between


Intra-
›
› 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


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


3 main “types” of intermolecular forces
1.
London forces (also called dispersion forces or instantaneous induced dipole forces
2.
3.
dipole-dipole forces (polar molecules) a stronger type of dipole-dipole bonding called hydrogen bonding strength increases
* Van der Waals and London forces are often used interchangeably– more later on this.



 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

›
1.
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
2.
shape of molecules
 molecules with shapes that have more contact area have greater forces between them than those don’t

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




 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 dipoledipole forces to be significant the more polar the molecule, the greater the dipole-dipole force stronger than London forces






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


 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






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

 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

› H
2
O vs. H
2
S?
 In H
2
O , 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 H
2
S, H is 2.1 and S is 2.5, only a difference of 0.4
 H
2
S can dipole-dipole bond (and London forces) to each other
 BP is -60°C H-NOF ?
no
H-NOF ?
yes


CH
3
OCH
3 vs. CH
3
CH
2
OH ?

-24°C 78°C dipole – dipole has hydrogen bonding

NH
3 vs. PH
3
?

-33°C -87°C has hydrogen bonding dipole – dipole


CH
3
CH
2
CH
3 vs. CH
3
CHO vs. CH
3
CH
2
OH ?
only London forces
(VDW) low BP
VDW & dipole-dipole medium BP
VDW, D-D, & hydrogen bonding highest BP