November 6, 2009

Chemistry Teaching Musings (VII)

Posted in Chemistry at 1:26 am by faith786

بسم الله الرحمن الرحيم

Assalamu Alaikum,

This post is primarily dedicated to Mally as she is the only other chemistry major in our high school class to my knowledge.

WARNING: There maybe *technical* chemistry jargon so don’t read ahead if you can’t handle the exothermic reaction… shameless joke. And I stop caring about grammar. I am sleepy.

Today’s Chemistry Teaching Musings: Lab.

Yes I said it, Lab. With a capital L. (Sounds kinda creepy especially if you are reading this post two in the morning….)

I was in lab today working on recording the heat of a reaction before and after it occurs and seeing if the experimental value of the heat change matches the literature value. (In language for Noreen and Spiderman: that means I am trying to get the book answer in my lab.) So I did the experiment three times, twice ‘old school’ (literally having a thermometer in the reaction and writing down the temperature every minute) and once on the LabPro (some computer software that does it for me). As I was waiting in between minutes or waiting for the computer to do its stuff, I started to think about the procedure and why I always mess up every lab.

According to my chemistry professor, he once said (I may be poorly quoting this) that there are three reasons why ‘labs go wrong’–the person doing the lab is messing up, the procedure itself is faulty or the chemistry law/principle only is oversimplified for the class you are in (therefore your results don’t match up to it). But I was wondering–what can procedural errors mean?

I am fully aware I messed up that lab and that I am part to blame (Faith786: I am doing another trial because I think I messed up the first one. Chemistry Professor: You probably did. XD) but I think there are MAJOR holes with the procedure and different types of holes so I guess I am kind of elaborating on how ‘labs go wrong’ based on procedural errors.

1. The actual procedure is wrong in the assumptions of technique: In the lab I did, I had to mix hydrogen peroxide (H2O2–used as a disinfectant and possibly for bleaching) with Iron (III) nitrate (Fe(NO3)3–don’t try to drink it, that is all I know) as a catalyst for decomposition. So what does that mean? Fe(NO3)3 should only help H2O2 decompose–it should NOT change or do crazy things. But when you initially mix the Fe(NO3)3 with the H2O2, an immediate cloudy dark brown solution forms for couple minutes–that means the Fe(NO3)3 turned into **something else** temporarily. What it is, I don’t know but for a temporary period of time, Fe(NO3)3 was NOT a catalyst–this could affect the results as heat could have escaped or been absorbed by that temporary reaction and change the heat you measure from H2O2 decomposing. You are assuming that only heat change is coming from H2O2 NOT Fe(NO3)3.  (For people who want to say according to Hess’ law that the overall reaction heat equals the sum of the reactions to get the same reaction heat: well, yeah, but there are bound to be oopsies in that too–especially if Fe(NO3)3 permanently changed.)

2. The method in calculating your final results is flawed even if the procedure is ‘sound’: So let us say that the procedure is fine–then we should take the change in temperature, the specific heat and the amount of H2O2 used and start finding out how much heat was released (q=mst and then more stuff). That is fine–but if you try to compare it to the ‘generic’ or ‘textbook’ heat released from H2O2, that heat was measured at 25 degrees C and 1 atm (pressure). The actual pressure and temperature in the lab are NOT 25 degrees C and 1 atm so you are comparing it to a sample that is not on the same standards and that can make your ‘lab go wrong.’

3. The chemistry law you are using might be actually true–in ideal settings: So in theory, PV=nRT is true. The previous point oversimplified how to calculate change in heat by neglecting temperature of formation or the pressure of the settings which the reaction occurs in. But PV=nRT follows properly experimentally determined trends but you can never hit the *exact* amount because either your gas is not ideal (probably) or the conditions you are working in aren’t ideal (but you can get awfully close). So when you get a percent error with PV=nRT, it may not be you or your calculations; it can just be that the gas you are working with is NOT an ideal gas. It happens. Maybe another gas another day. =) And I still love you, PV=nRT. May you save other people from detention like you did for me.

4. The procedure isn’t nuts about precision: I may be being a little hypocritical in this point because I personally don’t care about my decimal places or making sure the compound is completely dry or something–but that doesn’t mean the procedure can be. For example, the lab procedure for measuring temperature said that the temperature should be measured to the tenths (one decimal place) but if you do that, it will appear that for 7 minutes, your temperature didn’t change but it really did. Ex: My results: 29.0, 29.0, 29.0, 29.0 (degrees C). What it could have been: 29.08, 29.06, 29.04, 29.01 (degrees C). There is a HUGE difference. In the former there is no change. In the latter there is an actual decrease. But that isn’t my fault–the procedure said only one decimal place. And when I want to do a linear relationship, in one there is no change and the other there is and that can make a WORLD of a difference when I do more calculations later on. Blast you procedure!

Enough rambling today. I need to read for history and chemistry. Aaahhh yummy yummy.


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