9-9 Intermolecular Forces (IMFs) ~ (Section 14.3) And you

Intermolecular forces are the attractions between molecules and are responsible for many of the physical properties we observe: phase changes (melting, boiling), surface tension, and viscosity. These forces are far weaker than the ionic, covalent, and metallic bonds. The three IMFs are listed in order from weakest to strongest: Dispersion < Dipole/dipole < Hydrogen bonding

Dispersion forces Also called London or Van der Waals forces. Result from a momentary shift in electrons that creates an instantaneous dipole. Occurs in all molecules - especially large ones, usually weakest IMF (unless it is a big molecule) Ex: H 2 bp = -253 C, weak IMF = (Δ to gas at low T)

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Once again: These weak forces of attraction occur in all molecules, even nonpolar ones. They can, however, become quite strong if the size of the molecules increases (more electrons to shift).

Dipole – Dipole Molecules that have permanent dipoles due to electronegativity differences between the atoms can attract each other like little magnets, with the δ+ end of one molecule near the δ- end of the other molecule.

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Example: HCl bp = -85 C Still a fairly low bp!

Hydrogen bonding This IMF is a special type of very strong dipole- dipole, about 10x stronger than most dipole attractions (however, it is still 1/10 as strong as ionic or covalent). Two conditions are necessary for a hydrogen bond to form: Both molecules need either an N, O, or F. One molecule’s N, O, or F has to be attached to an H. No FON = no hydrogen bonding!!!

The three atoms: N, O, and F are very electronegative (δ-) and when bonded to H cause the H to be very δ+. The small H can get close to the partial negative NOF, creating a strong Coulombic (+/-) attraction N-H, O-H, or F-H bonds = very polar bonds and hydrogen very small and can get close to other atoms Ex: HF bp = 20 C

Examples:

Note how the hydrogen bonding in ice creates a six membered ring of water molecules with a lot of empty space in the middle. This increased volume makes ice less dense than liquid water.

Finally Breaking inter molecular forces = physical change therefore, IMFs affect B.P. and M.P. Breaking intra bonds = chemical change