1. Intermolecular Forces... explaining the properties of liquids

2. Let’s get one thing straight... Intra*molecular forces include: Non-polar covalent bonds Polar covalent bonds Ionic bonds (not technically within a molecule, but anyway...) Metallic Bonding *intra = within/between (pick one) within

3. Intermolecular forces exist between molecules Intermolecular forces are the glue that holds molecules together Consider the following data:

4. CompoundFormulaMolar mass (g) mp ( o C)bp ( o C) methaneCH 4 16-183-164 ammoniaNH 3 17-77.7-33.3 waterHOH180100 neonNe20-249-246 methanolCH 3 OH32-93.964.9 hydrogen sulfide HSH34-85.5-60.7

5. Why is the boiling point of water (and its mp) so much higher than that of other small molecules? Due to the high strength of intermolecular forces.

6. Types of Intermolecular Forces 1. van der Waals Forces include: Dipole-dipole Ion-dipole Induced Intermolecular Forces Ion-induced dipole Dipole-induced dipole Dispersion (London) Forces 2. Hydrogen Bonding

7. Let’s look more closely at each type … Dipole-Dipole Forces exist between polar molecules --or represet as--

8. polar molecules will have a stronger attraction for one another than similarly- sized non-polar molecules this results in ____________ boiling points for polar molecules compared to similarly- sized non-polar molecules (higher)

9. Ion-Dipole Forces exist between a polar solvent and an ionic solute

10. eg. NaCl dissolved in water Remember: Ion-dipole attractions only occur with soluble ions.

11. Induced Intermolecular Forces What does induced mean? 1.To lead or move, as to a course of action, by influence or persuasion. Synonym: ______________. 2.To bring about or stimulate the occurrence of; cause: a drug used to induce labour. (thanks to dictionary.com)

12. Ion-induced dipole Occurs when an ion induces a nearby non-polar molecule to become polar ie. The ion distorts the electron density of the nearby non-polar molecule

13. eg of Ion-induced dipole Bonding between Fe 2+ ion and O 2 in hemoglobin  Vital for O 2 transport in the body.

14. dipole-induced dipole Similar to ion-induced dipole. A polar molecule induces a nearby molecule to become polar by distorting its electron cloud.

15. example of dipole-induced dipole Explains the—limited, but significant— solubility of O 2 in water.

16. Dispersion (London) Forces Seems like nonpolar molecules have no way of attracting each other. BUT: nonpolar gases (O 2, N 2, CH 4 ) can be liquefied. This indicates that if kinetic energy of molecules reduced, some type of attractive force can predominate.

17. Fritz London (1930) suggested that the motion of electrons within an atom or non-polar molecule can result in a transient dipole moment. (transient = short-lived)

18. A Model To Explain London Dispersion Forces: Consider two Helium atoms (each with 2 electrons)

19. Explanation of Dispersion Forces At a given instant, there may be an uneven distribution of e - s around an atom or molecule. Think electron “pile-up”.

21. How does the strength of dispersion forces correlate to molecular size?

22. as the size of a non-polar molecule increases, so does its electron cloud this will increase the strength of the temporarily induced dipole ie. as non-polar molecules get larger, their bp increases

23. Consider the following bp data

24. What’s with the unusually high bp of HF NH 3 H 2 O ? What does this suggest about the strength of intermolecular bonding in these compounds? Very strong...

25. Hydrogen Bonding HF, HOH, NH 3 all have significantly higher bp than predicted from the trend in each family. (bp of H 2 O ca. 200 o C greater than expected) due to the very polar H-F, H-O, H-N bonds

26. Simple molecules which exhibit H-bonding:

27. For Hydrogen Bonding to occur... Hydrogen must be attached to one of the most electronegative elements (N,O,F) very polar bonds dipole vectors don’t cancel

28. In these v polar bonds  H acquires a significant amount of positive charge. Remember: O has 2 lone pairs of e - s

29. The H must be bonded to either N, O, F. These atoms each have at least one "active" lone pair.

30. “Active” lone pairs in second E level are contained in a relatively small volume  results in high density of negative charge. (Lone pairs at higher levels are more diffuse and not so attractive to positive charges.)

31. Consider five water molecules close together

33. Review of Intermolecular Bonding

34. Another Review

35. Homework From Chemistry 12 (Mustoe et al) This is posted as “intermolecular forces explanation” From this handout: P 208 SR 1, 2, 3abc (omit “network”) 4, 5 (omit “network”), 6, 7 Also: Illustrate the intermolecular forces that exist in NH 3 (aq). Draw a pseudo 3-D picture of NH 3, decide on molecular polarity and intermolecular forces.