11/4/13-11/8/13

This week in AP Chemistry I continued to study intermolecular forces. The topics covered this week included viscosity, surface tension, cohesive and adhesive forces, different forms of solids, vapor pressure, and lattice energy.

Viscosity is a liquid's resistance to flow. It is the ease by which the molecules in a substance move past each other. The higher the viscosity, the less a liquid moves. Viscosity increases with stronger intermolecular and decreases with higher temperature. This topic was covered in the Liquids lecture.

Surface tension results from an imbalance of intermolecular forces at the surface of a liquid. Molecules on the surface are subject to only inward forces, whereas molecules in the center of a substance are surrounded on all sides by molecules and thus are subject to a full cadre of intermolecular forces. Of all the topics covered this week, I had the the weakest understanding of surface tension. I did not comprehend the portion in the Liquids lecture about surface tension well. I also feel as though I am unable to complete the questions in the worksheets I was given in class that regard surface tension. This website gave me a little clarity on the subject, although I was unable to find a good source that could give me a more in depth explanation that I could understand well.

Cohesive forces are intermolecular forces between molecules of the same kind. Adhesive forces are forces that are intermolecular forces between two molecules of a different kind. The strength of cohesive and adhesive forces can determine how a substance acts on a certain surface. For example, if you put a drop of water on a waxed surface and a drop of water on glass, the water on the waxed surface beads up (the cohesive forces between water molecules are greater than the adhesive forces between water and the waxed surface), while the water on the glass spreads out (the cohesive forces between water molecules are weaker than the adhesive forces between the water and the glass). Cohesive and adhesive forces were covered in the Liquids lecture as well as in the Intermolecular Forces II worksheet. I felt I had a strong understanding of this topic.

Another example of cohesive vs. adhesive forces. In the tube with water, the adhesive forces between water and glass are greater than the cohesive forces between the water molecules. Thus, water has a concave meniscus. In the tube with mercury, the cohesive forces between mercury molecules are greater than the adhesive forces between mercury and glass. Therefore, mercury has a convex meniscus.

Additionally, I studied various properties of solids this week. Solids generally fall into two groups: crystalline and amorphous. Crystalline solids are solids in which the molecules have a highly ordered arrangement. Amorphous solids are solids in which there is no order in the arrangement of particles. Furthermore, there are other classifications of solids including molecular solids, ionic solids, and covalent network solids. Molecular solids are solids where molecules are held together by van der Waals forces. They tend to be soft and usually have low melting and boiling points. Ionic solids are the solid forms of ionic compounds. In ionic solids, ions pack themselves in a position that maximizes attraction between ions. Ionic solids typically have very high melting and boiling points. Covalent network solids are solids in which atoms are covalently bonded to each other in a large network. Substances like diamond, graphite, quartz, and glass are all network solids. These solids tend to be strong and have high melting and boiling points.

A vapor is the gas form of a substance that is normally liquid at room temperature. Pressure is the force by which gas particles hit a surface. Vapor pressure is simply the pressure of a vapor at a given temperature. The greater the number of gas particles, the greater the pressure of a gas. As temperature increases, the kinetic energy of molecules in a liquid increases and more particles "escape" the liquid and evaporate. Thus, the pressure of the surrounding atmosphere is increased. When the vapor pressure equals the atmospheric pressure, a substance boils (at sea level, this pressure is 1 atm). However, intermolecular forces influence boiling point. The greater the intermolecular forces, the greater the boiling point. Boiling point and vapor pressure are linked; the greater the boiling point, the lower the vapor pressure. The lower the boiling point, the higher the vapor pressure. All substances have the same vapor pressure at their boiling point. Initially, this concept threw me off. When we studied properties like viscosity and boiling point, as the intermolecular forces grew stronger, the viscosity and boiling point of that given substance increased as well. But with vapor pressure the relationship is different. Other than that small caveat, I understood this concept well. I completed several assignments that related to vapor pressure, including a lecture and the Intermolecular Forces II worksheet.

