This document discusses intermolecular and intramolecular forces. It describes that intramolecular forces are the forces within a molecule that form covalent bonds. Intermolecular forces are the weaker forces between molecules and include London dispersion forces, dipole-dipole forces, and hydrogen bonding. London dispersion forces exist between all molecules and depend on the number of electrons. Dipole-dipole forces exist between polar molecules. Hydrogen bonding is the strongest intermolecular force and occurs when hydrogen is bonded to fluorine, oxygen, or nitrogen. Intermolecular forces influence properties such as melting point, boiling point, solubility, and surface tension.

1. INTERMOLECULAR FORCES
AND POLARITY

2. INTRAMOLECULAR FORCES
• Forces of electrostatic attraction within a molecule
• Occurs between the nuclei of atoms & their electrons making
up the molecule (i.e. covalent bonds)
• Must be broken by chemical means
• Form new substances when broken

3. INTERMOLECULAR FORCES
• Forces of attraction between two molecules
– London dispersion
– Dipole-dipole
– Hydrogen bonds
• Much weaker than intramolecular forces so much easier to break
• Physical changes break or weaken these forces
• Do not form new substances when broken

4. Type #1: London Dispersion Forces
• Based on the simultaneous attraction of the electrons of one
molecule by the positive nucleus of neighbouring molecules
• Strength of force is directly related to number of electrons &
protons in a given molecule
• The greater number of electrons & protons, the greater the
force
INTERMOLECULAR FORCES

5. Type #1: London Dispersion Forces
INTERMOLECULAR FORCES

6. Type #1: London Dispersion Forces
INTERMOLECULAR FORCES

7. Type #1: London Dispersion Forces
INTERMOLECULAR FORCES

8. Type #1: London Dispersion Forces
INTERMOLECULAR FORCES

9. Type #1: London Dispersion Forces
INTERMOLECULAR FORCES

10. Type #1: London Dispersion Forces
INTERMOLECULAR FORCES

11. Type #2: Dipole – Dipole Forces
• Occurs between polar molecules having dipoles
• Molecules with dipoles are characterized by oppositely
charged ends that are due to an unequal distribution of
charge on the molecule
• Polarity is determined by both the polarity of the bond & the
shape of the molecule
INTERMOLECULAR FORCES

12. • Based on the simultaneous attraction of the electrons of one
dipole by the dipoles of neighbouring molecules
• Strength of the force is related to polarity of the given
molecule
Which molecule will make stronger dipole-dipole forces?
Type #2: Dipole – Dipole Forces
INTERMOLECULAR FORCES
I Cl I F
2.5 3.0 2.5 4.5
ΔEN = 1.5 ΔEN = 2.0

13. Type #2: Dipole – Dipole Forces
INTERMOLECULAR FORCES

14. • Type of dipole – dipole interaction
• Occur between H atoms in one molecule & highly EN atoms
(N, O or F) where there are usually unshared pairs of electrons
present
• H bonds are the strongest intermolecular force & are 1/10th
the strength of a covalent bond
Type #3: Hydrogen bonds
INTERMOLECULAR FORCES

15. Type #3: Hydrogen bonds
INTERMOLECULAR FORCES

16. SUMMARY
INTERMOLECULAR FORCES

17. Intermolecular forces affect:
– Melting point
– Boiling point
– Capillary action
– Surface tension
– Solubility
INTERMOLECULAR FORCES

18. • Molecules that are isoelectronic (same # of electrons) have the
same strength of London dispersion forces
• More polar molecules have stronger dipole – dipole interaction 
Higher melting & boiling points
• The more electrons per molecule, the stronger the London forces
& the higher the melting & boiling point
PREDICTING BOILING POINTS

19. • Solution is formed when a solute dissolves in solvent
• Solution formation is independent on the intermolecular
forces of all molecules involved
• Remember: ‘Like Dissolves Like’
POLARITY AND SOLUBILITY

20. • Polar liquids dissolve in other polar liquids
– Share same type of intermolecular forces
• Example: methanol in water
– Both have hydrogen bonds between each
SOLUBILITY: COVALENT MOLECULES

21. • Example: carbon tetrachloride in water
– Will not dissolve as it is nonpolar & does
not contain the same intermolecular
forces
• Solids behave similarly to liquids
• Example: sugar in water
– Sugar is slightly polar (H bonds) &
dissolves in water (H bonds)
SOLUBILITY: COVALENT MOLECULES
CCl4 in water
sugar in water

22. SOLUBILITY: COVALENT MOLECULES
Soluble: Sugar (glucose) in water

23. SOLUBILITY: COVALENT MOLECULES
Insoluble: Hexane in water

24. • Rarely soluble in nonpolar or low polar solvents
• Sometimes soluble in water
• Example: NaCl
– Consists of Na+ & Cl- ions
– When placed in water the partial negative charges from O
in the water are attracted to the Na+
– Partial positive charge from H is attracted to Cl-
SOLUBILITY: IONIC MOLECULES

25. SOLUBILITY: IONIC MOLECULES
NaCl in water

26. SOLUBILITY: IONIC MOLECULES
NaCl in water

27. • Surface tension
• Capillary action
• Viscosity
OTHER PHYSICAL PROPERTIES

28. Homework
• Page 247 #1 – 8
• Complete pre-lab for access to lab tomorrow
INTERMOLECULAR FORCES & SOLUBILITY