Week 9

This week in AP chemistry we delved into greater detail on the electrostatic forces between atoms and their effects on the different characteristics of the substance they make up. Each of these separate classifications has its own particular set of rules determining which of its properties are affected by its particular structure. The separate classifications are atomic bonding structure, bonds formed, boiling/ melting point and vapor pressure.

The way in which a substance is made up overall will typically have the largest magnitude in determining the properties of that substance. First, with the lowest melting and boiling points in most cases, covalently bonded compounds are molecules made up of nonmetal elements who share their electrons in order to complete their octets. Such as carbon dioxide, nitrogen trifluoride and sulfur dichloride, these elements have the weakest total molecular forces, resulting in smaller quantities of energy required to pull them further apart. Next, substances with ionic bonds generally have much higher melting and boiling points than compounds with covalent bonds, and for this reason they are more commonly encountered as solids at room temperature, such as sodium chloride, better known as table salt, a combination of sodium cations and chlorine anions. The final group with the highest melting and boiling points, are often solids that can not be melted at all. These substances are known as network solids. They are composed of elements with covalent bonds, but no particular distinctions of individual molecules. Instead, these substances are composed of  "macromolecules," a single lattice structure of a very large quantity of atoms, all with covalent bonds between those next to it. Common examples of this include diamond and graphite, both structures of only carbon atoms bonded together in one piece, and quartz, composed of a large network of silicon dioxide. Their molecular bond strength comes from the continuous network of bonds within the entire substance, and that melting them would require breaking all of the covalent bonding (See figures below).

Quartz

Many characteristics of substances are further determined by the specific types of bonding that occur within the molecule. Especially present in covalent bonding, the presence of extra van der Waal forces each increase melting and boiling points. Typically the weakest, London Dispersion Forces (LDFs) are present in all substances, although to varying degrees. The strength of this force depends on the polarizability of the molecule, which increases with the size of the molecule, factoring in the number of elements and their sizes, which is ultimately the area with which they may form temporary dipoles with those next to it. Next, dipole interactions increase melting and boiling as a result of increasing the attraction between the positive and negative sides of separate atoms and requiring more energy to pull them apart. Most significant when present, hydrogen bonding can greatly increase the melting and boiling points of these compounds. The particular reasoning behind this trend was not explicitly described to us, but I have concluded that this may only occur between hydrogen and the elements nitrogen, oxygen and fluorine because only these elements have greater electronegativity values than hydrogen, making it negative and the hydrogen positive, and more likely to bond with the typically negative outside of another molecule.

For an additional recap of molecular trends, try the link below:
http://www.masterorganicchemistry.com/2010/10/25/3-trends-that-affect-boiling-points/

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