• Intermolecular attractions determine how
tightly liquids and solids pack
• Two important properties that depend on
packing are compressibility and diffusion
• Compressibility is a measure of the ability
of a substance to be forced into a smaller
volume
• Solids and liquids are nearly
incompressible because they contain very
little space between particles
Question 11.125
The ‘rotten-egg’ odor is caused by hydrogen sulfide, H2S.
Most people can detect it at a concentration of 0.15 ppb (parts
per billion), meaning 0.15 L of H2S in 109 L of space. A
typical lab is 40 x 20 x 8 ft (1m3 = 1,000L; 1 ft = 0.30 m)
a) How many liters of H2S could be present in a typical lab to
have a concentration of 0.15 ppb?
b) At STP, how many milliliters of 0.100 M Na2S would be
needed to generate the amount of H2S in “part a” by the
following reaction with hydrochloric acid?
N2S(aq) + 2HCl(aq)  H2S(g) + 2NaCl(aq)
• Diffusion occurs more rapidly in gases than
in liquids and solids
Diffusion in a gas (a)
and liquid (b). Gas
molecules move a
much greater distance
than liquid molecules
between collisions. As
a result, diffusion
occurs more rapidly in
the gas.
• The strength of intermolecular attractions
determine many physical properties
– Volume and shape
• Attractions in gases are not strong enough to retain
either volume or shape
• Attractions in liquids and solids are strong enough
so they retain their volume
• Attractions in solids are stronger than for liquids so
that solids also retain shape
– Surface tension is the tendency of a liquid to
take a shape with minimum surface area
– Molecules at the surface have higher potential
energy than those in the bulk of the liquid
– The surface tension of a liquid is proportional
to the energy needed to expand its surface area
– In general, liquids with strong intermolecular
attractions have large surface tensions
Surface tension holds moist particles of sand together.
Separation is resisted because the surface area of the
water would increase.
– Wetting is the spreading of a liquid across a
surface to form a thin film
– For wetting to occur, the intermolecular
attractive force between the surface and the
liquid must be about as strong as within the
liquid itself
– Surfactants are added to detergents to lower
the surface tension of water
– The “wetter” water can then gets better access
to the surface to be cleaned
– Viscosity is the resistance to changing the form
of a sample
• Gases have viscosity, but respond almost instantly to
form-changing forces
• Solids, such as rocks, normally yield to forces acting
to change their shape very slowly
• Liquids are what most people associate with
viscosity
– Viscosity is also called internal friction
because it depends on intermolecular attractions
and molecular shape
WHICH IS MORE VISCOSE?
• A change in state is called a phase change
• Evaporation is the change in state from
liquid to gas
• Sublimation is the change from solid to gas
• Both deal with the motion of molecules
• You have also probably noticed that the
evaporation of liquids produce a cooling
effect
Molecules that are able
to escape from the
liquid have kinetic
energies larger than the
average. When they
leave, the average
kinetic energy of the
remaining molecules is
less, so the temperature
is lower.
• The rate of evaporation depends on the
temperature, surface area, and strength of
the intermolecular attractions
At higher
temperature, the
total fraction of
molecules with
kinetic energy
large enough to
escape is larger so
the rate of
evaporation is
larger.
• For a given liquid, the rate of evaporation
per unit surface area is greater at a higher
temperature