1 Intermolecular Forces Chemistry. 2 Intermolecular Forces Intermolecular forces are attractive...

Intermolecular Forces

Chemistry

Intermolecular Forces

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.

“Measure” of intermolecular force

boiling point

melting point

Types of Intermolecular Forces1. 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 Forces

2. Ion-Dipole Forces

Attractive forces between an ion and a polar molecule

Ion-Dipole Interaction

Types of Intermolecular Forces

3. Dipole-Dipole Forces

Attractive forces between polar molecules

Orientation of Polar Molecules in a Solid

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

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.

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 is nonpolar: dispersion forces.

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

Comparing Intermolecular ForcesComparing Intermolecular Forces

1 Intermolecular Forces Chemistry. 2 Intermolecular Forces Intermolecular forces are attractive...

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