11-11-13

This week we learned about vapor pressure, lattice energy, and reviewed for the test this Tuesday.

Gas particles hit surfaces with a certain amount of force. They all have mass and must stop when they hit a surface (which is acceleration). The more gas particles, the more force. Vapor is the gas state of a liquid at room temperature, and it exhibits a pressure just like any gas. As temperature rises, the fraction of molecules that have enough energy to escape increases (warmer liquid evaporate faster). The more molecules that escape, the higher the pressure they exert. A liquid reaches boiling point when the temperature at which it's vapor pressure equals atmospheric pressure. Vapor pressure increases as temperature increases because more and more molecules at the surface have enough kinetic energy to escape the surface. At higher altitudes, water boils at a lower temperature so cooking and baking lengths are longer. The higher the boiling point, the lower the vapor pressure and the lower the boiling point the higher the vapor pressure. Vapor pressure decreases with molecular weight, but boiling point increases. If the intermolecular forces increase, vapor pressure decreases.

Lattice energy is the energy required to completely separate a mole of a solid ionic compound into its gaseous ions. The energy associated with electrostatic interactions is led by Coulomb's law. Lattice is periodic and predictable because charge and ion size are periodic in nature. It increases with the charge of ions. It also increases with decreasing size of ions. As lattice energy increases, so does melting point. Smaller ions lead to increased lattice energy. Greater charge also leads to increased energy, and the effect of charge is greater than the effect of distance.

We also did an activity where we tested the conductivity of substances with LED conductivity testers that light up when a substance or solution is conductive. Through this activity we determined that steel was the only one that conducted electricity because the electrons are loosely held due to metallic bonding. Water, acetone, ethanol, and nonane were all poor conductors because they are covalently bonded, making the electrons unable to move around. Another activity we did was identifying six unknowns by comparing surface tension and viscosity. My group was fairly close and predicted 4/6 of the substances.

I'd give my understanding this week about an 8. I definitely understand lattice energy and surface tension and how they affect bonding and was able to explain it to my classmates when they asked. I'm still not quite totally sure about everything on vapor pressure and I especially noticed while doing the task chains. The easiest part to me is the water phase change diagram and identifying what's happening in the diagram at a certain point and when a change is endothermic or exothermic like in the task chains. 

11-4-13

This week we began learning about intermolecular forces, and did a POGIL on water.

Molecules attract each other, and the force of attraction increases as intermolecular distance decreases. In liquids, molecules are very close to each other and are constantly moving and colliding. In a gas, molecules are much further apart than in a liquid. Boiling points and melting points are largely determined by intermolecular interactions in the liquid. As molecular weight increases, intermolecular forces get much stronger as well. Intermolecular forces are also much weaker than intramolecular bonds. London dispersion forces are the weakest intermolecular force. They exist in every molecule. The larger the molecule the larger the polarizeablility of the molecule. Dipole induced dipole are the next weakest. It occurs in a molecule when it has a very small dipole moment. Next is dipole dipole. The strength of a dipole dipole interaction on the dipole moment and how closely the molecules approach one another. In a solid, molecules are held close together in a regular pattern by dipole dipole forces to minimize repulsions and maximize attractions. Dipole-dipole forces only occur if the molecule is polar. The strongest of the intermolecular forces is hydrogen bonding. Hydrogen bonding can only occur with Nitrogen, Oxygen, and Fluorine. These intermolecular forces are all called van der waals forces.

In the water POGIL we learned that covalent bonds occur in a single molecule of water. These bonds are intramolecular. We used the femto beaker of water and the molecules with magnets to represent water. Unlike other molecules, when water freezes the volume increases. When pressure is applied to ice, the volume decreases and it becomes liquid. When you apply pressure to ice, the structure breaks and you melt ice due to pressure, so you're able to skate on ice.

We also learned about a fifth type of force - ion dipole interactions. The strength of these forces are what make it possible for ionic substances to dissolve in polar solvents. If cation-anion attractions are stronger than ion dipole attraction, the compound will not be soluble.

I'd give my understanding this week about an 8. I definitely understand the different types of intermolecular forces and how melting and boiling point goes up as molecular weight goes up. I also understand that the higher molecular weight means more polarizability, which means a higher boiling point. I understand that every molecule has London dispersion forces. I'm still a little unclear on how to determine which gas is more soluble, but I think if I go over the Powerpoints and lectures I'll be able to understand it.