1. Intermolecular Forces Topic 4.4
between

3. Intra vs. Inter molecular forces
strong forces that hold the atoms in a molecule together
takes 464 kJ/mol to break the H-O bonds within a water molecule
responsible for chemical properties

4. Inter- weak forces that holds molecules to one another
takes only 19 kJ/mol to break the bonds between water molecules
the strength of the intermolecular forces determines the physical properties of the substance
melting, boiling, solubility, conductivity, volatility

5. 3 main “types” of intermolecular forces
London forces (also called dispersion forces or instantaneous induced dipole forces
dipole-dipole forces (polar molecules)
a stronger type of dipole-dipole bonding called hydrogen bonding
* Van der Waals and London forces are often used interchangeably– more later on this.
strength increases

6. 1. London forces

9. van der Waals’ YouTube (:20) occurs in non-polar molecules
the distribution of electrons around an individual atom, at a given instant in time, may not be perfectly symmetrical
this can produce temporary/instantaneous dipole (polar molecule)
this can then induce a nearby molecule to be polar and therefore a very weak attraction between the two molecules

11. magnitude of the force depends on…
number of electrons and size of the electron cloud
with more electrons, valence electrons are farther away from the nucleus and can be polarized more easily
shape of molecules
molecules with shapes that have more contact area have greater forces between them than those don’t

12. boiling point increases
these round shapes do NOT allow them to stick to one another
this flat shape allows it to stick to one another better
boiling point increases

13. Dipole-Dipole Forces
attractive forces between the positive end of one polar molecule and the negative end of another polar molecule
must be in close proximity for the dipole-dipole forces to be significant
the more polar the molecule, the greater the dipole-dipole force
stronger than London forces

17. Hydrogen Bonding
The hydrogen atom has no inner core of electrons, so the side of the atom facing away from the bond represents a virtually naked nucleus
This positive charge is attracted to the negative charge of an electronegative atom in a nearby molecule
Because the hydrogen atom in a polar bond is electron-deficient on one side (i.e. the side opposite from the covalent polar bond) this side of the hydrogen atom can get quite close to a neighboring electronegative atom (with a partial negative charge) and interact strongly with it (remember, the closer it can get, the stronger the electrostatic attraction)

18. H-NOF YouTube Hydrogen Bonding (1:40)
YouTube Hydrogen Bonding Video (:58)
a specific type of dipole-dipole type interactions
stronger than other dipole-dipole and London forces
the hydrogen (H) in a molecule is intermolecularly bonded to a small, highly electronegative element (usually an N, O or F atom) on another molecule
H-NOF

21. Van der Walls vs. London Forces
the term London forces is used for instantaneous induced dipole – induced dipole force in non-polar molecules
Van der Waals is a more inclusive term, for all intermolecular attractions

22. Physical Properties Related to Bonding Type

23. Melting point (mp) - solid to liquid
Boiling point (bp) - liquid to gas
Volatility - how easily it is converted to gas
Conductivity (conducts electricity)
depends on whether the substance contains freely moving charged particles
Solubility - solute’s ability to dissolve in solvent

24. Intermolecular forces affect on boiling point
the greater polarity of a molecule, the higher the boiling point
In HF, H is 2.1 and F is 4.0, difference of 1.9
In HCl, H is 2.1 and Cl is 3.0, difference of 0.9
BP is 20°C
BP is -85°C

25. H-NOF ? H2O vs. H2S? In H2O, H is 2.1 and O is 3.5, difference of 1.4
water molecules can hydrogen bond (and London forces) to each other
BP is 100°C
In H2S, H is 2.1 and S is 2.5, only a difference of 0.4
H2S can dipole-dipole bond (and London forces) to each other
BP is -60°C
H-NOF ? no
H-NOF ?
yes

26. H-NOF ? CH3OCH3 vs. CH3CH2OH ? -24°C 78°C
dipole – dipole has hydrogen bonding
NH3 vs. PH3 ?
-33°C °C
has hydrogen bonding dipole – dipole

27. CH3CH2 CH3 vs. CH3CHO vs. CH3CH2OH ?
only London forces
(VDW) low BP
VDW & dipole-dipole
medium BP
VDW, D-D, & hydrogen bonding
highest BP