Intermolecular Forces, Liquids,
and Solids
Chapter 11 BLB 11th
Questions?
 Why does water have such anomalous
properties?
 Why do you add salt to the water before
cooking pasta?
 How can ice melt below 0°C?
 How does antifreeze work?
Answer: Solutions have different
properties than pure solvents.
11.2 Intermolecular Forces
 The attractive forces between molecules or
between ions and molecules
 Most properties of liquids and solids (and
solutions) are due to the strength of the
intermolecular forces present.
 Boiling point
 Melting (freezing) point
 ΔHvap°
Types of Intermolecular Forces:
In order from strongest to weakest:



Ion-dipole – attraction of an ion and the
partial charges of a polar molecule (hydration
of ionic salts)
Dipole-dipole – attraction between two polar
molecules (water, HF, alcohols)
London Dispersion (induced dipole) –
instantaneous attraction between two
nonpolar molecules due the polarizability of
the electron cloud.
Ion-dipole
Dipole-dipole
London dispersion
Hydrogen “bonding”
 Special type of dipole-dipole interaction
 Occurs between a H atom attached to an
electronegative atom (N, O, F) on one
molecule and a lone pair on an
electronegative element on another
molecule. (whew!)
 Common in water(!) and alcohols (C–OH)
 Very strong collectively
Hydrogen-bonding examples
Between same molecule
Between different molecules
Protein structure
(α-helix)
DNA
(double helix)
Hydrogen-bonding between
base pairs in DNA
Why is water so weird?
 High specific heat (only NH3 higher)
 High ΔHfus° (only NH3 higher)
 Highest ΔHvap°
 Highest surface tension
 Higher boiling point than expected
 Ice floats on water.
Answer: Strong network of hydrogen bonding
Hydrogen bonding in ice
Boiling Points (See Table 11.3)
11.3 Some Properties of Liquids
 Viscosity – resistance to flow
 Surface tension – a measure of the inward
forces that must be overcome in order to
expand the surface area of a liquid
 Cohesive – between molecules
 Adhesive – attraction between substance and
a surface (glass ⇒ water)
 Capillary action
11.5 Vapor Pressure
 In an open container:
liquid → gas (evaporation)
 In a closed container:
liquid ⇌ gas (dynamic equilibrium)
 Vapor pressure – pressure exerted by the
vapor in equilibrium with the liquid or solid in
a closed container at a given temperature
Vapor Pressure
Vapor Pressure, cont.
 As T ↑, vapor pressure ↑.
 When vapor pressure = atmospheric pressure,
boiling occurs.
 When pressure = 1 atm: normal boiling point
 Volatile substances have a higher vapor
pressure.
Vapor Pressure
11.6 Phase Diagrams
 Show graphically the relationship between
the different phases of a substance.
 Solid, liquid, gas regions
 Normal melting and boiling points
 Triple point
 Critical point
 Slope of solid-liquid line
Generic Phase Diagram
H2O
vs.
CO2
H2O