1. Intermolecular Forces
What is the difference between an intramolecular force and an intermolecular force? How might molecules attract each other?
Intermolecular Forces

2. Intramolecular Force Forces of attraction WITHIN molecules
IONIC BONDING
Ex: NaCl
COVALENT
(nonpolar and polar)
Ex: H2, HCl
METALLIC
Ex: Cu, brass

3. INTERMOLECULAR FORCES (IMF’s)
Attractive forces between molecules.
Much weaker than chemical bonds
within molecules.

4. Strength of IMF’s is linked to phase of matter
Stronger IMF’s = higher bp and mp

5. B. Types of IMF
Video Link

6. Covalent Bonds vs Intermolecular Forces
Icing within holding cookies together = analogy for covalent bond
Butter Bonds = analogy for IMF weak forces between cookie molecules
Solid line = covalent bonds (strong)
Dashed/dotted lines = Intermolecular Force (Weak)

7. Dipole-Dipole Forces Weak attractive forces between POLAR molecules.
Partial POSITIVE end of one molecule is attracted to partial NEGATIVE end of another molecule. + -
View animation online.

8. Dipole-Dipole IF IF
SCl2
SCl2

9. HW 10-3, p. 89 IMF problem set, #1
Draw a correctly oriented dipole-dipole attraction:
1A) ICl and ICl

10. HW 10-3, p. 89 IMF problem set, #1
Draw a correctly oriented dipole-dipole attraction:
1A) ICl and ICl

11. IMF problem set, #1
Draw a correctly oriented dipole-dipole attraction:
1B) NH3 and CHCl3

12. IMF problem set, #1
Draw a correctly oriented dipole-dipole attraction:
1B) NH3 and CHCl3

13. Hydrogen Bonding
Video Link
An especially strong type of dipole-dipole attraction between molecules

14. Requirements for H-bonds: HFON and FON
H covalently bonded to F, O, or N; AND
F,O, or N present on second molecule (N must have nonbonding pair)
E1 – H E2
Where
E1 and E2= F, O, N

15. Effect of Size of Charge and Distance Between Charges on Strength of Attraction

16. Hydrogen Bonds are an especially strong dipole-dipole attraction
F,O,N have high electronegativity values therefore form very polar covalent bonds (large difference in EN in bonds)
Small size (valence electrons are in second energy level) means atoms on different molecules can approach each other closely.

18. Example: Which pairs of molecules can form hydrogen bonds?
1) H2O and NH3
2) CH2O and CH2O
Ammonia Fountain
No H-bond H is not directly attached to F,O,N

19. Homework 10-3, #4
3) What atoms in addition to H are necessary for hydrogen bonding? How does the small size of the hydrogen atom contribute the unusual strength of the dipole-dipole forces involved in hydrogen bonding?
F, O, N
Smaller size of e- cloud of H atom allows it to more closely approach e-clouds of F, O, or N.
Closer distance between charges = strong attraction

20. HW 10-3, #5
For each of the following pairs of molecules indicate whether or not hydrogen bonds will form. If hydrogen bonds will form, draw the molecules in their correct orientation.
H2O and CH4 H-bond formed?
NO; CH4 has no F,O,N

21. HW 10-3, #5
For each of the following pairs of molecules indicate whether or not hydrogen bonds will form. If hydrogen bonds will form, draw the molecules in their correct orientation.
NH3 and NH H-bond formed?
YES

22. HW 10-3, #5
For each of the following pairs of molecules indicate whether or not hydrogen bonds will form. If hydrogen bonds will form, draw the molecules in their correct orientation.
HF and CH3OH H-bond formed?
YES

23. Which molecule has the strongest IMF’s? Which has the weakest?
HW 10-3, #6
H2O molecules form H-bonds with each other; Other molecules in graph do not

24. Surface Tension explained by Hydrogen Bonding

25. Hydrogen Bonding in Ice
Open space creates a LESS dense structure (Water expands when it freezes)

26. Characteristics of Hydrogen bonding important for Biology
Bond strength- weak compared to covalent bond ; excellent for bonds that need to be temporary
Specificity- requirement that δ+ and δ- are precisely aligned.

27. Hydrogen Bonding in DNA
Bond Strength – bonds are strong enough to hold strand together but weak enough to separate strands without too high an energy cost during replication and transcription
Specificity – only correct pairing of A with T and C with G positions partial + across form partial – to form base pair.

28. DNA Replication – copying DNA

29. DNA transcription

30. HW 10-3, #7
CORRECT
ADENINE
THYMINE

31. HW 10-3, #7

32. HW 10-3, #7
NOT CORRECT
CYTOSINE
ADENINE

33. HW 10-3, #7
CORRECT
GUANOSINE
CYTOSINE

34. B. Types of IMF London Dispersion Forces View animation online.
Video Link
London Dispersion Forces
View animation online.

35. Basis of London Force

36. London Dispersion Force (LDF) also called Van Der Waals
Instantaneous (momentary) dipole attraction between NONPOLAR molecules.

37. Van der Waals also called London or Dispersion (LDF)
Momentary shift in electrons in one NONPOLAR molecule can induce (create) a similar shift in nearby molecules.
Draw this

38. London Dispersion Forces become more significant as the size of the molecule increases.
LDF attraction between two Xe atoms is STRONGER than LDF attraction between two He atoms because Xe has LARGER electron cloud. He Xe
BIGGER electron cloud = STRONGER LDF

39. IMF Summary
Video Link

40. HW 10-3, #8 What are London dispersion forces and do they arise?
Temporary (instantaneous) dipoles that arise from momentary shift of electrons in a molecule. The only force of attraction between non-polar molecules