Intermolecular Forces

1. Ionic - result of electrostatic forces between ions
   Coulomb's law:
   examples: NaCl(s), solid sodiumnitrate, NaOAc(s)
2. Ion-dipole - interaction of an ion (cation or anion) with a polar molecule
   examples: dissolving any ionic compound in water
3. Dipole-dipole - Interaction of polar molecules with other polar molecules
   examples: acetone in acetone, triethyl amine in acetone
   
   Hydrogen bonding - special case of dipole-dipole when there is a H bonded to a N, O, or F.
   examples: water, acetic acid, acetone in water
   
4. Dipole-induced dipole - a polar molecule interacting with a nonpolar one
   examples: carbondioxide in water (note: this may not be a good example because we could envision hydrogen bonding occurring), carbondioxide in ethylene (Dr. Abrash in our department studies the interactions of such molecules at low temperatures using lasers.)
5. London (also called instantaneous dipole-induced dipole)
   These are the very weak interaction that hold two nonpolar molecules together.
   Many general chemistry textbooks say that London forces are always present because these are forces that are inherent in anything that has mass.

These last two forces are collectively known as Van der Waals forces and are in general very weak.

Notes:

1. Clearly you must be able to first determine whether something is ionic, polar covalent, or nonpolar covalent.
2. Note that ionic forces decrease with increasing size but the other increase.
3. Once you have identified the type of intermolecular forces present, you should be able to make predictions about boiling point (volatility).
4. You should also be able to predict whether one substance will be miscible or soluble with another.

Substances soluble in water

1. ionic compounds
2. organic compounds with less than five carbons and one polar functional group, e.g., containing O or N (more than five carbons and the molecule is too nonpolar)
3. polyfunctional compounds

Substances soluble in diethylether

1. not ionic compounds
2. Most organic compounds are soluble - polar and nonpolar

Question: Is diethylether soluble in water based on above criteria? Answer

One way to think about solubility

If we list the type of intermolecular interactions a pure substance has in increasing order of strength:

A substance will in general be soluble in a substance directly above of below itself

• ionic in hydrogen bonded (NaCl in water)
• hydrogen bonded in polar (water and diethylether)
• polar and London (diethylether and pentane)

but will not be soluble in a substance two away from itself

• ionic will not dissolve in polar (diethylether) or nonpolar (pentane)
• hydrogen bonded will not dissolve nonpolar (water and pentane)