Week 8

This week in AP chemistry we were introduced to the correlation of intermolecular forces and characteristics of different substances. The new vocabulary we were given included distinguishing between intermolecular forces and intramolecular forces. Intermolecular forces are those that form between separate compounds, including London Dispersion Forces, hydrogen bonds, dipole- dipole bonds, dipole- induced dipole bonds and ion- dipole bonds. Intramolecular bonds are those which form inside of the compound, holding it together, including molecular and ionic bonds. The combination of these bonds is the largest contributing factor in the melting and boiling points of any compound.

The intermolecular forces, most prevalent in covalently bonded compounds have a significant influence on the melting and boiling points of the compound. The weakest of these forces is London Dispersion Forces, in which a temporary dipole in one compound due to an increased or decreased electron density creates a temporary dipole in the compound adjacent to it, followed by a weak attractive force between the two partially positive and partially negative ends (See figure below). This particular force is found in all compounds, while magnitude can vary greatly due to polarizability. Polarizability is a compounds capacity to form temporary induced dipole- induced dipole bonds, determined majorly by its contact or surface area with the compounds around it.

London Dispersion Forces between two atoms

Another force of similar strength is the dipole- induced dipole force. This bond forms between a molecule with a significant dipole and a non-polar molecule in which the electron density is influenced by the dipole of the other, forming a temporary dipole along with attractive forces. Next and the most simple of this group, dipole-dipole interactions form between the positive dipole end of one molecule and the negative dipole end of another, these interactions are significantly stronger as a result of permanent partial charges belonging to both molecule. The last of the van der Waal forces is hydrogen bonding, typically stronger than all of the rest. These bonds form in substance with hydrogen atoms bonded to nitrogen, oxygen or fluorine. Only these three elements have electronegativity values greater than that of  hydrogen and are able to form these bonds. They are the main reason why water takes its rigid hexagonal rings shape as ice, with a greater volume and lesser density than liquid water (See link below for practice with these).

Intramolecular forces within particular substances have a larger impact melting points and boiling points than intermolecular forces. Most importantly, ionic compounds with large charges can have incredibly high melting points and boiling points. It has been observed that these characteristics increase along with bond strength and opposite bond length as a result of atomic radius. Increasing the charge of the ions also has a great impact on these characteristics, such as in the case of sodium chloride have lower melting and boiling points than magnesium sulfide. Covalent bonds also have much more strength than van der Waal forces and increasing the molecular weight of a molecule increases its melting and boiling points, such as adding additional carbon layers to alkanes.

Molecular Forces and Melting Points worksheet with useful graphs:
http://www.dublinschools.net/Downloads/Key-boiling%20points%20and%20IMF.pdf