2. KINETIC MOLECULAR THEORY AND
INTERMOLECULAR FORCES
Objectives:
1. Use the kinetic molecular model to explain properties of liquids and solids.
2. Describe and differentiate the types of intermolecular forces.
3. Predict the intermolecular forces possible for a molecule.
4. Design a simple investigation to determine the effect on boiling point or freezing point
when a solid is dissolved in water.
Chapter 1:
4. KINETIC MOLECULAR THEORY
ARRANGEMENT OF
PARTICLES
KINETIC ENERGY OF
PARTICLES
PARTICLE MOTION
ATTRACTIVE FORCES
BETWEEN PARTICLES
INTERMOLECULAR
FORCES
• closely packed
• orderly
• less closely packed
than in a solid
• disorderly
• very far apart
• disorderly
Solid Liqui
d
Gas
i
n
in
in
in
such as
• particles
vibrate &
rotate about a
fixed position
• particles slide
over each other
• particles move
about at great
speed
• very low • low • high
• very strong • strong • very weak
• ion-dipole
• dipole-dipole
• London dispersion forces
• hydrogen bonding
11. Intermolecular Forces of Attraction
How is intermolecular forces defined by nature of
particles?
12. INTRAMOLECULAR
FORCES OF
ATTRACTION
Intramolecular Forces
Intramolecular forces – exist inside the molecule and are
relatively strong because their charges are larger and closer.
Ex. Ionic bonding – attraction between cations and anions
Covalent bonding – sharing of electrons
Metallic bonding – attraction between metal cations and
delocalized valence electrons.
13. INTERMOLECULAR
FORCES OF
ATTRACTION
Intermolecular Forces
- occurs between neighboring molecules as a
result of partial charges or between ions and
molecules.
- usually called van der Waals forces
(Johannes van der Waals, Dutch scientist).
Types of van der Waals forces:
1. Ion-dipole
2. Dipole-dipole
3. London dispersion forces
4. Hydrogen bond
14. INTERMOLECULAR
FORCES OF
ATTRACTION
Ion-Dipole Forces
- results when an ion and the partial charge found
at the end of a polar molecule attract each other.
- are important in solutions of ionic substances,
such as NaCl, in polar liquids like water.
16. INTERMOLECULAR
FORCES OF
ATTRACTION
Dipole-Dipole Forces
- attractive forces that exist between polar
molecules.
- polar molecules attract each other when unlike
charges are close together and repel each other when
like charges are close together.
18. INTERMOLECULAR
FORCES OF
ATTRACTION
Dipole-Dipole Forces
• The more electronegative atom becomes partially
negative and has most electron density whereas the
least electronegative becomes partially positive and
has a lesser electron density.
24. Molar Mass, Melting Points, and Boiling Points of Halogens
Halogen Molar Mass (g/mol) Melting Point (K) Boiling Point (K)
𝐹2 38 53.5 85.0
𝐶𝑙2 71 172.2 239.1
𝐵𝑟2 168 265.9 331.9
𝐼2 254 387.7 457.9
Instantaneous dipole – exerts an influence on nearby particles which become an induced dipoles.
Polarizability – the ease with which a dipole can be induced.
- increases with increased molecular mass. The electrons in large molecules are less
firmly held and are easier to be polarized.
• As polarization and London forces increase, the boiling and melting points of covalent
substances increase with increasing molecular mass.
25. INTERMOLECULAR
FORCES OF
ATTRACTION
LONDON DISPERSION
FORCES
• The strength of instantaneous and induced dipoles depends on
molecular shape. Small and compact structures molecules are
more difficult to polarize than molecules with elongated
structures.
bp: 282.5 K bp: 309 K
27. INTERMOLECULAR
FORCES OF
ATTRACTION
HYDROGEN
BONDING
Hydrogen bond – is an attractive interaction between a hydrogen
atom bonded to an electronegative F, O, or N atom and an unshared
electron pair of another nearby electronegative atom.
Ex. Hydrogen bonds occur in H2O, NH3, HF, CH3OH
28. INTERMOLECULAR
FORCES OF
ATTRACTION
HYDROGEN
BONDING
Hydrogen bond – a special type of dipole-dipole interaction.
Consequences:
• Increase in boiling point with increasing molecular weight due
to increased dispersion forces.
• Low density of ice compound (0.917g/mL) compared to that
of water (1.00g/mL).
32. What type of intermolecular force will act in the following
substances? Justify your answer.
1. sulfur dioxide (SO2)
2. nitrogen gas (N2)
3. hydrogen fluoride (HF)
4. carbon dioxide (CO2)
5. neon gas (Ne)
6. magnesium chloride(MgCl2)dissolved in water (H2O)
33. Identify the dominant intermolecular forces for the following substances.
Select the substance with the higher boiling point in each pair.
a. Ne and Ar
b. CH3OH and CH3CH2OH
c. HF and CH3F
d. BaCl2 and PCl3
