part 23: FORCES OF ATTRACTION

Attractive forces

Forces necessary for molecules to cohere

Repulsive forces

Forces necessary to prevent the molecules from interpenetrating and annihilating each other

Intramolecular force

Forces within molecules

Ionic bond

• Transfer of electrons

• Metal + Non-metal

• Example: NaCl

• Bond energy: > 100 Kcal/mol

Covalent bond

• Sharing of electrons

• Non-metal + Non-metal

• Bond energy: 50-100 Kcal/mol

• Classified as polar and non-polar

Intermolecular force

Forces between molecules

Van der Waals Forces

Relate to non-ionic interactions between molecules, yet they involve charge-charge interactions

1. Dipole-Dipole

2. Dipole-Induced Dipole

3. Induced Dipole-Induced Dipole

Van der Waals Forces

Dipole-Dipole

• In polar molecules which have permanent dipoles, the Van der Waals forces are mainly due to electrostatic interaction between the dipole

• 2 polar/2 permanent dipoles

Dipole-Induced Dipole

• A molecule which is already has polarity, may sometimes polarize a nonpolar molecule, which comes in its close vicinity and in this way induced polarity is developed in that molecule

• 1 polar, 1 non-polar

Induced Dipole-Induced Dipole

• A non-polar molecule may become momentarily self polarized. This polarized molecule may induce a dipole moment in a neighboring molecule with antiparallel orientation

• 2 non-polar

Willem Hendrik Keesom

Discovered Dipole-Dipole

Keesom Forces

Other name of Dipole-Dipole

1-7 kcal/mol

Bond energy of Dipole-Dipole

Orientation/Alignment

Effect of Dipole-Dipole

1. HCl

2. Acetone

3. Phenol

4. Alcohol

Example of Dipole-Dipole

Peter Debye

Discovered Dipole-Induced Dipole

Debye Forces

Other name of Dipole-Induced Dipole

1-3 kcal/mol

Bond energy of Dipole-Induced Dipole

Induction (of Polarity)

Effect of Dipole-Induced Dipole

1. Ethyl acetate

2. Methylene Cl

3. Ether

Example of Dipole-Induced Dipole

Fritz London

Discovered Induced Dipole-Induced Dipole

Induced Dipole-Induced Dipole

• Weakest Van der Waals Force

• Responsible in liquefaction of non-polar gases

London Forces

Other name of Induced Dipole-Induced Dipole

0.5-1 kcal/mol

Bond energy of Induced Dipole-Induced Dipole

Dispersion

Effect of Induced Dipole-Induced Dipole

1. Carbon disulfide

2. Carbon tetrachloride

3. Hexane

Example of Induced Dipole-Induced Dipole

Ion-Dipole

It occurs between ionic and polar molecules because ions have strong charge so a partial charge end of the dipole will be attracted to the ion

Ion-Induced Dipole

• It occurs between ionic molecules and non-polar molecules

• Involved in the formation of the iodide complex

Hydrogen bridge

Hydrogen bond is also known as?

Hydrogen bond

• The interaction between a molecule containing a Hydrogen atom and a strongly electronegative element (F, O, N)

• Accounts for many of the unusual properties of water including:

  • High dielectric constant

  • High boiling point

  • Abnormally low vapor pressure

2-8 kcal/mol

Bond energy of Hydrogen bond