Student Comments Fall 2008

Student Comments Fall 2008

Kinetics

When choosing a pipette, make sure the rubber gasket is a newer rubber that is pliable. The older one with dried out rubber will not hold a seal on the pipette. Also, blow out your pipette before you ever use it. Some of them are wet inside, which can end up dripping down your (clean) pipette, and potentially causing contamination errors in your results. MSub

Due to the nature of this lab experiment it is easy to get glassware and pipettes mixed up. Be careful to keep everything separate and don't be afraid to label beakers or flasks. NL

For this experiment, the higher the temperature, the faster the reaction takes place. When you change the solvent to a more polar protic solvent, the amount of time for the reaction to occur decreases. For a faster reaction, use the 80/20 water and acetone. KM

Decide which chemicals you and your lab partner will work with before starting the experiment. If you only work with one set of chemicals and your partner works with the other set you can more easily keep the glassware separate, you will make the measurements more uniform,and you will perform the experiment more efficiently. CA

Having a good idea/foundation about SN1 and SN2 reactions (i.e. effects that temperature, solvent, leaving group and structures on each reaction) to lab really helps this lab run smoothly because you know what to expect. Plus, this can save you a lot of time because there are some reactions that don't occur due to the above mentioned (like structure). So that way you aren't just sitting and waiting for a reaction to take place (when its not supposed to), but you would have already known that if you have a good foundation with substitution reactions. AG

This lab was the one I and my hood partner had the most trouble with. The main problem was not knowing how to correctly pipet certain amounts of liquids. Be sure to read over this technique and practice it. If you are still unsure, ask the lab professor or aid to make sure you are doing it correctly before you begin. This technique is not as easy and direct as it looks!

Also, make sure all glassware is thoroughly cleaned. Even the smallest amount of contamination will make a large difference. And do extra trials if the data you have varies a great amount. RR

Make sure that you record the time correctly with a starting time when you first start mixing. Even if some of the reactions take a really short time, count the seconds instead of watching the clock because you might miss the color change occuring.
I recommend doing more trials in case there is contamination, you can choose the closer times from all of your trials. PO

My lab partner and I had trouble with this one experiement because we did not know how to pipette using these pipettes. Make sure to count the liquids right or you will not finish the lab.

Also, it was difficult to get the right amount of OH- in the pipette because the amount used was very small...it could be a source of error if not pipetting right. I'd start with the OH before you add everything else to the flasks NT

If possible, try to have all the necessary chemicals and equipment under your hood. If you and your lab partner utilize the glassware from both drawers your will have enough to continue with very little interruptions. HI

First, bring a stopwatch. Second, this lab tends to be one of the longer ones so work in groups and you might also consider running two trials at once (especially for the cold water bath). Also, make sure you have all the necessary chemicals before you start so you do not have to pace back and forth getting the chemicals and wasting time. I would also recommend rinsing the pippets with distilled water instead of acetone which seemed to mess with my results. BL

This lab can be quite headache-inducing, but don't get sloppy towards the end! I found myself trying to rush and not being as careful with my pipetting which led to inaccurate results and ultimately another trial. The sloppiness/impatience led to a lot of extra time. It's key to be patient and precise EVERY time. KC

Have one person keep time while the other partner adds the liquids together.(Switch off and take turns doing the pipetting) If you decide to use the same pipette for a solution consistently throughout the experiment, label it! JS

Working in a group was the best way to do this lab. More heads make for greater understand in this lab. BES

The best way to complete this lab I felt was ina group and making sure that everything was explicitly labeled. It can become very confusing (and detrimental to your experiment) if you don't label correctly. I felt thatmore prep work upfront really pays off in this experiment. DF

When using the capillary tubes make sure to tap them gently or they will break ( this happened several times to me). It may help if you tilt the capillary tube and tap on it with your finger to get your substance down. LC

I suggest to future students for the Extraction from Beverages Lab not to use a milk product such as Chocolate Milk or Yahoo because it will curdle during the process of the experiment. Also I highly recommend energy drinks, because a high recovery rate is generated. BR

During the procedure, be sure to write down how many drops of indicator you used during the experiment and try to keep it uniform during all of the different procedures. Be patient during the procedures, however, if you've waited longer than 15 min or so, you probably did something incorrect so re-do the procedure to obatin the correct results. Also, during the 'leaving group' procedure, 2-bromo-2-methylpropane was used instead of 2-chloro-2-methylpropane, so if another solution is used be sure to write it down because this can effect your results. SF

