Intermolecular Forces

1. Intermolecular Forces Part II Chapter 11

2. Dipole-Dipole • Dipole-dipole is and attraction of molecules with a dipole moment. The strongest of these attractions are the hydrogen bonding forces. • These forces are only 1% of ionic or covalent bonds.

3. Hydrogen Bonding • Molecules with hydrogen bound to oxygen, nitrogen, and fluorine show the greatest dipole moments and hence the strongest hydrogen bonding. • Two factors that create this situation are: 1. The large electronegativity between these two atoms. 2. The relative small size of the O, N, and F which allows a close approach of other partially charged molecules.

4. Hydrogen Bonding • The result is that molecules with these qualities have a very high boiling point and high heat of vaporization.

5. London Dispersion Forces • We know that molecules without a dipole moment exert a force on one another because even a noble gas a low enough temperature will form a liquid. • The forces that exist between non-polar molecules and noble gases are known as London Dispersion Forces.

6. London Dispersion Forces • Electrons may not always be evenly distributed around the nucleus of an atom or the nuclei of a molecule. • As electrons move about, a non symmetrical electron distribution may occur resulting in a temporary dipole. • This dipole may influence the electron distribution of the electrons of another atom or molecule, resulting in IMF’s.

8. Polarizability • “Squashiness” is the electron cloud distortion that resulting in an instantaneous dipole.

9. The ability of an atom or molecule to “polarize” increases with size (mass). The larger the electron cloud the more easily it “squashes”. Larger molecules exhibit larger London Forces. Element Freezing Pt. C° He -269.7 Ne -248.6 Ar -189.4 Kr -157.3 Xe -111.9 Larger London

10. Water • Water is highly polar, exhibiting strong IMF’s. But are those forces stronger than the larger London Forces?

11. Water • Yes Hydrogen Bonding is the strongest of the IMF’s. • Water exhibits a larger surface tension than large nonpolar molecules.

12. Surface tension • Surface tension is the energy required to increase the surface area by a certain amount. • Compare the meniscus of water and mercury. How are they different? • Why? See capillary action.

13. Water exhibits capillary action • Water molecules, and other polar liquids, are attracted to the walls of glass tubing or other materials. • This attraction allows the molecules to rise to a higher level. • Why?

14. Water • Water has both cohesive and adhesive properties. • Water adheres to the water of the tube. • Water is cohesive. Its IMF’s pull other water molecules along. • Mercury is not attracted to the walls of the tube. It has metallic bonds.

15. Water • Due to the intermolecular attractions, water molecules are closer together as a liquid and slide in relation to one another. • As a solid, the molecules spread out to form a crystal structure. • The less dense solid floats in its own liquid.

16. Viscosity • Viscosity is the measure of a liquid’s resistance to flow. • Molecules with large IMF’s tend to be more viscose, such as glycerol. • The larger more complex molecule tends to have the larger viscosity such as grease with 20-25 chain hydrocarbons. • This is due to the molecules’ tendency to become entangled. • At higher temperatures, viscosity is reduced as the larger average kinetic energy allows the molecules to overcome their IMF’s.

17. Viscosity

18. The structural model for liquids • Gases have molecules spread far apart and particles moving rapidly. • Solids have molecules in a relatively stationary position. • Liquids are sort of a disorganized solid with molecules interacting with IMF’s and movement relative to one another.