Friday, February 25, 2011

Intermolecular Lab Review

        In this blog I'll be giving my review from the Intermolecular Lab that my chemistry class has been preforming these past few days. The first day we did a simple Pre-Lab. In this preparation lab we were given a list of all of the substances that we would be working with and their formula. With this information we created a structural formula for each one. We also found each of these substances molecular weight (graphs x-axis) and whether or not it had a hydrogen bond or not. The next day when we came to class, we did the first part of the lab which dealt with ethanol and methanol. For each, we had a test tube of the substance (compared 2 at a time) and a probe wrapped with filter paper at the tip held on by rubber bands (separate for each substance). We repeated the process (for the other substances) of dipping the probes into each substance for 15 seconds and then pulling them out simultaneously as we watched the computers, which the probes were hooked to, to see which substance temperature would decrease the most (graphs y-axis). Our data chart read this informations:

SUBSTANCE:               INTIAL TEMP:           LOWEST TEMP:        CHANGE IN TEMP:
ethanol                                 21.3 C                                8.4 C                           13.4 C
methanol                              23.7 C                                1.7 C                           22.0 C
propanol                              21.8 C                               14.5 C                             7.3 C
butanol                                22.7 C                               18.1 C                              3.6 C
pentane                                21.9 C                                 -.9 C                            21.0 C
hexane                                 21.4 C                                4.2 C                            17.7 C

Between each of these testings we made predictions of each substances temperature change. We were never right on, but several of them came pretty close. We based our predictions off the previous experiment testings of ethanol and methanol. We figured that the bigger the mass the less the temperature changes, which was correct, but as I said, none of our numbers were right on.
     Finally, after we had collected all of our data, we did a Post-Lab where we processed our results. We used our data from the pre-lab and from our data table (shown above) to make a graph of the substances to show how molecular weight affects the change in temperature. As we had predicted, the larger the mass of the substance the less it changed in temperature and that was proven by this graph of our data (pentane and hexane not included in this explanation).

        To explain this graph and this whole lab a bit more and wrap up, I would like to tie all of this together back with the term intermolecular forces. As I stated before, we concluded that the larger the mass a molecule has the less the molecule changed in temperature and this is because of intermolecular forces. Here you can picture something really sticky vs. something easy to break, a rice crispy vs. a dry cookie. This relates to intermolecular forces because the larger the substance, the more complex it is, meaning the more atoms it has to hold it together, therefore being a lot like the rice crispy and more resistant to fall apart. A substance more like methanol however, tends to be a lot more like the dry cookie. Methanol's structural formula contains one carbon and a few hydrogens hanging on and connected to an oxygen and a hydrogen. This is a fine structural formula, but it isn't sticky. The bonds between these substances aren't as strong causing the substance to be much easier to just break apart, resulting in evaporation taking some of the heat energy with it and larger temperature changes. As for how intermolecular forces deal with temperature change it's like this: if a molecule is larger and "stickier" then during the reaction more energy is used trying to break down the substance rather than changing its temperature and if a molecule is smaller and "less sticky" then it doesn't take the reaction long at all the break down the substance so it has more energy to work on changing the temperature.
        Overall, I liked this lab. I thought it was a fun hands on way to understand how molecular weight can change the temperature. I understand the concept that a molecular weight affects the change in temperature of a substance. For example, the lighter the substance the more its temperature will change and the heavier a substance the more likely it is for its temperature to be fairly close to its original. From this blog I would really like to achieve a 4 in communication and lab skills, because I feel I understood this well enough to share it in an easy-to-understand blog and my lab skills were EXCELENTE! :)

Monday, February 7, 2011

Silver Copper Replacement Lab

Copper Coil
     Copper wire reacts with aqueous silver nitrate. The relative amounts of the reactant and product are determined from the mass loss of copper wire, the starting mass of silver nitrate, and the mass of silver metal obtained. In the experiment, copper changed from its elemental form, Cu, to its blue aqueous ion form, Cu2+(aq). At the same time, silver ions (Ag+(aq)) were removed from solution and deposited on the wire in the elemental Ag metallic form.

     This lab took our class about 3 days to complete. Day number one, we got our materials ready and weighed out. We then took the silver nitrate (AgNO3) and mixed in distilled water until the AgNO3 had dissolved. After that, we placed the copper coil in the test tube as well and let it set until day two. Finally, we went back to class and used math with the balanced equation 2AgNO3 + Cu ----> Cu(NO3)2 + 2Ag and we formed predictions for how much silver should be formed and how much copper became Cu(NO3)2 in the reaction. Day number two rolled around and we observed a sort of crystal looking structure that had formed around the copper wire. We gently shook the test tube to dislodge this formation like the instructions requested. Then we set up a funnel with filter paper with a waste beaker underneath and we lifted the wire from the solution and dumped the remaining solution & silver mixture into the filter paper. Once it had drained we set both the wire & filter paper with the silver under the fume hood to dry. Day three and this was just to finish up the lab. We weighed the copper coil and recorded it's mass and did the same with the silver and filter paper. Now we had all the measurements needed to make out figures of how much Ag was formed & how much Cu was lost.

We used alot of moles in this lab.
There was lots of converting
grams to moles and back again.
     Our predictions said that we should expect to form about .7703g of Ag and loose about .2269g of Cu. With our measurements we did some more math to see if this prediction was correct. First was changed the number of moles of silver produced to moles and got .0103 mol. we did the same for number of copper consumed and that was recorded at .0036 mol. When divided by the smaller mole we got a 1 to 3 ratio of Cu to Ag. This information was compared to our predictions which had stated we would have a 1 to 2 ratio of Cu to Ag. When compared, we got 144 percent yield meaning that we got a considerable amount more silver than we had expected, but for the amount of copper used, we got 100 percent yield so we were right on target there. Overall, this lab, the Silver/Copper Replacement Lab, was a fun way for our class to learn more about mole ratios in a hands-on way.