Chapter 11: Intermolecular Forces, Liquids, and Solids

1. Chapter 11: Intermolecular Forces, Liquids, and Solids Tadas Rimkus Period 2 AP Chemistry

2. 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

3. 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

4. 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

5. 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)

6. 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

7. 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

8. 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

9. London Dispersion Forces Pentane bp=309.4 K • 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 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

10. 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.

11. 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

12. Distinguishing Between Forces http://itl.chem.ufl.edu/2041_f97/matter/FG11_012.GIF

13. Phase Changes http://www.alterniawhatif.com/HPS%20Project/Phase%20Changes_files/phase_change.jpg

14. 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

15. 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

16. Phase Diagrams D D B A C http://ltl.tkk.fi/research/theory/TypicalPD.gif

17. 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)

18. 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

19. Phase Diagrams of H2O and CO2 http://www.cbu.edu/~mcondren/water-phase-diagram.jpg http://www.teamonslaught.fsnet.co.uk/co2%20phase%20diagram.GIF • 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