Interactions between molecules
• Three phases of matter
– Solid
• ordered structure
• fixed volume and shape independent of container
• Strong interaction between molecules
– Liquid
• loosely ordered
• fixed volume, but not fixed shape - dependent on container
• Moderate interaction between molecules
– Gas
•
•
•
•
No order
Volume dependent on container
Always occupies all of its container
Almost no interaction between molecules
Role of Kinetic Energy and types
of matter
• Kinetic energy is the amount of energy
needed to bring a body to rest or impart
motion on a resting body
• On the molecular level kinetic energy is
dependent only on temperature
• Heating a compound will raise its kinetic
energy (the temperature will rise)
Phase Transitions - change from one
physical state to another
• When enough energy is added to a solid, the
molecules are no longer rigid and the solid melts
(turns from solid to liquid)
– The reverse (liquid to solid) is called freezing.
• When enough energy is added to a liquid, the
molecules are no longer moderately attracted to
each other vapourises (turns from liquid to gas)
– The reverse (gas to liquid) is called condensation.
• When enough energy is added to a solid, the
molecules sometimes sublime (go directly to the
gas phase)
– Dry Ice, Iodine (lab)
• Question: Why do do molecules have
different melting and boiling points?
– Water (H2O) boils at 100°C and is a liquid at
room temperature
– Methane (CH4), with almost the exact
molecular weight, boils at -164°C and is a gas
at room temp.
• Answer: Interactions between water
molecules are stronger than methane’s.
Attractive Forces Between
Molecules (Van der Waal’s)
• Dipole-Dipole Interactions
– Attraction between polar molecules
• Hydrogen Bond (strongest)
special type of dipole-dipole
– Found in molecules with HF, HO
or HN bonds
• difference between electronegativities
of these molecules is large
• Induced Dipole dipole (weakest)
- Attraction from temporary dipole
moments
• temporary uneven distribution of
electrons
• Increase with size of molecule
Like Dissolves Like
• Why don’t oil and water mix?
– A) Oil’s molecular mass is much greater than water’s.
– B) Oil has only induced dipole forces and water has Hbonding.
• Water will dissolve polar compounds (H-bonding or dipoledipole) but not non-polar.
• Oil will dissolve non-polar compound (induced dipole forces)
but not polar.
• When attractive forces between different
molecules are similar, they will dissolve
• When attractive forces between different
molecules are not similar, they will not dissolve
Soap
• The polar part of soap is hydrophilic (water
loving) and interacts with water
• The non-polar part is hydrophobic (water hating)
and interacts with oil & grease