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Intermolecular Forces of Attraction

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Intermolecular Forces of Attraction

  1. 1. What are Intermolecular Forces of Attraction?• We’ve discussed ionic and covalent bonds • involve attractions between atoms • involve the exchange or sharing of electrons between two or more atoms• The resulting molecules have different forces of attraction for one another • These are intermolecular forces
  2. 2. Why are Intermolecular Forces Important?• They often determine the phase a substance is more likely to be. • Solid, liquid, or gas• Life science applications: DNA
  3. 3. Types of Intermolecular Forces of Attraction• London (or van der Waals) Forces• Permanent Dipoles• Hydrogen Bonds
  4. 4. London Forces• Weakest force• Instantaneous Dipole• Electrons in constant motion• No permanent positive/ negative region• Strength of force depends on total # of electrons
  5. 5. London Forces• Weakest force• Instantaneous Dipole• Electrons in constant motion• No permanent positive/ negative region• Strength of force depends on total # of electrons
  6. 6. London Forces• Weakest force• Instantaneous Dipole• Electrons in constant motion• No permanent positive/ negative region• Strength of force depends on total # of electrons
  7. 7. London Forces• Weakest force• Instantaneous Dipole• Electrons in constant motion• No permanent positive/ negative region• Strength of force depends on total # of electrons
  8. 8. London Forces• Weakest force• Instantaneous Dipole• Electrons in constant motion• No permanent positive/ negative region• Strength of force depends on total # of electrons
  9. 9. London Forces• Weakest force• Instantaneous Dipole• Electrons in constant motion• No permanent positive/ negative region• Strength of force depends on total # of electrons
  10. 10. London Forces• Weakest force• Instantaneous Dipole• Electrons in constant motion• No permanent positive/ negative region• Strength of force depends on total # of electrons
  11. 11. London Forces• Weakest force• Instantaneous Dipole• Electrons in constant motion• No permanent positive/ negative region• Strength of force depends on total # of electrons
  12. 12. London Forces• Weakest force• Instantaneous Dipole• Electrons in constant motion• No permanent positive/ negative region• Strength of force depends on total # of electrons
  13. 13. London Forces• Weakest force• Instantaneous Dipole• Electrons in constant motion• No permanent positive/ negative region• Strength of force depends on total # of electrons
  14. 14. London Forces• Weakest force• Instantaneous Dipole• Electrons in constant motion• No permanent positive/ negative region• Strength of force depends on total # of electrons
  15. 15. London Forces• Weakest force• Instantaneous Dipole• Electrons in constant motion• No permanent positive/ negative region• Strength of force depends on total # of electrons
  16. 16. London Forces• Weakest force• Instantaneous Dipole• Electrons in constant motion• No permanent positive/ negative region• Strength of force depends on total # of electrons
  17. 17. London Forces• Weakest force• Instantaneous Dipole• Electrons in constant motion• No permanent positive/ negative region• Strength of force depends on total # of electrons
  18. 18. London Forces• Weakest force• Instantaneous Dipole• Electrons in constant motion• No permanent positive/ negative region• Strength of force depends on total # of electrons
  19. 19. Permanent Dipoles• More durable than London Force• Permanent imbalance of e- in molecule• Positive and Negative end• Example: Acetone Acetone
  20. 20. Hydrogen Bonds• Strongest of all of the intermolecular forces• Must have...• H-N; H-F; or H-O• N, F, O very high EN values• Higher than expected BP, MP Water
  21. 21. Electronegativity andIntermolecular Forces
  22. 22. Are There Exceptions?• Yes, the total number of intermolecular forces present is very important• Some waxes (CH -CH -CH ....CH ) have 3 2 2 3 very high melting points despite having only London Forces• SbH has a higher BP than H O 3 2 • How can this be explained?
  23. 23. A Few Closing Questions...• Describe the bonding in CH3Cl?• CH3-CH2-CH2-OH• What about CH3-CH2-CH2-CH2-CH3?• What force holds the base pairs (C,G,A,T) together in DNA? • Covalent bonds? London forces? Hydrogen Bonds? • Why?

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