Intermolecular Forces

• Why do some solids dissolve in water but others do not?

• Why are some substances gases at room temperature, but others are liquid or solid?

• What gives metals the ability to conduct electricity, what makes non-metals brittle?

• The answers have to do with …

Intermolecular forcesIntermolecular forces

Questions

Intermolecular Forces(IMFs)Intermolecular forces are attractive forces between molecules.

Intramolecular forces hold atoms together in a molecule

Intermolecular vs Intramolecular

• 41 kJ to vaporize 1 mole of water (inter)

• 930 kJ to break all O-H bonds in 1 mole of water (intra)

• Generally, intermolecular forces are much weaker than intramolecular forces.

Intermolecular force• Ion-dipole• dipole - dipole • H-bonding • London dispersion forces

Intramolecular force• Ionic• covalent (network solids)• metallic

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

Capillary ActionCapillary Action: : Spontaneous rising Spontaneous rising

of a liquid in a of a liquid in a narrow tube.narrow tube.

Properties of Liquids

Cohesion is the intermolecular attraction between like molecules

Adhesion is an attraction between unlike molecules

Adhesion

Cohesion

attracted to glass

attracted to each other

Properties of WaterSurface Tension is the amount of energy required to stretch or increase the surface of a liquid by a unit area.

The stronger the IMFs the molecules have with each other, the higher is its surface tension.

This is because of the cohesive forces inside the liquid (Polar-Polar forces) become much greater than the adhesive forces between the liquid (polar) and the air (non-polar) molecules outside.

Properties of Liquids

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

Strong intermolecular

forces

High viscosity

• Nonpolar Cl2

(Electronegativity

difference is zero)

• Polar HCl (Electronegativity difference

is not zero)

• Ionic NaCl

Bond Polarity

Types of Intermolecular Forces

1. Ion-Dipole Forces

Attractive forces between an ion and a polar molecule

Ion-Dipole Interaction

Types of Intermolecular Forces

2. Dipole-Dipole Forces

Attractive forces between polar molecules

Orientation of Polar Molecules in a Solid

Types of Intermolecular Forces3. Hydrogen Bond (strongest)

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. IT IS NOT A BOND.

A H…B A H…Aor

A & B are N, O, or F

Hydrogen Bond

Why is the hydrogen bond considered a “special” dipole-dipole interaction?

Decreasing molar massDecreasing boiling point

Types of Intermolecular Forces4. Dispersion Forces – van der Walls forces/London forces (weakest)

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

ion-induced dipole interaction

dipole-induced dipole interaction

Non-polar molecules do not have dipoles like polar molecules.So London dispersion force exists in non polar molecule.

Dispersion forces usually increase with molar mass.

Intermolecular Forces4. Dispersion Forces Continued

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.

Types of IMF• The hydrogen bonds in water explain its relatively high boiling

point, considering that it is a small molecule. The H-bonds hold the water molecules together as a liquid, so you have to heat it a lot before it will change to a gas. Compare boiling points of these molecules:

Molecule IMF (s) present Molar Mass (g/mol)

Boiling Point (oC)

CH4 16.05 - 164

HCl 36.46 - 85

H2O 18.02 100

London Disp.

London Disp.

Dipole-DipoleLondon Disp./Dipole-Dipole/Hydrogen Bonding

Water, H20

Which is related to…

And Glycerine (Glycerol)

Compare the three structures

Which of these molecules has the greatest polarity?

Which will exhibit the strongest intermolecular forces?

Ethanol. Polar or Non-Polar?

Methanol. Polar or Non-Polar?

Methane. Polar or Non-Polar?

Chloro-Methane. Polar or Non-Polar?

Sulfur Hexafluoride. Polar or Non-Polar?

Nitrogen Trihidride, aka Ammonia. Polar or Non-Polar?