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