Van der Waals Forces

1. Van der Waals Forces Johannes Diderik van der Waals

2. Polarity Separation of charge An asymmetrical difference in electronegativity along a bond or in a molecule

3. Cl Al O N Cl H H H Cl H H Cl S O C O Cl C Cl H H Cl Circle the polar molecules. Label d+ and d- d- d- d+ d- d+ d+

4. Non-Polar 104.5o C. __________ molecules are symmetrical  D. What is the bond angle in H2O? _______  E. The motion of particles in these phases: Solid Liquid Gas http://itl.chem.ufl.edu/2045_s00/lectures/FG11_001.GIF

5. Van der Waals Forces Small, weak interactions between molecules

6. Van der Waals Forces Intermolecular: between molecules (not a bond) Intramolecular: bonds within molecules (stronger)

7. What is being attracted? d+ attracted to d-  electrostatic attraction e- s of one atom to another atom’s nucleus e- + + e-

8. O O O O O O O O O O O O C C C C C C C C C C C C O O O O O O O O O O O O Evidence of VDW Forces? Non-polar molecules can form gases, liquids and solids. Ex: CO2

9. 3 Types of Van der Waals Forces 1)dipole-dipole 2)dipole-induced dipole 3) dispersion

10. Dipole-Dipole Two polar molecules align so that d+ and d- are matched (electrostatic attraction) Ex: ethane (C2H6) vs. fluromethane (CH3F)

11. Fluoromethane (CH3F) – boiling point = 194.7 K H H H H H C C H H C C H H H H H Dipole-Dipole NOT Dipole-Dipole d- d+ d+ d- polar or non-polar? H H H C F H C F H H Ethane (C2H6) – boiling point = 184.5 K polar or non-polar?

12. Br Br K K Try This: Draw two KBr molecules and draw their dipole-dipole interactions with a dashed line.

13. What does to “induce” mean? • To cause or bring about Ex: Induced vomiting Induced labor Induced coma

14. Dipole-Induced Dipole A dipole can induce (cause) a temporary dipole to form in a non-polar molecule The molecules then line up to match d+ and d- charges

15. Cl d+ d- H Example e- e- e- e- Ar e- e- d- d+ e- e- e- e- e- e- e- e- e- e- e- e- INDUCED DIPOLE non-polar A DIPOLE (it’s polar) Dipole – Induced Dipole (weak and short-lived)

16. O H H O O C C O O Draw CO2 (aq) What does (aq) mean? dissolved in WATER So…draw CO2 (g) in H2O (l) d+ d- d+ d- d+ d-

17. Where is CO2 (aq) seen? Carbonated water CO2 is not very soluble… 1 CO2 in 1000 H2O molecules http://www.packaging-technology.com/contractor_images/venus/4_rinser.jpg

18. Dispersion Forces A temporary dipole forms in a non-polarmolecule… which leads to… a temporary dipole to form in ANOTHERnon-polar molecule Dispersion is the ONLY intermolecular attraction that occurs between non-polar molecules

19. d- d+ d+ d- Dispersion Forces e- e- e- e- e- e- Cl-Cl e- e- Cl-Cl e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- INDUCED DIPOLE TEMPORARY DIPOLE non-polar non-polar Dispersion (weakest and very short-lived)

20. Tokay Gecko:Dispersion Forces!

21. Review Dipole – Dipole between two polar molecules Dipole – Induced Dipole b/w a polar & a non-polar molecule Dispersion between two non-polar molecules

22. Hydrogen Bonding STRONGEST Intermolecular Force!! A special type of dipole-dipole attraction Bonds form due to the polarity of water Draw 3 H2O molecules in your notes Ice Liquid

23. Hydrogen Bonding con’t Hydrogen bonds keep water in the liquid phase over a wider range of temperatures than is found for any other molecule of its size

24. Hydrogen bonds account for the high boiling point of water

25. Expansion of Ice Ice expands when water freezes compared to most substances that contract when freezing Ice bomb video

