4.4 INTERMOLECULAR FORCES (IMFs)
* IMFs are forces that exist between ___________________. Remember, a molecule is an entity consisting of
two or more atoms that are ______________________ bonded to one another. Hence, these forces are NOT
present in ________________ and ________________ substances, as well as most covalent ____________!
1. ____________________ Dispersion Forces(LDFs) (aka  van der Waals forces)
(instantaneous, TEMPORARY dipole – ________________ TEMPORARY dipole attraction)
 Induction =
 the simultaneous attraction between negative electrons and surrounding positive nuclei
 relatively _____________ intermolecular force
 exists between ALL molecules
 The only type of attraction present in ______________molecules.
 The strength of the London force depends on the number of ________________ in the molecule. (i.e. The greater the number of
electrons, the ___________ the London forces.)
e.g. Boiling Point of First 5 Straight Alkanes
Formula
Name
CH4
methane
C2H6
ethane
C3H8
propane
C4H10
butane
C5H12
pentane
Boiling Point (°C)
-161
-89
-44
-0.5
36
*Describe and explain the trend in boiling points of the first 5 straight alkanes.
Explaining London Dispersion Forces:
2. a)___________________-___________________ Forces (DDF’s)
(permanent dipole – ________________ dipole)
 oppositely charged ends of polar molecules ______________
 stronger than _______________ Forces
 The strength of the dipole-dipole force is dependent on the ____________ of the molecule.
(i.e. The greater the polarity, the _________________the dipole-dipole force.)
 only about _____% as strong as a covalent bond
b)
Bonding (HB)
(permanent dipole – _______________ dipole)
Used initially to explain the unexpectedly high _____________ ____________ of group 15, 16 & 17 hydrides. (i.e. NH3, H2O and
HF)
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*Hydrogen bonding is a particularly strong intermolecular force that involves the attraction of the
________________hydrogen end, of a water molecule for example, to the ________________
LONE PAIRS at the oxygen end. In order for hydrogen bonding to occur, the hydrogen atom must
be BONDED to either a ______________, ____________, or ______________ atom. Remember:
hydrogen bonding is just another name for an ESPECIALLY STRONG ________________ ________________ attraction (about 5x stronger).
*Reasons for stronger dipole – dipole attraction:
i) N – H, O – H, and F – H bonds are very _______________. (i.e. large __________________
difference)
ii) Because of the ___________ size of the atoms involved, the charge densities at the dipole ends are relatively concentrated.
iii) A ____________ pair of electrons on another O, N or F atom, with which the positively charged H atom can line up.
REVIEW: Notice the similarities between hydrogen bonding and coordinate covalent bonding.
*Examples of coordinate bonding: (ammonium, hydronium, carbon monoxide)
How do you determine the type(s) of IMF present?
- Is there a H atom bonded directly to a F, O or N? If “yes”, hydrogen bonding is present.
- Is the molecule non-polar? (low ∆EN or symmetrical)? If “yes”, only London forces are present.
- All others are dipole-dipole.
London Forces
 non-polar molecules
Examples:
- hydrocarbons such as CH4,
C2H6 etc.
- any molecular element like
Cl2, Br2, O2, P4, S8
Hydrogen Bonding
 H-F, hydrogen fluoride,
…only one molecule
 H-O…water, alcohols,
carboxylic acids
 H-N…ammonia, amides,
amines
The Layering of Intermolecular Forces
H2O
CH3Cl
Strong
- hydrogen bonding
Medium - dipole/dipole
- dipole/dipole
- London forces
- London forces
weak
Dipole-Dipole
Any other molecule that
is not covered by previous
two. Only present in
POLAR molecules.
CH4
- London forces
Try the following:
1. Identify the IMFs in the following: He, Cl2, CH3(CH2)4CH3, NF3, (CH3)2O, CH3F, CH3CH2OH
2. Which of the following can form hydrogen bonds with water. Draw diagram for those that can.
 NH3, CH3CH2CH3, CH3COOH
3. The boiling points of CCl4 and CH3F are 77°C and -78°C, respectively. Explain the difference.
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