Mark the time and quickly pour the acetone solution into the water solution, swirl for a second, and immediately pour the solution back into the other flask (ensuring complete mixing). Continue swirling while waiting for a color change, Before beginning each experiment, the two flasks should be rinsed with acetone and drained. NMR

Make sure that you review how to read the meniscus with the calibration mark pipetting the solution in. Inaccurate data of how many millimeters were added could skew your results. KJ

Make sure when using the pipettes that you measure the correct amount of liquid. I used a little too much and I had to perform the procedure again to make sure my results were correct. LC

Use the same amount of indictor for each trial to keep your experiment constant. Also do not be afraid to run a fourth trial to obtain results.I know for our experiment, the reaction time varied during a particular part, giving us mixed results. Therefore,we ran the reaction again. NNM

I calibrated disposable pipettes using the mass of water. I weighed out 1g of water and that is 1 mL. I then marked the pipette so that I could replicate the volume. This is a better way to do the lab. The pipette bulbs were not very good. You might use fixed volumes to speed the process even more. BES

As mentioned in the comment above, the bulbs are certainly challenging to use. However, if you take a few minutes at the beginning of lab time to practice with water, your technique will be improved. Taking 5 or 10 minutes on the front end will save you valuable time during the lab itself. I wouldn't recommend using the disposable pipettes--even if you attempt to calibrate them they have not been manufactured with the intent that you will be using them for precise measuring. This experiment requires precise measurement ofthe reactantsso take your time and do it right! KV

The reaction times vary. Some of the reactions occur very fast. Be attentive to the color change otherwise you might miss it. LM

make sure you keep a close eye on the color change and do not get impatient because some reactions take a very long time. JVil

Having students practice using the pipettes before labmight help avoid measuring errors. LM

My lab partner and I used a syringe to draw up the sodium hydroxide solution. This made the small amount of 0.1 mL easier to handle and a lot more accurate. AJohn

it would be best for maybe a talk on how to use the pipettes before starting this experiment because not everyone remembers how to use them. if working with your partner, make sure he/she knows how to use them. This will save a lot of time in the experiment. NT

I agree that a review of pipette use is in order and would improve this experiment -- what I'd like to add is a source for better student prep: a short demo on YouTube that I found about pipetting. It's helpful not only in reminding us what equipment we need but also how to measure accurately using a pipette.
The video is 10 minutes long; minutes 2:30-4:30 speak specifically to the equipment and glassware we used in lab:
"SUNY-ESF TV/Chemistry - Using Pipettes"
JW

Why did the kinetic experiment results vary so much? JBF

There are a couple of factors that might have caused this, such as: varying room temperatures, different experimental techniques (such as swirling the reactants before pouring them back into flask A), or contaminated glassware.sources:
and experience NL

It could also be a chaos because of the volume. Many people found it hard using the pipets. JJ

Maybe having more Erlenmeyer flask to perform the trials would be beneficial. I say this because washing the flask between trials can leave water in the erlenmeyer flasks and this can alter results observed. VP

In the kinetic lab, what does the change in color of the mixture from blue to green to yellow mean? JBF

Bromophenol blue indicator changes color between pH's 3.0 to 4.6. At pH 3.0 the mixture should be yellow and at pH 4.6 the mixture should be purple.Reference: AM

Why did we run only one trials each for 1-bromo and 2-bromobutane in the kinetics lab? JBF

We only ran 1 trial for 1-bromobutane and 2-bromobutane because both of the compound have primary and secondary structures and do not react with NaOH as fast as the tertiary structure, therefore after 5min wait the solutions could be discarded. No more than 1 trial is needed to determine this difference. PO

What is the purpose for and or reasons for utilizing the bromophenol blue in this experiment? HI

Bromophenol blue was used in the experiment to see where the end point of the reaction was. When 10% of the 2-chloro-2-methylpropane was used up the bromophenol blue changes from blue to yellow because for every hydroxide on ion hydrolysis for each 2-chloro-2-methylpropane reacted thus the color change occurs after only 10% is used up. KB

When we plotted the information for this experiment why did we get an upside down graph and a negative slope? JVil

The graph for this lab was negative because the points being plotted were 1/T on the x and lnk on the y. This is according to the Arrhenius equation which states that k= Ae^(-Ea/RT). In order to have this equating in a form that makes the Activation energy (Ea) easily accessible the natural log of the equation is taken which yields lnk= -(Ea/1.99)(1/t)+ constant. This equation resembles the equation for a straight line with the Ea as the slope however from this equation it is evident that Ea is negative. In order for the slope to be negative the graph must be decreasing. Taking the lnk gives a negative value also making the graph negative. KB