Intermolecular Forces

Chapter 11.2 Pages 385-392

Set-up the following chart in your notesBonding Force

Inter- or Intra-

Molecular

Attractive Force

Properties Bond Strength

Examples

8 Rows

Forces of AttractionIntramolecular forces – forces of attraction that hold particles together.

chemical bonds – ionic, covalent and metallic are intramolecular forces. covalent bonds hold the hydrogen and

oxygen atoms together – intramolecular

Intermolecular forces – forces of attraction between neighboring molecules.

there is an attraction between one water molecule and another water molecule, which causes water to be a liquid at room temperature.

Intermolecular forces are weaker than intramolecular forces

INTRAMOLECULAR FORCESAre within a

molecule itself such as:

Covalent bonds Ionic bondsMetallic bonds

Ionic bonds are solids at room temperature - very strong bonds.

Metallic bonds - attractive forces between fixed positive ions and the moving valence electrons in a metal.

+ + + + + + + + + fixed positive ions

moving valence electrons

Metallic bonds are strong bonds which is why most metals are solids at room temperature.

Covalent bonds form molecules. Molecules can be solids, liquids or gases at room temperature.

Chemical Bonds

ORDER OF STRENGTH OF INTRAMOLECULAR FORCESMetallic – strongestIonicCovalent Network (difference in

electronegativity is “0”)Molecular

Polar Non-Polar

Bonding in Solids

Solid Particles Attraction PropertiesSystem Forces

Molecular Atoms or(Covalent) Molecules

DispersionDipole-DipoleHydrogen Bond

Covalent AtomsNetwork

Covalent Bonds

Ionic Ions Ionic Bonds

Metallic Metal Atoms Metallic Bonding

Soft, low m.p.,poor electrical conductors

Very hard, high m.p., poor conductors

Hard & brittle, high m.p., poor conductors,nonvolatile

Shiny, malleable, ductile, good conductors

ice

diamond

NaCl

Al

What are the intramolecular forces?

Choices to consider are metallic, ionic, covalent, or molecular crystals.

1. C2H6 __________ 2. Na2O ____________3. SiO2 ______________ 4. CO2 ______________5. N2O5 ____________6. NaNO3 ______________ 7. Al ________________ 8. C(diamond) _______9. SO2 ________________

INTERMOLECULAR FORCES

Are the forces between one molecule and another

Are weak forces compared to “bonding forces” Involves the attraction of the electrons of one

atom for the protons of another atom Are called van der Waals forces

KINDS OF van der WAALS FORCES dipole – ion (dissolving) dipole – dipole London dispersion forces

(temporary dipole forces) hydrogen bonding

Dipole – ion forces Ionic compounds dissociate into cations

and anions because water is a polar molecule and the negative end of the water molecule is attracted to the cation and the positive end of the water molecule is attracted to the anion.

Ion Dipole

Dipole-Dipole Forces Electrostatic attraction between polar molecules

δ - δ + δ - δ +

Polar molecules form a dipole (one end of the molecule has a partial positive charge and the other end of the molecule has a partial negative charge) because of difference in electronegativity.

ClCl

Cl Cl

Cl

H H

H

H

H

δ-

δ-δ-

δ-

δ-δ+

δ+

δ+ The dipole-dipole force is the attraction that the δ+ end of one molecule has for the δ- end of a neighboring molecule.

Dipole-dipole Forces

Dipole-Induced Dipole A polar molecule forces a dipole in a nonpolar

molecule. “Likes dissolve likes”

δ - δ +

- +

Distribution of electrons is not always even. When electrons become unevenly distributed the one end of the atom becomes -(the end with more electrons) and the other end becomes +. Causing one atom to be attracted to another.

+ - + -

Attractive force between the + of one atom and the - of a neighboring atom.

This is a very weak intermolecular force.

Dispersion

Dispersion (London) Forces

The random movement of electrons creates temporary dipoles in nonpolar molecules.

These movements occur in all molecules to some extent.

+ - + -

All of the noble gases are gases at room temperature, however, at extremely low temperatures and/or high pressure, they can be condensed. This indicates that there is some attraction between the particles.

Dispersion forces - intermolecular force of attraction between induced or temporary dipoles.

atom of neon with electron evenly distributed in the electron cloud.

atom of neon with uneven distribution of electrons.

London Dispersion Forces

+ -

Hydrogen Bonding Dipole-Dipole attraction between

hydrogen and a highly electronegative element such as F, O, or N.

H

+ -

F

Hydrogen bond is a dipole-dipole attraction that occurs between molecules containing a hydrogen atom bonded to N, O or F.

The difference in electronegativity results in particularly strong dipole-dipole attraction.

Water exhibits hydrogen bonding. A hydrogen bond is the strongest of all of the intermolecular forces, which is why water is a liquid at room temperature.

Hydrogen Bonding

ORDER OF STRENGTH OF INTERMOLECULAR FORCEShydrogen bonding – strongestdipole – iondipole – dipoleLondon dispersion forces

Comparing Intermolecular Forces

Are ionsinvolved?

Are polar molecules

involved?

Are polarmolecules and ions

involved?

Are hydrogenatoms bonded to N,

O, or F atoms?

Dipole-dipole

Hydrogen bonding

Ion-dipole Ionic bondingLondon forces only

YesNo

NoYes

Yes

Yes

No

No

Interacting molecules or ions

Identify the Intermolecular Attraction Forces

1. H2Dispersion

(London)

2. NeLondon

3. C2H4Cl2Dipole-Dipole

4. HFHydrogen

(Dipole-Dipole)

5. NF3

Dipole-Dipole

6. C6H6

London

7. HClDipole-

Dipole

What intermolecular forces? HCl I2 CH3F CH3OH H2O H2S SiO2 SO2 Cu

Fe Kr CH3OH CuO NH3 CH4 HCl(g) NaCl SiC

Properties affected by intermolecular forces Boiling point Retention of volume and shape Surface tension Evaporation Vapor pressure Viscosity

Properties are affected by forces Physical properties depend on these

forces. The stronger the forces between the particles,

(a) The higher the melting point. (b) The higher the boiling point. (c) The lower the vapor pressure (partial

pressure of vapor in equilibrium with liquid or solid in a closed container at a fixed temperature).

(d) The higher the viscosity (resistance to flow).

(e) The greater the surface tension (resistance to an increase in surface area).

Surface Tension

Vapor Pressure