The final subject I studied this week was lattice energy. Lattice energy is the energy required to completely separate a mole of a solid ionic compound into its gaseous ions. Lattice energy is associated with coulomb's law; it increases as charge magnitude increases and increases as the size of ions decreases (distance between oppositely charged ions decreases, so the charge between them is stronger). This process is very exothermic; it takes a considerable amount of energy to break ions from the solid.

I felt I had a strong grasp on most of the topics covered this week with the exception of surface tension. I will continue to look for a source that could give me some clarity in that subject. I also feel relatively prepared for the upcoming test on Tuesday, but none the less I will be furiously studying and stressing on Monday night.

10/28/13-11/1/13

This week in AP Chemistry I learned about intramolecular forces and intermolecular forces. Intramolecular forces are forces between atoms in a molecule such as covalent and ionic bonds. However, intramolecular forces were not the main focus this week. Intermolecular forces were.

Intermolecular forces are attractive and repulsive forces between molecules. Intermolecular forces are weaker than intramolecular forces but are strong enough to influence physical properties of substances. There are several different types of intermolecular forces. Most fall under the category of "van der Waals forces." In class, I learned of four important van der Waals forces: dipole-dipole interactions, dipole induced dipole interactions, induced dipole-induced dipole interactions (a.k.a. London dispersion forces), and Hydrogen bonding.

Dipole-dipole interactions are attractive forces between the partially negative end of a polar molecule and the partially positive end of another molecule. Although the strength of dipole-dipole interactions are dependent on the magnitude of a substance's dipole moment and the proximity of its molecules, in general dipole-dipole attractions are relatively weak. Dipole-dipole interactions are only present in polar substances.

Hydrogen bonds are closely related to dipole-dipole interactions. Hydrogen bonds occur when partially positive hydrogen atoms are attracted to very electronegative partially negative atoms like oxygen, fluorine, and nitrogen. Although in essence they are the same as dipole-dipole interactions, they have their own classification because they are much stronger than typical dipole-dipole forces. Hydrogen bonds are the strongest of all of the van der Waals forces.

When a polar molecule approaches a nonpolar molecule, the electrons in the nonpolar molecule, usually shared evenly, are attracted to the polar molecule. This results in an induced dipole in the nonpolar molecule. This interaction is aptly named a dipole-induced dipole interaction. Much like the dipole-dipole interaction, it too is relatively weak.

Induced dipole-induced dipole interactions are the most common. Electrons are constantly in motion. Even in nonpolar molecules, there is a probability that at some point there will be an imbalance of electrons (this distortion in electron clouds is called polarizability). When such an imbalance occurs, a temporary dipole is produced. This temporary dipole induces a temporary dipole in other molecules. This intermolecular force, also called the London dispersion force, is present in all substances.

Additionally, I learned this week about ion-dipole interactions. Ion-dipole interactions are forces between ions and molecules. They are not van der Waals forces. Ion-dipole interactions make it possible for some ionic substances to dissolve in water. If the forces of attraction between atoms in an ionic compound are greater than the ion-dipole force between the ionic compound and water, the substance will not dissolve in water. If the forces of attraction between atoms in an ionic compound are lesser than the ion-dipole force between the ionic compound and water, the substance will dissolve in water.

Intermolecular forces have a great effect on the physical properties of substances. For example, the stronger the intermolecular forces, the higher the boiling and melting point of a substance. Intermolecular forces also affect the vapor pressures and viscosity. To help learn the concepts associated with intermolecular forces, I completed three lectures (one on dipoles and induced dipoles, one on hydrogen bonds, and one on ion-dipole interactions) as well as a the Intermolecular Forces POGIL and the Water POGIL. In addition, we went over the Intermolecular Forces I worksheet. Initially I found the concept of intermolecular forces to be confusing. I found the scanned textbook chapter you gave us in class on Tuesday to be challenging reading. I did not comprehend the text well (the fact that I was running on a minuscule amount of sleep certainly did not help in this regard). However, I found that the lectures brought me to understand these topics well.

A demonstration from class this past week. Butane is lit on fire. In the can, butane is in liquid form because it is under very high pressure. However, when it was sprayed into a test tube, it immediately started to boil, as butane normally has a very low boiling point. When lit on fire, the butane gas burned.