Vapor Pressure

1. Vapor Pressure

2. Vapor Pressure Vaporization - change from liquid to gas at boiling point. Evaporation - change from liquid to gas below boiling point Heat of Vaporization(DHvap ): the energy required to vaporize 1 mol at 1 atm. (Aka: Enthalpy of Vaporiz.)

3. Vaporization is an endothermic process - it requires heat. Energy is required to overcome intermolecular forces.

4. Condensation Change from gas to liquid. Molecules achieve a dynamic equilibrium with vaporizing molecules in a closed system. A closed system means matter can�t go in or out. Sealed! But What is Dynamic Equilibrium?

5. Dynamic equilibrium When first sealed the molecules escape the surface of the liquid As gas molecules build up, some condense back to a liquid. As time goes by the rate of vaporization remains constant but the rate of condensation increases because there are more molecules to condense. Equilibrium is reached when

6. Rate of Vaporization = Rate of Condensation Molecules are constantly changing phase �Dynamic� The total amount of liquid and vapor remains constant �Equilibrium� Dynamic equilibrium

7. Vapor pressure Defined as the pressure above the liquid at equilibrium. Liquids with high vapor pressures evaporate easily. They are called volatile. Vapor Pressure decreases with increasing intermolecular forces. Bigger molecules (bigger LDF) More polar molecules (dipole-dipole)

8. Vapor pressure Smaller Molecules with weak IM forces tend to be most volatile. (High Vapor Pressures) Vapor Pressure also ? with ? temperature. Easily measured in a barometer.

9. A barometer will hold a column of mercury 760 mm high at one atm

10. A barometer will hold a column of mercury 760 mm high at one atm. If we inject a volatile liquid in the barometer it will rise to the top of the mercury.

11. A barometer will hold a column of mercury 760 mm high at one atm. If we inject a volatile liquid in the barometer it will rise to the top of the mercury. There it will vaporize and push the column of mercury down.

12. The mercury is pushed down by the vapor pressure. Patm = PHg + Pvap Patm - PHg = Pvap 760 - 736 = 24 torr

13. Effects of Temperature on Vapor Pressure NRG is needed to overcome intermolecular forces. As temperature (average KE) is increased, more of these forces are broken creating more vapor. Vapor Pressure increases

14. Mathematical relationship ln is the natural logarithm DHvap is the heat of vaporization in J/mol

15. R = 8.3145 J/K mol. Surprisingly this is the graph of a straight line. (actually the proof is in the book) Mathematical relationship

16. Changes of state The graph of temperature versus heat applied is called a heating curve. The temperature at which a solid turns to a liquid is the melting point. The energy required to accomplish this change is called the Heat of Fusion DHfus (or Enthalpy of Fusion)

19. Melting Point Melting point is determined by the vapor pressure of the solid and the liquid. At the melting point the vapor pressure of the solid = vapor pressure of the liquid

20. Boiling Point Reached when the vapor pressure equals the external pressure. Normal boiling point: is the boiling point of a liquid at 1 atm pressure. Super heating - Heating above the boiling point. Supercooling - Cooling below the freezing point.