Intermolecular Forces (IMFs)
Intramolecular force: bond between atoms
Intermolecular force: attractive force between molecules
Also referred to as a particulate interaction
Chemical processes—involve breaking and/or forming chemical bonds (intramolecular)
Physical processes—involve changes in intermolecular forces of attraction (weaker than bonds)
Three types:
London Dispersion Forces (LDF)
 Instantaneous dipole created by movement of electrons within a molecule
 All molecules and atoms have these
 Buzz word—polarizable
 Molecules with more electrons are more polarizable  stronger LDFs
Dipole-dipole forces
 Between atoms in polar molecules
Hydrogen bonding
 Only in molecules containing F—H, O—H, N—H bonds
 Strongest type of IMF
Properties determined by IMFs
 states
solids > liquids > gases
 melting point/boiling point
Stronger IMF means higher melting point or boiling point
 surface tension
Strong IMFs cause molecules to ‘clump’ together
 Vapor pressure
Stronger IMF cause lower vapor pressure b/c molecules cannot escape from the liquid phase
Ionic
Evaluate the strength of the electrostatic charge using Coulomb’s Law
Lattice energy describes strength of attraction between ions LE = q1 q2
r2
Same charge with smaller size means stronger lattice energy  higher melting point, boiling point
Lower charges with about same size means weaker lattice energy  lower melting point, boiling point
(Strong attractive forces need large top and small bottom: think Barbie)
Multiple Choice
1. Which of the following molecules is predicted to have the greatest molecular dipole moment?
A) CO2
B) HCl
C) HBr
D) HI
2. Which of the following lists salts with increasing lattice energy?
A) NaCl < NaI < KI < MgO < MgS
B) KI < NaCl < NaI < MgO < MgS
C) NaI < KI < NaCl < MgS < MgO
D) KI < NaI < NaCl < MgS < MgO
3. In the dissolving of solid CaCl2 in water to form an aqueous solution,
A) bonds in CaCl2 between ions are broken as ion-dipole bond form
B) bonds in CaCl2 between ions are broken as covalent bonds form
C) bonds in CaCl2 between molecules are broken as ion-dipole bond form
D) bonds in CaCl2 between molecules are broken as dipole-dipole bonds form
4. In general, the melting points of ionic solids are higher than the melting points of molecular solids. Which
accounts for this difference?
A) Attractions between particles with opposite charge are stronger than covalent bonds.
B) Repulsions between ions with the same charge are negligibly small.
C) The distance between oppositely charged ions is less than the corresponding distance between bonded
atoms.
D) Attractions between ions with opposite charge are stronger than intermolecular forces.
5. Rank the following from lowest to highest boiling temperature.
C2H6
C2H5OH
C2H5Cl
A)
B)
C)
D)
C2H6 < C2H5OH < C2H5Cl
C2H6 < C2H5Cl < C2H5OH
C2H5OH < C2H5Cl < C2H6
C2H5Cl < C2H6 < C2H5OH
6. The energy absorbed when dry ice sublimes is required to overcome which type of interaction?
A) ion-dipole forces
B) dipole-dipole forces
C) dispersion forces
D) hydrogen bonds
7. The molar masses of a series of similar polar molecules increases in this order: A < B < C < D < E. The
boiling points , in degrees Celsius, of molecules A, B, C, D and E are respectively 20°, 50°, 150°, 100° and 200°.
Which molecule is likely to form hydrogen bonds?
A) A
B) B
C) C
D) D
Free Response
1. Account for each of the following observation about pairs of substances. In your answers, use appropriate
principles of chemical bonding and/or intermolecular. For each part, your answer must include references to
both substances.
a) Even though NH3 and CH4 have similar molecular masses, NH3 has a much higher normal boiling point
(-33⁰C) than CH4 (-164⁰C).
b) At 25⁰C and 1.0 atm, ethane (C2H6) is a gas and hexane (C6H14) is a liquid.
c) Si melts at a much higher temperature (1,410 ⁰C) than Cl2 (-101⁰C).
d) MgO melts at a much higher temperature (2,852⁰C) than NaF (993⁰C).
e) NH3 boils at a higher temperature (-33°C) than AsH3 which boils at a higher temperature (-62.5°C) than
PH3 (-87.7°C).
2.
c) SO2 melts at 201 K, whereas SiO2 melts at 1,883 K. Account for the difference in melting points. You must
discuss both of the substances in your answer.
d) The normal boiling point of Cl2 (l) (238K) is higher than the normal boiling point of HCl (l) (188K). Account
for the difference in normal boiling points based on the types of intermolecular forces in the substances.
You must discuss both substances in your answer.
3. Two types of intermolecular forces present in liquid H2S are London dispersion forces and dipole-dipole
forces.
i.
Compare the strength of the London dispersion forces in liquid H2S to the strength of the London
dispersion forces in liquid H2O. Explain.