Chapter 11: Intermolecular Forces, Liquids, and Solids Tadas Rimkus Period 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.

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 ab.union.edu/c hem101_110_i ntermolecular_ forces/n- pentane.jpg Neopentane bp=282.7 K d.wikimedia. org/wikipedi a/commons/ b/b4/Neope ntane-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

Phase Changes ges_files/phase_change.jpg

Heating Curves  Heating curves incorporate the phase diagram but also show the energy required for the phase change phs/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 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 H 2 O and CO 2  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: //ww w.te amo nslau ght.f snet. co.u k/co 2%2 0pha se% 20di agra m.GI F http: //ww w.cb u.ed u/~ mco ndre n/wa ter- phas e- diagr am.j pg