Kinetic Molecular Theory and the the Nature of Fluids

1. Kinetic Molecular Theory and the the Nature of Fluids A model for liquids/Evaporation Vapor Pressure Boiling Point

2. Kinetic Molecular Theory • States that that all substances are made of tiny particles (atoms and molecules) that are in constant motion • The kinetic energy and motion of these particles can help us to understand the nature of fluids and phase changes

3. A model for Liquids and gases- Kinetic Theory • Liquids and Gases have kinetic energy---allows flow • No attraction between gas particles • Intermolecular attraction between liquid particles keeps them together • Interplay between disruptive motions of particles in a liquid and attractions among particles determines physical properties of the liquid.

4. Phase Changes using Kinetic Theory Solid  Liquid  Gas Low KE Medium KE Highest KE KE Forces overcome the IM forces to escape IM Forces greater than KE so substance stays together

5. Density and Pressure • Liquids much more dense than gas due to intermolecular attraction • Increased pressure has little effect on liquids and solids volume • Solids and Liquids are then called condensed states of matter

6. Evaporation • Conversion of liquid to gas = vaporization • Most molecules don’t have enough kinetic energy to break free • When vaporization is at the surface without boiling = evaporation • During evaporation only those molecules with a certain minimum kinetic energy can escape from the surface of the liquid

7. Further Evaporation • Some escaping particle rebound back in off of air particles • Heating increases kinetic energy which increases evaporation • Removal of these higher energized particles leaves a lower average energy • THEREFORE: evaporation is a cooling process

8. Evaporation example • Sweating uses evaporation as a cooling process • Liquid on your skin takes heat energy • Liquid evaporates taking that energy with it • Leaves you with a lower temperature

9. Vapor Pressure • Vapor Pressure – measure of the force exerted by the gas leaving the surface of a liquid • Over time they increase and particles condense ---eventually return to liquid state

10. Vapor Pressure • In a system of constant vapor pressure, a dynamic equilibrium exists between vapor and liquid • Equilibrium because • rate of evaporation = rate of condesation

11. Vapor Pressure • As temperature increases, vapor pressure increases • Particles in liquid have increased kinetic energy • More KE, able to escape surface • Collide with “walls” • Vapor pressure measured with a manometer • Vapor increases, difference between levels increases

12. Boiling Point • Rate of evaporation increases as a liquid is heated • KE increases as temperature increases • When a liquid is heated to a temperature at which particles in the liquid have enough KE to vaporize, the liquid begins to boil.

13. Boiling Point • Boiling point- temperature at which the vapor pressure of the liquid is equal to the external pressure on the liquid • Not all liquids have same boiling point • Changes in altitude also affect boiling point

14. Boiling Point • Boiling similar to evaporation • Particles with most KE rise to surface and break free • Temperature of boiling liquid never rises above boiling point • Escaping particles take growing energy with them

15. Boiling Point • Interesting fact: • Burn from steam more severe than a burn from water • Steam particles can carry more energy than water particles • Collides with skin harder

16. Boiling Point • NORMAL BOILING POINT table

17. ASSESSMENT • What factors determine the physical properties of liquids?

18. ASSESSMENT • Explain how evaporation lowers the temperature of a liquid.

19. ASSESSMENT • What is vaporization?

20. ASSESSMENT • Define vapor pressure.

21. ASSESSMENT • What is a normal boiling point?