1. Mission B6 - Intermolecular Bonding
“Aim B6 – How do IMFs Change the Boiling Points”

2. Bonding in non-Molecules
Ionic solids
held by ionic bonds between all the ions
Metallic solids
are atoms of a metal bound to each other in a sea of mobile electrons
Network solids
diamond and sand have all their atoms bound together
All have high melting points due to these bonds

3. Intermolecular Forces
Individual molecules in a molecular solid are held together by covalent bonds
BUT – molecular solids are not!
There are three types of molecule - to - molecule attractions in molecular solids called intermolecular forces or IMFs
London forces (aka Van der Waals forces) – electrons of one atom can attract the protons of another in a very weak bond
Dipole-dipole interactions – the “mini magnet effect”
H-bonds – a special case of the “mini magnet effect”

4. The Weakest - London Forces
London Dispersion Forces
Protons have attractions for electrons
both their own and other atoms
A very weak bond occurs between atoms due to these attractions
Example
helium molecules
monoatomic molecules

5. The Weakest - London Forces
London Dispersion Forces
The larger the molecules
The more electrons / protons
The greater the attraction
between the two molecules
The greater the boiling point
Examples
BP of Neon = -246oC
BP of Argon = -186oC
BP of Krypton = -152oC
BP of Xenon = -107oC
Increasing molar masses

6. Which of these two molecules has stronger London forces?
Tend to raise melting and boiling points
Due in part to masses of molecules
RELATIONSHIP
The larger the molecules
the greater the London
forces, and the higher
the MP and BP
Which of these two molecules
has stronger London forces?

7. A little Bit Stronger - Dipole-Dipole Interactions
A polar covalent bond occurs
between each H and O in
water H2O
But not an even sharing
Due to electronegativity differences
Electrons spend more time around oxygen
Oxygen becomes slightly (-), H becomes slightly (+)
A “mini magnet” or dipole is formed

8. Dipole-Dipole Interactions
Positive ends of dipole
attracted to the negative ends of other dipoles
Again, electrostatic effect or “mini magnet effect”

9. Polarity and Molecular Symmetry
Polar molecules are dipoles – they are asymmetrical molecules
Shape affects polarity!
Linear shapes
with different atoms attached
Ex: hydrogen chloride (HCl)
Angular shapes
bent
Ex: water (H2O)
Pyramidal shapes
like a tripod
Ex: ammonia (NH3)
Note – all have polar bonds too!

10. Polarity and Molecular Symmetry
Nonpolar molecules are symmetrical
They DO NOT form dipoles!
Linear shapes
with same atoms on each end
Ex: carbon dioxide (CO2)
Tetrahedral shapes
Four sided pyramid shape
All corners have same elements
Ex: methane (CH4)
Note – even though they have polar bonds, the whole molecule is nonpolar due to symmetry!

11. Dipole-Dipole Interactions
Keep in mind!
London Forces occur in all molecules
Many molecules also have dipole-dipoles
Question: what will happen to the boiling point of similar sized particles, one that is nonpolar with only London Forces, and the other with polar molecules with dipole-dipole interactions?
HIGHER MELTING / BOILING POINTS

12. The ultimate force – HYDROGEN BONDING
Special form of dipole-dipole interaction
Occurs in molecules with:
Hydrogen atoms
And small atoms
Much stronger form of dipole bonding
Reason for the boiling point of water being only 100oC
Hydrogen sulfide molecule
Water molecule

13. Boiling points of hydrogen compounds

14. When Ions and Molecules Collide! Dissolving - Molecule-Ion Attractions
NaCl dissolves in water (a given)
This is due to the attraction of the (+) and (-) ends of the polar water molecules to the Na+ ions and Cl- ions dissolved in the water
Hydration - water molecules surround the ions, separating them and thus dissolving the solid

15. Molecule-Ion Attractions – Dissolving
Sodium chloride and other ionic substances don’t dissolve in oil!
No hydration occurs
Oil is nonpolar
Main reason for oil and water not mixing
Always remember: “Like dissolves like”
Polar dissolves polar
Salt, sugar, in water
Nonpolar dissolves nonpolar
Oil paint in turpentine, styrofoam in acetone