few reach the gas phaseenough so few reach the gas phase

4.4. Fewer as a gas means less pressure Fewer as a gas means less pressure provided.provided.

Boiling Point is different from Boiling Point is different from Vaporization…..Vaporization…..

The temperature at which the vapor The temperature at which the vapor pressure of a liquid equals the pressure pressure of a liquid equals the pressure exerted on the liquidexerted on the liquid is called the is called the boiling pointboiling point..

The formation of the bubble is opposed by The formation of the bubble is opposed by atmospheric pressure.atmospheric pressure.

Boiling PointBoiling Point

The The normal boiling pointnormal boiling point is the is the boiling point at 1 atm.boiling point at 1 atm.

Why would the boiling point of a Why would the boiling point of a liquid vary throughout the world?liquid vary throughout the world?

The higher altitude The higher altitude the lower the lower the atmospheric pressure, the atmospheric pressure, therefore, a lower boiling point therefore, a lower boiling point will occur since a lower vapor will occur since a lower vapor pressure is requiredpressure is required