Phase Changes

• Vaporization- The process of changing from a liquid to a gas.

• Endothermic- lower energy liquid goes to a higher energy gas.

• When this occurs only at the surface the process is called evaporation.

Phase Changes• Evaporation is how our body controls its

temperature. As high energy molecules leave the surface of the water, the cooler ones are left behind.

• A substance that evaporates very easily is said to be volatile.

• condensation—opposite of vaporization. When the energetic steam molecules generated by your morning shower hurl themselves across the bathroom and collide with the cold mirror, they lose energy and return to the liquid phase.

• Exothermic - goes from high energy gas to a low energy liquid.

Phase Changes

• As temperature increases the amount of evaporation increases, if evaporation is taking place in a closed container. The vapor will exert a pressure on the liquid called vapor pressure.

Phase Changes

Two things affect the vapor pressure:•Temperature- increase temp increase vapor pressure•Intermolecular forces- the weaker the force, the higher

the vapor pressure.

Equilibrium vapor pressure• equilibrium vapor pressure—reached

when the rate of evaporation equals the rate of condensation in a closed container.

• Moleucles leave and enter the liquid phase at the SAME RATE.

Temperature and Vapor Pressure

• High Vapor Pressure = Low IMF

• The liquid takes energy from the surroundings (warm room) and vaporizes.

• Increase the temperature and more molecules have sufficient KE to escape.

• Slope of the ln Pvapor versus 1/T tells the ∆Hvaporization. Water has a greater slope than diethyl ether, therefore, it has a higher ∆Hvaporization.

Increase the temperature and more molecules have enough energy to

escape

Vapor Pressure and Molar Mass

• In general, as molar mass increases, vapor pressure DECREASES.

• This is due to polarizability.

• As MM increases, the number of electrons increases. Greater number of electrons increase the probability of an induced dipole moment.

Clausius- Clapeyron equation

• If you know VP and ∆Hvaporization at one temp, you can calculate VP at a different temp and you know you want to do that.

122

1 11ln

TTR

H

VP

VP vap

122

1 11ln

TTR

H

VP

VP vap

Exercise 6 Calculating Vapor PressureThe vapor pressure of water at 25°C is 23.8 torr, and the heat of vaporization of water at 25°C is 43.9 kJ/mol. Calculate the vapor pressure of water at 50°C.

Phase Changes

• Boiling Point- the temperature where the vapor pressure of a liquid is equal to atmospheric pressure.

• To determine the boiling point, a vapor pressure curve can be used

Phase Changes

• Sublimation- Changing directly from the solid phase to the gas phase. Dry Ice, Iodine, Smoke in your freezer are examples.

Phase Changes That Remove Energy

• Phase changes that release energy

• Freezing - Removing heat from liquid molecules, allows them to slow down and form intermolecular bonds.

• How does Melting Point and Freezing point of the same substance compare?

Phase Changes Cont’d

• Condensation- Going from a gas to a liquid. Bonds are forming. Energy must be released.

• Deposition- going directly from a gas to a solid. Snowflakes are an example

Supercooling• Supercooled liquids can stay liquid

below melting point because doesn’t achieve level of organization needed to make solid.

Superheated

• A substance is at a temperature above its BP, yet it remains a liquid. Usually happens when heated very rapidly [microwave oven] and bubbles form in the interior with high internal pressures. They often burst before reaching the surface.

Phase Changes Cont’dQuick Review

If two liquids are at the same temperature, which liquid:

• probably contains a nonpolar compound?

• has would have a lower vapor pressure?

• is more volatile?• has a greater evaporation

rate?• which has stronger attractive

forces?Draw the vapor pressure curve

and label the lines for liquid A and B.

Phase Diagrams

• Phase Diagrams:• Normal Melting Point/freezing point – The

temperature at 1 atm crossing the liquid/solid line.

• Normal Boiling Point – The temperature at 1 atm crossing the liquid/gas line.

• Triple point – The temperature and pressure point when all three phases exist

• Critical point – The temperature and pressure where the substance cannot exist as a liquid.

Phase Diagrams

PHASE DIAGRAMS• The AD line is the interface between liquid and solid.

– The melting point at each pressure can be found along this line.

– Below A the substance cannot exist in the liquid state.

• Along the AC line the solid and gas phases are in equilibrium; the sublimation point at each pressure is along this line.

Warming/Cooling Curves

q = mc∆T

q= mHv

q = mf