Intermolecular forces

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