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Chapter 11: Intermolecular Forces, Liquids, and Solids
Tadas RimkusPeriod 2
AP Chemistry
Intermolecular Forces Intermolecular forces are the forces
that exist between molecules. They include:
Ion-dipole forces Dipole-dipole forces London Dispersion forces Hydrogen bonding
The last 3 are called van der Waals forces as well
Intermolecular Forces The strengths of
intermolecular forces vary greatly depending on the substance, but they are generally much weaker that intramolecular forces.
http://www.chem.ufl.edu/~itl/2045/matter/FG11_002.GIF
Boiling and Melting Point
The boiling point of liquids and melting point of solids are dependent on intermolecular forces The higher the temperature at the
boiling/melting point, the stronger the forces
Ion – Dipole Forces
Exist between an ion and the partial charge on the end of a polar molecule Polar molecules are dipoles
Ion-dipole forces are important for solutions of ionic substances in polar liquids (NaCl in water)
Dipole – Dipole Forces
Exist between neutral polar molecules They are effective only when the
molecules are very close together and are generally weaker than ion-dipole forces
The strength of these forces tends to increase with increasing polarity of the molecules involved
London Dispersion Forces Exist between ALL molecules
The molecules create an instantaneous dipole that causes the non-polar molecules to attract or repel
Like dipole-dipole, these forces are only significant when the molecules are very close together
Tends to increase with increasing molecular weight
London Dispersion Forces
Polarizability is the ease with which the electrons in a molecule can be distorted (the “squashiness” of the electron cloud)
More polarizable molecules have stronger London dispersion forces
London Dispersion Forces The shape of the
molecules also affects the strength of the dispersion force
The larger the surface area of the molecule, the stronger its dispersion force
Pentane bp=309.4 K
http://genchmlab.union.edu/chem101_110_intermolecular_forces/n-pentane.jpg
Neopentane bp=282.7 K
http://upload.wikimedia.org/wikipedia/commons/b/b4/Neopentane-3D-balls.png
Comparing Intermolecular Forces
When the molecule have similar molecular weights and shapes, dispersion forces are basically equal. In this case, the differences are due to dipole-dipole attractions, with the most polar molecules having the strongest attractions.
When molecules vary greatly in their molecular weights, dispersion forces tend to be the decisive forces.
Hydrogen Bonding
Hydrogen bonding is a special type of intermolecular force that exists between the hydrogen atom in a polar bond and a fluorine, oxygen, or nitrogen atom. H-F, H-O, H-N
It is the strongest of the intermolecular forces
Distinguishing Between Forces
http://itl.chem.ufl.edu/2041_f97/matter/FG11_012.GIF
Phase Changes
http://www.alterniawhatif.com/HPS%20Project/Phase%20Changes_files/phase_change.jpg
Heating Curves Heating curves incorporate the phase
diagram but also show the energy required for the phase change
http://library.thinkquest.org/C006669/media/Chem/img/Graphs/HeatCool.gif
Critical Temperature and Pressure
The critical temperate is the highest temperature at which a substance can exist as a liquid.
Critical pressure is the pressure required to liquefy the substance at this critical temperature
The greater the intermolecular forces, the higher the critical temperature and the more easily is liquefies
Phase Diagrams
http://ltl.tkk.fi/research/theory/TypicalPD.gif
D
D
B
C
A
Phase Diagrams A phase diagram is a visual used to
explain the conditions under which equilibria exist between the different states of matter The line from A to B is the liquid vapor-pressure
curve It ends at the critical point (the critical
temperature and pressure of the substance) Beyond this point, the liquid and gas phases are
indistinguishable (supercritical fluid)
Phase Diagrams The line from A to C represents the
vapor pressure of the solid as it sublimes at different temperatures
The line from A to D represents the change in melting point of the solid with increasing pressure
Point A is known as the triple point. All 3 phases are at equilibrium at this
temperature and pressure
Phase Diagrams of H2O and CO2
The phase diagram of carbon dioxide (right) follows the typical behavior, with its melting point increasing with increasing pressure
The phase diagram of water (left) shows that the melting point decreases with increasing pressure
http://www.teamonslaught.fsnet.co.uk/co2%20phase%20diagram.GIF
http://www.cbu.edu/~mcondren/water-phase-diagram.jpg