Super Important Concepts

Intermolecular Forces

Intermolecular forces are not chemical bonds. They are weaker than chemical bonds. They determine the melting point, boiling point, and solubility of chemical compounds.

Molecules with lots of strong intermolecular forces, like hydrogen bonds, will have a high boiling point and a high freezing point. Compounds that have weak intermolecular forces will often be gasses at room temperature, or liquids with low boiling points.

Table I – The four types of intermolecular forces.

Intermolecular Force	Strength	Example	Requirements
Hydrogen Bond	Strongest	Water	H attached to N, O, or F
π-cation	Strongest	Cobalt and Acetylene	Transition metal and double or triple bond.
dipole-dipole	Medium	Water, CHCl3	Must be polar.
Van Der Waals	Weak	Methane, Butane, Propane	All molecules do this.

You can tell that a molecule is polar if it has atoms with very different electronegativities and it is asymmetrical. Atoms on the right side of the periodic table are very electronegative. Atoms on the left are very electropositive.

Chemists often use intermolecular forces to separate and purify chemicals. For example, they often use nonpolar solvents to extract nonpolar chemicals from plants.

Solubility

Compounds dissolve other compounds that have similar intermolecular forces. A polar compound that can hydrogen bond will dissolve other polar compounds that can hydrogen bond.

The number that tells you how well something will dissolve is called K_sp. The higher the K_sp is, the more soluble the compound will be.

Thermodynamics

A reaction that gives off heat energy is exothermic. Exothermic reactions have a negative DH. That means that heat is leaving them.

A reaction that sucks up heat is endothermic. Endothermic reactions have a positive DH. That means that heat is getting sucked up and used by them.

Entropy is how disordered something is. If your room is messy, it has a high entropy. A well folded protein has a low entropy. An unfolded, denatured, protein has a high entropy.

Free energy is a measure of how stable something is. The more energy something has, the less stable it is. Aromatic compounds like benzene are very stable. A negative Gibbs Free Energy means a reaction is spontaneous. G is Gibbs free energy. H is enthalpy. T is temperature in degrees Kelvin. S is entropy.

G = DH - TDS

Kinetics

The speed of the reaction is called the reaction kinetics. A catalyst is a chemical that can lower the activation energy of a reaction without being used up during the reaction. That makes the reaction go much faster, because the molecules do not need to go over as high of an energy barrier. Most catalysts are organometallic compounds. That means they have organic ligands and a metal center.

Reactions go faster at high temperatures because it is easier for them to get over the energy barrier. In other words, at higher temperatures, they have enough energy to easily get above the activation energy.

Ziegler-Natta catalysts are used to make polyethylene and polypropylene. Polymerization is a reaction that turns a monomer, a small molecule, into a polymer, a huge molecule that is a chain of lots of small molecules. Polyethylene and Polypropylene are plastics that are often used for food and drink containers.

Palladium on carbon is used as a hydrogenation catalyst. It adds hydrogen to unsaturated molecules. Unsaturated molecules have double or triple bonds.