Aim:

I.O.SWBAT:

1) Define vapor pressure

2) Interpret phase change diagrams

3) Describe critical temperature and pressure

4) Draw a phase change diagram

MOTIVATION: What happens ice at -25oC if your add heat to it?

Do-Now: How does the behavior of a liquid in a closed container differ from that in an open one?

Vapor: Gas form of a substance that is a solid or a liquid at room temperature.

LIQUID GASDynamic equilibrium: Vaporization and condensation occur at the same rate.

As more molecules escape the liquid, the pressure they exert increases.

Vapor pressure: partial pressure of the vapor over the liquid measured at equilibrium

• At any temperature, some molecules in a liquid have enough energy to escape.

• As the temperature rises, the fraction of molecules that have enough energy to escape increases.

Boiling point: is the temperature at which the vapor pressure of a liquid equals the surroundings pressure.

Vapor Pressure Curves

The normal boiling point is the temperature at which its vapor pressure is 760 torr.

Volatile liquids: Liquids with a high vapor pressure and weak intermolecular forces. (have low boiling points )

 

Volatility: The tendency of a substance to evaporate easily.

PRACTICE:

1)Why does the vapor pressure of a liquid depend on intermolecular forces?

2)Why does it take longer to hard boil an egg at high elevations?

3)How does a pressure cooker cook food faster?

Do-Now: Complete hand-out.

Phase diagram: Is a graphic representation of the relationships between the physical state of a substance and its pressure and temperature.

• The AB line is the liquid-vapor interface.

• Each point along this line is the boiling point of the substance at that pressure.

• Triple point (A): solid, liquid and gas are in nequilibrium.

• Critical point (B): Beyond this temperature gas cannot be liquefied by increasing pressure.

• 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)

Phase Diagram of Water

• The slope of the solid–liquid line is negative.– As the pressure is increased

at a temperature just below the melting point, water goes from a solid to a liquid.

• The high critical point of water is due to the strong van der Waals forces between water molecules.

Phase Diagram of Carbon Dioxide

• Carbon dioxide cannot exist in the liquid state at pressures below 5.11 atm.

• CO2 sublimes at normal pressures.

Isotherm: constant temperatureIsobar: constant pressure

PRACTICE:

Complete Hand-Out

HW#44