INTERMOLECULAR FORCES

WHY DO GASES DIFFER FROM SOLIDS AND LIQUIDS? physical properties depend on forces of attraction between molecules which are strong in liquids and solids and are weak in gases

TYPES OF INTERMOLECULAR FORCES Van der Waal’s Forces a) dipole-dipole forces b) London forces Hydrogen bonding

SECRET!!! the strengths of the attractions within molecules (intramolecular) (the chemical bonds) determine chemical properties. The attractions between molecules (intermolecular) determine the physical properties of substances.

1. DIPOLE-DIPOLE FORCES

DIPOLE-DIPOLE FORCES Influence of dipole-dipole forces is seen in the boiling points of simple molecules CompoundMolar Mass (g/mol) Boiling Point ( o C) N2N CO Br ICl16297

ION-DIPOLE FORCES Force of attraction between an ion and a dipole Reason why most ionic solids are soluble.

2. LONDON FORCES

FACTORS AFFECTING LONDON FORCES 1)Size of Electron Cloud *The larger the atom the greater the London forces because larger clouds are more easily deformed. Therefore dipoles are more easily formed. AtomBoiling Point ( o C) He Ne Ar Kr Xe-107.1

Factors continued… 2) Number of atoms in a molecule The longer a molecule, the more places along its length where it can be attracted to other molecules. MoleculeBoiling Point ( o C) CH C2H6C2H C3H8C3H C 4 H

3. HYDROGEN BONDING

DOMINANT FORCE In terms of strength: H bonds > Dipole-Dipole > London

LEARNING CHECK Identify the dominant Intermolecular forces in these molecules H 2 O, OF 2, CH 4, CF 4, CH 3 F

BOILING POINT

✔

✔

✔

INTRAMOLECULAR AND INTERMOLECULAR FORCES

OTHER PHYSICAL PROPERTIES OF LIQUIDS Surface Tension Shape of meniscus Capillary action