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

Ionic Compound Covalent Compound

Strong ionic bonds hold ions in lattice

formation

Non-polar moleculesPolar molecules

Relatively strong intermolecular

forces

Relatively weak intermolecular

forces

Melting point tends to be higher

than non-polar

Melting point tends to be lower

than polar

Extremely high melting point

NaClSodium chloride

801oC

H20Water 0oC

CO2

Carbon dioxide –57oC

HOMEWORK

• Read pages 109 – 115

• Complete page 115 # 1, 2, 4, 5, 7