VSEPR Lab

You were given the VSEPR lab yesterday in class. The lab will be due next Friday, November 11th, 2011. The lab that you need to access is posted on the Events Calendar on the AP Chemistry web-site. VSEPR is an acronym for Valence Shell Electron Pair Repulsion. It is the repulsion of the electrons that create the geometric angles between the bonding and non-bonding ligands. Remember, non-bonding ligands repel more than bonding ligands. That is why the bond angles decrease as bonding ligands become non-bonding ligands within a particular type of parent geometry. Remember on the parent geometry trigonal bypyramidal, the equatorial bonding ligands transform to non-bonding ligands first because of the greater bond angles (120^o vs 90^0) allows for more space between the non-bonding ligands. (Non-bonding ligands repel more than bonding ligands.)

Molecular polarity - is a molecule symmetrical (same charge distribution all around the molecule) or asymmetrical (charge distribution is unequal around the molecule)? Symmetrical is a non-polar molecule and asymmetrical is polar. When you build your molecules, any difference in color for pipe cleaners used will cause the molecule to be asymmetrical, thus polar.

Please see me if you have any questions concerning the lab or concepts needed for the lab.

Halloween Take-Home-Quiz

Please complete the two questions shown below that deal with the Blowing-Up-Pumpkins demo done in class. Use your hand-out or book for the heats of formation and use this link for a table of bond energies.

Take-Home-Quiz

Below are the two images for the take home quiz that needs to be completed before class on Thursday, October 27th, 2011.

Take-Home-Quiz and Remedial Session Reminder

If you would like to earn back half of your points on the Chapter 5 Quiz, attend a remedial session from 6:30 a.m. to 7:15 a.m. tomorrow morning (Friday, October 7th,2011).

Below is the take home quiz that needs to be completed for tomorrow (Friday, October 7th, 2011). You have three choices as to what scenario could break the bond in question. The question does ask if more than one of the scenarios could break the bond, so all scenarios must be analyzed or you could miss out on receiving full credit. We will grade the quiz tomorrow in class. Have a good night.

Chapter 5 Quiz

Tomorrow is a quiz over Chapter 5, Thermochemistry. If you experienced success with the WebAssign set and you understood the pre-lab exercise for the Hess's Law lab , you should be fine with the material on the quiz. Below is a solution to the problem that I had for #8 on WebAssign. I was given three reactions to add together to get a target reaction. I did not use the third equation because two of the substances, water and hydrogen sulfate, are not part of the target equation and could not be factored out. The second equation is reversed and multiplied by two to achieve the correct factoring out. Remember, if a reaction is reversed, the enthalpy sign must be changed. Also, if a reaction is multiplied by some variable, the enthalpy must also be multiplied by that variable. Come and see me tomorrow if you have any questions.

Hess's Law Lab

Below are some of high points that I want to discuss about the Hess's Law lab.

• The water of the solutions is what captured the heat of the reaction. This remained constant at 100.0 mL (50.0 mL of one solution added to 50.0 mL of the other solution) for each trial of the lab. For calculation #1 of the analysis, mass is needed for q = s x m x delta T. Since the solutions are not pure water, the mass is determined by multiplying the total volume of solutions (after adding them together) with the density of the combined mixture. Since the mixture was mostly water, we did use the specific heat of water.


• The intent of the lab was not to prove the conservation of energy. There was a systematic error of heat being lost to the surroundings. This was true for all trials, so this factors itself out for the purpose of the lab. The only intent of the lab is to prove that achieving a heat of a reaction can be achieved through one step or multiple steps, given that the net of the multi-step approach is the same as the direct approach in terms of the same reactants and products.


• As indicated by the pre-lab, hypothetically, the sum of the heats of reaction for reactions #2 and #3 are supposed to equal the heat of reaction for reaction #1. Systematically across the board, this did not turn out to be true. The reaction #3 (hydrogen ions with ammonia) went way off the chart. I have a theory as to why. The ammonia solution used to make the 2.0 M solution for the lab was very old. Because water has evaporated from the solution over time, traces of some precipitate can be seen floating in the solution. It is my theory that as the reaction occured, the precipitate from the ammonia solution dissolved. As you will find out later this semester, some processes of dissolving are quite exothermic. This is my proposition for the undesirable results.


• I will be performing the experiment myself with new solutions. I will be giving you a write-up to place with this lab in your portfolio describing the inadequacies of the lab. This way, any criticism of your results can be placed upon the situation, not your abilities.


• IT IS NOT YOUR RIGHT TO FUDGE THE RESULTS TO MAKE IT FIT THE THEORY. IT IS YOUR RESPONSIBILITY TO PRESENT THE RESULTS "AS IS" AND EXPLAIN ANY DISCREPENCIES IN HOW THE THEORY AND DATA DO NOT COINCIDE. THE RESULTS ARE WHAT THEY ARE. SCIENCE HAS NEVER PROGRESSED BY MANIPULATING RESULTS TO GET THE DESIRED AFFECT. YOU WILL GET PAID MONEY IN THE FUTURE TO FIGURE OUT THE PROBLEM AND FIX IT, SO GET USED TO IT!! And now Mr. Brueckner is stepping off his soap box. :)

Key for Mole Ratio Lab Practice Quiz

Go to the AP Chemistry Calendar page and under today's date is a pdf file of the key for the mole ratio lab practice quiz. Remember, tomorrow you will be taking a quiz for real.