CHEM1201 Module 6: Intermolecular Forces Summary

Intermolecular vs Intramolecular Forces

• Intermolecular forces are forces between molecules.
• Intramolecular forces are forces within molecules (bonds).

Intermolecular Forces and States of Matter

• Molecular liquids and solids are held together by intermolecular forces.
• Supplying energy increases molecular vibration and can overcome intermolecular forces, leading to phase changes (e.g., liquid to gas).

Polarity and Intermolecular Forces

• Polar molecules have dipole moments due to unequal sharing of electrons.
• Non-polar molecules have no dipole moment due to equal sharing of electrons.
• The more polar the molecule, the more energy it takes to break the intermolecular forces.

Types of Intermolecular Forces

• Hydrogen Bonds: Strong dipole-dipole attraction between H and electronegative atoms (N, O, F). Moderate strength.
• Dipole-Dipole Forces: Attraction between molecules with permanent dipoles. Moderate strength.
• Dispersion Forces (London Forces): Temporary polarity due to unequal electron distribution; present in all molecules. Weak.
• Dipole-Induced Dipole Forces: Weak attraction between a polar molecule and a nonpolar molecule where the polar molecule induces a dipole in the nonpolar molecule.
• Ion-Dipole Forces: Electrostatic attraction between an ion and a polar molecule. Strong.

Consequences of Intermolecular Forces

• Boiling Point & Melting Point: Stronger intermolecular forces lead to higher boiling and melting points.

Additional Points

• Dispersion forces are present in all molecules.
• More than one type of intermolecular force can be operational in a substance.
• "Like dissolves like": Polar molecules interact with polar molecules, and non-polar molecules interact with non-polar molecules.