1. Intermolecular ForcesH Cl H Cl
The attractions between molecules are not nearly as strong as the covalent bonds that hold atoms together.
covalent bond (stronger)
intermolecular attraction (weaker)
They are, however, strong enough to impact physical properties such as boiling and melting points, vapor pressures, and viscosities.
These intermolecular forces as a group are sometimes referred to as van der Waals forces.

2. Johannes van der Waals (1837−1923).
van der Waals Forces
London dispersion forces
Dipole-dipole interactions
Hydrogen bonding
Johannes van der Waals (1837−1923).
Fritz London (1900−1954).

3. Three Types of Dipoles
1. Permanent Dipoles: Polar molecules which are the result of electronegativity differences between
atoms and/or molecular geometry.
Examples: HCl, H2O, NH3 H Cl δ+ δ-
2. Instantaneous Dipoles: A temporary dipole which results from a fluctuation in the electron cloud. May also occur in noble gases. δ+ δ- N N
3. Induced Dipoles: A temporary dipole forms due to the influence of a nearby dipole. H Cl δ- δ+ δ+ δ- N N

4. Dipole-Dipole Interactions
Polar molecules (permanent dipoles) are attracted to nearby polar molecules due to electrostatic forces and dipoles present. (stronger than London dispersion forces for molecules with similar sized electron clouds)
The molecules must be close together for dipole-dipole forces to take effect.
Which of these molecules would have dipole-dipole interactions? CO2, CO, ClF, N2
CO, ClF

5. Dipole-Dipole Interactions
propane
C3H8
ethanal
C2H4O
dimethyl ether
C3H8
acetonitrile
C2H3N
chloromethane
CH3Cl
All of these molecules have the same size electron cloud. Rank them according to increasing boiling point. Explain.
Propane (BP: 231 K; Dipole Moment: 0.1 D)
Dimethyl ether (BP: 248 K; Dipole Moment: 1.3 D)
Ethanal (BP: 249 K; Dipole Moment: 1.9 D)
Chloromethane (BP: 294 K; Dipole Moment: 2.7 D)
Acetonitrile (BP: 355 K; Dipole Moment: 3.9 D)

6. Instantaneous Dipoles
Due to fluctuations in the electron clouds of atoms and molecules, partial positive and partial negative regions of atoms and molecules form. The tendency of an electron cloud to distort is called polarizability. Larger electron clouds are described as being more polarizable. δ+ δ- δ+ δ-

7. Instantaneous Dipoles
An instantaneous dipole can induce a charge in a nearby non-polar molecule. This results in a type of London dispersion force called an instantaneous dipole-induced dipole attraction. δ+ δ- δ+ δ-
This is the type of attraction that holds liquid bromine molecules or solid iodine molecules together.

8. Study this table:
Halogen Electrons Boiling Point (K) F Cl Br I
Observations? Relationship? Explanation?
Direct relationship (increase in electron cloud size results in a higher boiling point) due to greater polarizability of the electron cloud which results in stronger dispersion forces and therefore more energy is needed to separate the molecules.

9. Same pattern for the noble gases:
Noble Gas Electrons Boiling Point (K) He Ne Ar Kr Xe
Larger electron clouds are more polarizable which results in stronger intermolecular forces.

10. Which Have a Greater Effect: Dipole-Dipole Interactions or Dispersion Forces?
If two molecules are of comparable size and shape, dipole-dipole interactions will likely be the dominating force.
If one molecule is much larger than another, dispersion forces will likely determine its physical properties.

11. Describe the relationship between the strength of the existing intermolecular forces and the following properties:
Melting Point:
Boiling Point:
Vapor Pressure:
Viscosity:
Enthalpy of Fusion:
Enthalpy of Vaporization:
Surface Tension:
Direct
Inverse

12. Observations?
H2O, HF, & NH3 are out of place.

13. Hydrogen Bonding
The dipole-dipole interactions experienced in molecules where H is bonded to N, O, or F are unusually strong.
We call these interactions hydrogen bonds.
The hydrogen bond is the attraction of the hydrogen of one molecule to a lone pair of electrons on a nearby molecule.
Which of these molecules would hydrogen bond to molecules identical to itself? HF, CH4, NH3, HCl
HF, NH3

14. Two essential requirements for the formation of a hydrogen bond:
One molecule must contain at least one H atom attached to a highly electronegative atom (F, O or N).
The other molecule must contain an F, O, or N atom that provides the lone pair of electrons.

15. Hydrogen Bonding
H F
H F
H N H H
H N H H

16. Hydrogen Bonding
H2O
Water undergoes extensive hydrogen bonding.

17. Each H2O molecule can be bonded tetrahedrally to four H2O molecules1 4 2 3

18. H N H H O H O H H H H N H H O O O H H Hydrogen Bonding
Hydrogen bonding can take place between dissimilar molecules:
H N H H
H O H
H O H
H N H H
H O H
O O

19. Hydrogen Bonding
Make a model of methanol (CH3OH) and water (H2O). Arrange the models to show two possible hydrogen bonding configurations. Show them to the instructor.
Why is this arrangement NOT a depiction of hydrogen bonding?
Hydrogens bonded to carbon lack the ability to hydrogen bond.

