Understanding Phase Changes: Evaporation, Boiling, and More

1. Unit 10- States of Matter III. Changes of State

2. A. Phase Changes

3. A. Phase Changes • Evaporation • molecules at the surface gain enough energy to overcome IMF • Volatility (how easily a substance can change to a gas) • Is a measure of evaporation rate • depends on temp. & IMF

4. # of Particles Temp. volatility IMF(Stronger) volatility A. Phase Changes Boltzmann Distribution p. 477 Kinetic Energy

5. A. Phase Changes • Dynamic Equilibrium • trapped molecules reach a balance between evaporation & condensation

6. temp v.p. IMF v.p. A. Phase Changes p.478 • Vapor Pressure • pressure of vapor above a liquid at equilibrium • depends on temp. & IMF • directly related to volatility (dotted vertical line = Boiling Point of the liquid, dotted horizontal line = Standard Pressure on the graph) v.p. temp

7. Patm b.p. IMF b.p. A. Phase Changes • Boiling Point • temp at which v.p. of liquid equals external pressure • depends on Patm & IMF • Normal B.P. - b.p. at 1 atm Pressure

8. IMF m.p. A. Phase Changes • Melting Point • equal to freezing point • Which has a higher m.p.? • polar or nonpolar? • covalent or ionic? polar ionic

9. A. Phase Changes • Sublimation • solid  gas • v.p. of solid equals external pressure • EX: dry ice, mothballs, solid air fresheners

10. B. Heating Curves – sketch this in your notes(add a page) Gas - KE  Boiling - PE  Liquid - KE  Melting - PE  Solid - KE 

11. B. Heating Curves Temperature Change Means a change in KE (molecular motion) depends on heat capacity • Heat Capacity • energy required to raise the temp of 1 gram of a substance by 1°C • “Volcano” clip - water has a very high heat capacity

12. B. Heating Curves Phase Change (while this is happening) Means a change in PE (molecular arrangement)  Temp. remains constant  • Heat of Fusion (Hfus) • energy required to melt 1 gram of a substance at its m.p.

13. B. Heating Curves Heat of Vaporization (Hvap) energy required to boil 1 gram of a substance at its b.p. usually larger than Hfus…why? • EX: sweating, steam burns, the drinking bird

14. C. Phase Diagrams Show the phases of a substance at different temps and pressures.

15. C. Phase Diagrams • The diagram on the last slide shows the phase diagram for water • Each region represents a pure phase • Line between regions is where the two phases exist in equilibrium • Triple point is where all 3 curves meet, the conditions where all 3 phases exist in equilibrium!

16. Phase changes by Name Critical Point Pressure (kPa) Temperature (oC)

17. C. Phase Diagrams • With a phase diagram, the changes in m.p. and b.p. can be determined with changes in external pressure • What are the variables plotted on a phase diagram?

18. Phase Diagrams: • representphasesdependent onpressureandtemperature. liquid solid Pressure gas Temperature

19. Triple Point(A): • T & P with all 3 states exist in equilibrium liquid solid Pressure triple point gas Temperature

20. supercritical fluid Critical Point(B): • above Tno liquid can exist (too energized) • above Pno gas can exist (too compressed) liquid critical point solid Pressure gas Temperature

21. liquid-vapor equilibrium(AB line): • every point along line is the boiling point(at that pressure) liquid =T↓P solid Pressure =P , ↑T gas Temperature

22. solid-liquid equilibrium(AD line): • every point along line is the melting point • (at that pressure) (freezing) =P , ↑T liquid solid Pressure =T↓P gas Temperature

23. solid-gas equilibrium(AC line): • every point is sublimation point • below A liquid cannot exist at such low pressure liquid solid Pressure gas Temperature