Intermolecular Forces and

Liquids and SolidsChapter 12

12.7 The compounds Br2 and ICl have the same number of electrons, yet Br2 melts at -7.2oC, whereas ICl melts at 27.2oC. Explain.

12.9 The binary hydrogen compounds of the Group 4A elements are CH4 (-162oC), SiH4 (-112oC), GeH4 (-88oC), and SnH4 (-52oC). The temperature in parentheses are the corresponding boiling points. Explain the increase in boiling points from CH4 to SnH4.

12.10 List the types of intermolecular forces that exist in each of these species: (a) benzene (C6H6), (b) CH3Cl, (c) PF3, (d) NaCl, (e) CS2.

Intermolecular ForcesDispersion Forces (London forces)

Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules

12.2

Instantaneous-induced dipole

The strength of dispersion forces tends to increase with increased molecular weight.Larger atoms have larger electron clouds, which are easier to polarize.

The tendency of an electron cloud to distort in this way is called polarizability.

Intermolecular ForcesDispersion Forces

12.2

Polarizability is the ease with which the electron distribution in the atom or molecule can be distorted.

Polarizability increases with:

• greater number of electrons

• more diffuse electron cloud

Dispersion forces usually increase with molar mass.

SO

O

What type(s) of intermolecular forces exist between each of the following molecules?

HBrHBr is a polar molecule: dipole-dipole forces. There are also dispersion forces between HBr molecules.

CH4

CH4 is nonpolar: dispersion forces.

SO2

SO2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO2 molecules.

12.2

• The strength of dispersion forces tends to increase with increaseing molecular weight.

• Increased strength of dispersion forces usually leads to an increased boiling point (see blue curve)

• Why do the binary hydrogen compounds of Group 6A not follow this trend?

Intermolecular ForcesHydrogen bonding (H-bonding)

A special kind of dipole-dipole interaction

• Why do the binary hydrogen compounds of Group 6A not follow this trend?

Intermolecular ForcesHydrogen bonding (H-bonding)

A special kind of dipole-dipole interaction

These molecules are polar so adipole-dipole interaction is alsooperative. H2O, in particular is capable of a special dipole-dipoleinteraction that is unusually strong.

Intermolecular ForcesHydrogen Bond

12.2

The hydrogen bond is a special dipole-dipole interaction between the hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N, or F atom.

A H…B A H…Aor

A & B are N, O, or F

Intermolecular ForcesHydrogen Bond

12.2

The dipole-dipole interactions experienced when H is bonded to N, O, or F are unusually strong.

H-bonding arises, in part, from the high electronegativity of nitrogen, oxygen, and fluorine.

Intermolecular ForcesHydrogen Bond

12.2

Fluorine is more electronegative than oxygen so it is reasonable to expect a stronger H-bond in liquid HF than in H2O. However, the boiling point of HF is lower than that of H2O, indicating a weaker H-bonding network in HF. Explain.

Each H-F can participate in two H-bonds while each H2O can make four H-bonds.

Summary of intermolecular forces

Ion-induced dipole and dipole-induced dipole are additional types ofdispersion forces.

Intermolecular forces affect properties of liquids

Surface tension is the amount of energy required to stretch or increase the surface of a liquid by a unit area.

Strong intermolecular

forces

High surface tension

12.3

Intermolecular forces affect properties of liquids

Surface tension is the amount of energy required to stretch or increase the surface of a liquid by a unit area.

12.3

Surface tension results from the net inward force experienced by the molecules on the surface of a liquid which causes the surface to tighten like an elastic film.

Surface tension allows water glider walk on water.

Properties of Liquids

Cohesion is the intermolecular attraction between like molecules

12.3

Adhesion is an attraction between unlike molecules

Adhesion

Cohesion

Surface tension is responsible for capillary action.

water mercury

Properties of Liquids

Viscosity is a measure of a fluid’s resistance to flow.

12.3

Strong intermolecular

forces

High viscosity

12.3

Structure and properties of water

• All life processes involve water

• Excellent solvent for many ionic compounds as well as other substances capable of forming hydrogen bonds with water (i.e., DNA)

• Moderator of climate owing to a high specific heat (absorbs heat in summer and gives off heat in winter with only small changes in the temperature of water)

• Unique substance in that solid ice is less dense than liquid water (usually solids are more dense than liquids)

Maximum Density40C

Ice is less dense than water

12.3

Water is a Unique Substance

Thermal expansion

H2O trapping incavities

Solids

Crystalline—high order

(regular repeating pattern)Amorphous—low order