20. Identify the hydrogen atoms of the following species that are capable of forming hydrogen bonding with water molecules.
Soluble in water
adenine
glucose

21. Hydrogen Bonding
The unusually strong dipole interactions found in hydrogen bonding arises from:
1. The high electronegativity of nitrogen, oxygen, and fluorine.
2. The small size of nitrogen, oxygen, and fluorine.
3. When hydrogen is bonded to one of those small and very electronegative elements, the hydrogen nucleus is exposed.

22. Effect of hydrogen bonding on DNA
The presence of intermolecular H-bonds helps maintain the double helical shape of DNA molecules.
hydrogen bonds

23. Hydrogen bonding in DNA
Cytosine Guanine (3 hydrogen bonds)
Thymine Adenine
(2 hydrogen bonds)

24. Strength of van der Waals’ forces
Depends on three factors (in decreasing order of importance) :
Size of molecule
Shape (Surface area) of molecule
Polarity of molecule

25. Size of electron cloud ­
1. Size of Molecule
Usually !
Molecule
Boiling point (oC)
Helium
Neon
Argon
-269
-246
-186
Fluorine
Chlorine
Bromine
-188
-34.7
58.8
Methane
Ethane
Propane
-162
-88.6
-42.2
Size of molecule ­
Rel. molecular mass ­
Size of electron cloud ­
Polarizability ­
Dispersion forces ­

26. 2. Surface area of molecule
The van der Waals’ forces also increase with the surface area of the molecule.
∵ van der Waals' forces are short-ranged forces
Atoms or molecules must come close together for significant induction of dipoles.

27. Pentane (C5H12)
Dimethylpropane (C5H12)
Boiling point: 36.1°C
Boiling point: 9.5°C
Observations:
Same # of electrons; both are non-polar; pentane has a higher boiling point
Explanation:
The shape of pentane (rod or plate) has more surface area (and contact area) than the shape (ball) of dimethylpropane. Pentane is more polarizable.

28. Dispersion and Dipole-dipole forces
Dispersion forces
32 electrons
boiling point = 50°C
34 electrons
boiling point = 0°C
Observations? Explain:

29. CH3Cl CH3Br CH3I
Boiling Point (°C )
Dipole Moment (D)
Observations? Explain:
Chloromethane is more polar; the effect of dispersion forces outweighs that of dipole-dipole forces.

30. Match the boiling point to the molecule:
Explain:
27.7°C
“Rod” or “plate” shaped molecules are more polarizable than “ball” shaped molecules.
36.1°C

31. Match the boiling point to the molecule:
F2 Cl ClF CH2Cl2
Match the boiling point to the molecule:
Explain:
39.6°C
Cl2 > ClF because:
Cl2 has more electrons and is more than polarizable than ClF.
Although ClF is polar, the effect of dispersion forces outweighs that of dipole-dipole forces.
-34.0°C
-100°C
-188°C

32. Substance
electron cloud size
Boiling point (°C)
NH3 10
-33.3 HF
19.5
H2O
100
Explain:
HF is more polar than NH3
H2O is able to form two hydrogen bonds per molecule (HF and NH3 only form one).

33. hydrogen bond
Each NH3 molecule has only ONE lone pair.
Þ On the average, each NH3 molecule can form only ONE hydrogen bond

34. Each HF molecule has only ONE hydrogen atom.
Þ On the average, each HF molecule can form only ONE hydrogen bond

35. hydrogen bond
Each H2O molecule has TWO hydrogen atoms and TWO lone pairs.
Þ On the average, each H2O molecule can form TWO hydrogen bonds

36. Compound
# of electrons
Boiling point (oC)
Propanal CH3CH2CHO 32 48
Ethanol C2H5OH 26 78
Methanol CH3OH 18 66
Explain the boiling point differences.
For molecules with similar structures, their boiling points depend on the polarizability of their electron clouds. As the relative size of the electron cloud of ethanol is greater than that of methanol, the boiling point of ethanol is higher. Methanol and Ethanol both have hydrogen bonding, propanal does not have hydrogen bonding.

37. Vapor Pressure Observations:
Diethyl ether (42 e-) has a lower vp and nbp than ethanol (27 e-), water (10 e-), or ethylene glycol (36 e-).
C4H10O
H2O
C2H6O
C2H6O2
Explanation:
Ethanol, water, and ethylene glycol have hydrogen bonding.

38. 2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
Name the types of bonds or intermolecular forces that are broken and formed in the following processes.
H2O(s) → H2O(g)
2Mg(s) + O2(g) → 2MgO(s)
H2(g) + Br2(l) → 2HBr(g)
HBr(l) → HBr(g)
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
CH3CH2OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l)
broken: H bond; formed: none
broken: metallic bond, covalent bond; formed: ionic bonds
broken: covalent bond, dispersion forces; formed: covalent bond
broken: dispersion forces; formed: none
broken: covalent, metallic & H bonds; formed: ion-dipole & covalent bonds
broken: covalent & H bonds; formed: covalent & H bonds

39. Chromatography Paper Gel A separation technique.
Many forms: paper, gel, gas, liquid.
Two phases: stationary & mobile
Based on molecule interactions with the phases.
Size, shape, nature of molecules (polar, non-polar) determine how quickly they move.
Applications: Forensics, DNA, Medicine, Analytical Chemistry.

40. Relative strength of London dispersion forces, hydrogen bond, and covalent bond
Phenomenon
Energy absorbed
(kJ mol-1)
Forces overcome
O2(s) ® O2(g)
3.63
dispersion forces
H2O(s) ® H2O(g)
46.90
hydrogen bonds
O2(g) ® 2O(g)
494.00
covalent bonds