3.4 INTERMOLECULAR FORCES

INTERMOLECULAR FORCE An attraction between molecules Weaker than the forces within the molecules

IONIC COMPOUNDS No intermolecular forces Ions are held together by ionic bonds Ionic bonds are very strong – accounts for high melting points

COVALENT COMPOUNDS Many are gases at room temperature Others are liquids with low mp or solids that melt easily Forces between molecules must be relatively weak

PHYSICAL PROPERTIES The strength of intermolecular forces determines the following physical properties: Physical state Melting point Boiling point Surface tension Hardness and texture solubility

TYPES OF INTERMOLECULAR FORCES Dipole-dipole Forces London Dispersion Forces Hydrogen Bonds

DIPOLE-DIPOLE FORCES Force between oppositely charged ends of polar molecules Occur between all polar molecules The more polar a molecule is, the stronger the dipole-dipole force

LONDON DISPERSION FORCES Temporary dipoles form for a fraction of a second When dipoles are momentarily formed in neighbouring molecules, an attraction is formed Very short-lived Exist between all molecules Larger molecules have stronger forces

LONDON DISPERSION FORCES

VAN DER WAALS FORCES Dipole-dipole forces and London Dispersion forces are together known as van der Waals forces

HYDROGEN BONDS Particularly strong dipole-dipole force Each molecule must have a hydrogen atom covalently bonded to a highly electronegative atom N, O, or F Occur due to the large differences in electronegativities, and the small size of hydrogen

HYDROGEN BONDS Play a large role in the structure and function of large, biologically important molecules Example: proteins and DNA

PROTEINS Very long molecules Chain of hundreds or thousands of atoms folds into 3D structures Retinoblastoma protein

DNA Double helix structure Two long chains of nucleotides Hydrogen bonds hold the two chains together Chains must break apart and re-form in order for the DNA to replicate

HOMEWORK Read pages 109 – 115 Complete page 115 # 1, 2, 4, 5, 7