26. Denisty vs Temperature of H2O 4 oC—max density of water – liquid! Solid Ice Liquid water

27. Hexagonal Ice http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/imgche/waterhex.gif http://www.gala-instrumente.de/images/44%20hexagonal%20ice.jpg

28. http://www.lummox.net/celestial/pics/ak1999-sundog.jpg Halos, Sundogs, & Pillars are caused by hexagonal ice crystals http://images.usatoday.com/tech/_photos/2006/09/12/cloud.jpg

29. Ponds Freezing Solid water (ice) has a lower density than liquid water

31. Why is this good? Ponds freeze from the top down, insulating the water below and keeping it from freezing solid Without this, ponds would freeze solid and thaw more slowly

32. Surface Tension Enhancement of the intermolecular attractive forces at the surface

33. Evidence Lab: Dixie cup Penny Capillary tube needle

34. What causes surface tension? The cohesive forces between molecules are shared with all neighboring atoms. Since the surface has no neighboring atoms above, they exhibit stronger attractive forces for their neighbors next to and below them

35. Surface tension is a result of cohesive intermolecular forces

36. How many drops can you get on a penny? Water? TTE? Why is there a difference??? Water has strong Hydrogen Bonds and TTE has weaker intermolecular forces http://www.msnucleus.org/membership/html/k-6/wc/water/1/images/penny.jpg

37. How is surface tension affected by soap? Breaks the surface tension! http://www.chemistryland.com/CHM107/Water/SoapDisruptsWater.jpg http://www.chemistry.nus.edu.sg/2500/micelle.jpg

38. Capillary Rise glass Water rises up the capillary tube because there are unbalanced forces between the water and glass and the water and gravity gravity H2O Hg

39. Which is larger? Adhesion or Cohesion? Adhesion: attraction between H2O (Hg) & glass Cohesion: attraction of H2O (Hg) molec. to each other Adhesion > Cohesion Cohesion > Adhesion

40. Do other liquids exhibit capillary rise? As long as they are attracted to glass and have enough cohesion

41. IM forces and interactions between liquids and surfaces Cohesion > Adhesion Liquid “Beads” on Surface Cohesion < Adhesion Liquid “Wets” the Surface

42. Evaporation Diagram the distribution of kinetic energy at a temperature 5oC 25oC 75oC # particles low KE ave KE high KE

43. Which molecules will evaporate? This lowers the total kinetic energy (temperature) of the entire system Only high energy molecules can vaporize # particles low KE ave KE high KE

44. Pvap Pvap P atm P atm P atm Pvap t = 0 min t = 5 min BOILING! t = 1 min Boiling Pvap= Patm

45. Boiling Boiling occurs when Vapor Pressure = Barometric Pressure When Vapor Pressure = 760 mmHg, Boiling Point = 100oC

46. Evaporation Questions • Why do we sweat? breaking water’s bonds has a cooling effect high energy molecules are lost

47. 2. Why does water stay cool in clay containers? Since clay is porous, high energy molecules escape leaving lower temperature water When the water added to the sand evaporates in the Pot-in-Pot Cooler, it pulls heat from the smaller pot, keeping vegetables cool. Refrigeration for the other 90% http://www.juneauempire.com/images/050406/13484_500.jpg http://www.npr.org/templates/story/story.php?storyId=11032381&sc=emaf

48. 3. Why can liquid water change to vapor at room temperature? High energy molecules escape Evaporation occurs at all temperatures # particles low KE ave KE high KE

49. 4. Define vapor pressure Force of particles leaving a liquid Pressure of molecules in their bubbles Can solids have a vapor pressure? Yes! Solid Gas Ex: ice, dry ice, plastics

50. 5. What is the difference between evaporation and boiling? Evaporation: occurs at any temperature; high energy molecules escape Boiling: occurs when atmospheric pressure = vapor pressure