Intermolecular Forces
2010 ©
Prem D. Sattsangi
&
Christopher L. Byers (programmer)
States of Matter (A balance between IMF & KE)
IMF hold molecules together
KE allows them to separate
Kinetic Energy depends upon temperature.
(s)
heating
cooling
<<KE
(l)
heating
cooling
 H2O (g)
<<KE
(g)
Intermolecular Force is equal. (Depends upon molecular structure)
Solid (ice) H2O
Own Shape
Definite Volume
Incompressible
Doesn’t Flow
Negligible Diffusion
Liquid H2O
Container’s Shape in part
Definite Volume
Incompressible
Flows
Slow Diffusion
Gas (vapor) H2O
Container’s Shape
Container’s Volume
Compressible
Flows
Rapid Diffusion
London Dispersion
London recognized that the motion of electrons in an atom or
molecule can cause instantaneous & momentary dipole.
This force is known as London Dispersion (LD).
LD is determined by the formula weight (FW).
If FW is the same, LD is determined by the surface area.
Too much weight
(Fgrav) will cause
the velcro to
break.
Force  
Force  
Less Velcro
surface area
Less Weight
can be held
More Velcro
surface area
More Weight
can be held
Dipole-Dipole Forces
Dipole: a bond with a positive and a negative end.
Strength depends on electronegativity difference (END).
END for HCl, [Cl(3.5) - H(2.1)] = 1.4
δ+
H
δCl
Dipole-Dipole Forces: The opposite ends of two or more
dipoles attract each other.
A.
B.
δ+
δ-
δ-
δ+
>>
δ+
A. is Stronger than B.
δ- δ+
δ-
Dipole Moments (DM)
A Dipole moment is the measure of the polarity of a molecule.
For compounds with similar FW, the DM determines the IMF.
As the Dipole moment increases, so does the boiling point (B.P.).
Substance
F.W.
D.M.
B.P. (K)
Propane (C3H8)
Dimethyl ether (C2H6O)
Methyl chloride (CH3Cl)
Acetaldehyde (C2H4O)
Acetonitrile (C2H3N)
44
46
50
44
41
0.1
1.3
1.9
2.7
3.9
231
248
249
294
355
Polar Molecules are Nonsymmetrical
Asymmetrical Polar
Tetrahedral
Symmetrical Non-polar
Symmetrical Non-polar
Tetrahedral
Hydrogen Bonding
Hydrogen Bonding is a special bond between the H atom in
a polar bond (H—N, H—F, or H—O) and an unshared
electron pair on a neighboring F, O, or N atom.
It is much stronger than Dipole-Dipole force or LD.
Of those bonds, H—O bond is the strongest, like that in
water.
Both H—N and H—O bonds are used in bonding DNA
molecules together.
H—O E.N.D. = 1.4, H—N E.N.D. = 0.9
Comparison of H-Bonds
(H—O & H—N)
H-bond in water
2 Electron Pairs (δ-) & 2 H ends (δ+).
After 1 H-bond, bonding sites increase.
3 Electron Pairs (δ-) & 3 H ends (δ+).
Water forms endless H-Bonds.
H-bond in ammonia
1 Electron Pair (δ-) & 3 H ends (δ+).
After 1 H-bond, H ends increase.
1 Electron Pair (δ-) & 5 H ends (δ+).
The (δ-) end is hard to find;
limited H-Bonds.
H-Bond in HF is limited for the same reason.
H-Bonding in DNA
H-Bonds
G
G, C, A, & T are
Bases
C
A
T
A
T
C
G
H-Bonds in Silk
H-Bonds
Ionic Bonding
Ions are formed by electron transfer.
They are held together by electrostatic forces in a lattice,
causing ionic bonding. E.g. NaCl
-
+
e-
Na+(g)
(g)atom
ion
Cl-(g)
(g)atom
ion
NaCl (s)
Sample Exercise 1
Consider the FW, Dipole Moment (DP), & Boiling Point (BP)
to answer the following questions.
Substance
CH3CN
CH3I
FW
41
142
DM
3.90
1.62
Which has greater London Dispersion?
CH3CN or CH3I
Which has greater dipole attraction?
CH3CN or CH3I
Which has better overall IMF?
CH3CN or CH3I
BP
354.8 K
315.6 K
Sample Exercise 2
Weaker IMF
Stronger IMF
London Dispersion << Polar << H-Bonding << Ion-Dipole << Ionic
Which has greater London Dispersion force?
(Periodic Table used to determine FW)
Why?
Xe (FW=131) or Ar (FW=40)
Xe has greater FW & therefore
more surface area.
HF b.p. = 20 °C and HCl b.p. = -85 °C.
Why does HF have a higher b.p.?
HF has H-bonding & HCl is only
polar. (See IMF figure above.)
Br2 b.p. = 59 °C and ICl b.p. = 97 °C.
Why does ICl have a higher b.p.?
ICl is polar & Br2 is non-polar.
(Both have FW ≈ 160.)
Sample Exercise 3
Weaker IMF
Stronger IMF
London Dispersion << Polar << H-Bonding << Ion-Dipole << Ionic
List the following substances in order of increasing boiling points.
BaCl2, H2, CO, HF, & Ne
1st highest Ionic Bonding:
2nd highest H-Bonding:
3rd highest Polar Bond:
4th highest Non-Polar:
5th lowest Non-Polar:
(Higher IMF = Higher BP)
BaCl2
HF
CO (slightly polar & higher FW than Ne)
Ne (higher FW than H2)
H2 (lowest FW)
Answer: H2 << Ne << CO << HF << BaCl2
Sample Exercise 4
List the substances N2, NH3, LiF, CO, & CH4 in order of increasing b.p.
LiF = Ionic Bond (strongest)
NH3 = H-Bond (2nd strongest)
CO = Polar (3rd strongest)
N2 & CH4 = Non-Polar (weakest)
N2 FW = 28
CH4 FW = 16
Answer: CH4 < N2 < CO < NH3 < LiF
Sample Exercise 5
Explain the nature of IMF present in each of the following components.
A.
B.
C.
LD only
Non-polar molecule.
LD and Dipole-Dipole
Polar molecule.
LD and H-Bonding
Polarity is present but H-Bond is
more dominant force.
Sample Exercise 6
Rank the following components in terms of (a) LD and (b) H-Bonding.
A.
(FW = 94)
B.
(FW = 93)
C.
(FW = 92)
LD A≈B≈C
Comparable FW and Shape.
H-Bonding A<B<C
H-Bonding increases with
the number of OH groups.