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 Walls and London forces are often used interchangeably– more later on this.



 van der Waals’ YouTube (:20) occurs in all molecules, even diatomic molecules (HOFBrINCl) 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






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






 held tight by strong electrostatic forces in between cations and anions non-volatile, high mp, high bp solid at room temp hard and brittle because of lattice/crystalline structure conductivity
›
›
› non-mobile e- as solid = no ions move freely when melted = yes cations and ions separate when dissolved in water = yes soluble in polar solvents like water



 strong intramolecular forces, weak intermolecular forces, usually liquids or gases at room temp or soft solid
› strength of polarity and strength of
London forces determine mp and bp
 greater polarity = higher mp and bp
 greater van der Waals’ = higher mp and bp often dissolve in nonpolar solvents but not in strong polar solvents like water do not conduct electricity




 high mp and bp
›
› decreases going down the periodic table
 harder for cations to attract the sea of electrons increases going across the periodic table
 atomic radii becomes smaller, easier to attract the sea of electrons low volatility not soluble in most solvents (polar or non-polar) conduct electricity well because of moving sea of electrons

 from highest to lowest
1.
5.
6.
metallic bonds
2.
4.
ionic bonds (cations and anions) hydrogen bonding (strong δ+ or δ-)
 very strong when H is bonded with NOF
( n itrogen, o xygen, or f luorine) dipole - dipole *δ+ or δ-)
London forces (weak, temporary δ+ or δ-)

 generally speaking
› the greater the intermolecular force (IMF) between the molecules, the higher the melting point, boiling point, and volatility
(evaporate) shape allows it to stick to one another better

 more electrons help increase the van der
Waals’ forces and keep the substance in the this flat liquid state molecules that can stick together better remain a liquid at higher temps.
these round shapes do
NOT allow them to stick to one another boiling point increases

Exampe: two Lewis structures for the formula
C
2
H
6
O. Compare the boiling points of the two molecules. hydrogen bonding can occur here which is the strongest type of dipole : dipole intermolecular force only normal dipole : dipole bonding can take place ethanol - higher BP dimethyl ether - lower BP



“like dissolves like”
›
› polar substances tend to dissolve in polar solvents (ex. water dissolves ionic compounds)
 dissociation of salt YouTube (:53) non-polar substances tend to dissolve in non-polar solvents (ex. alcohol dissolves covalent molecules) metals do not dissolve in most solvents

 +
 –
 +  +
 –
 +
 +
 –
 +
The dipoles of w a t e r attract, pushing the oil (with no partial charge) out of the way: attractions win out over the tendency toward randomness.
 +
 –
 +
 +
 –
 +
 –
 +
 +
 +
 –
 +
 +
 –
 +

 substances must possess Freely Moving
Charged Particles
› this occurs in…
 metals with their “sea of electrons”
 YouTube (1:05)
 molten ionic compounds (+ and – ions can move)
 http://www.dynamicscience.com.au/tester/solutions/ chemistry/bonding/bonding5.htm
 ionic compounds in aqueous solution
(dissolved in water)
 water pulls apart + and – ions and allows them to move

Type of
Bonding
Melting
Point
Boiling
Point
Volatilit y
Electrical
Conductivity
Low Low High No
Solubility in Nonpolar
Solvent
Yes
Solubilit y in
Polar
Solvent
No Nonpolar
Polar varies varies varies No No Yes
No No Yes Hydroge n bonding
Ionic
Bonding varies varies high high varies low
Metallic
Bonding high
Covalent varies high varies
Giant
Covalent
High High low varies
Low
Yes
(molten or aqueous)
Yes
No
No (except graphite)
No
No
No
No
Yes
(most)
No
No
No