Tuesday, October 28, 2014

Why I Hate Grading Lab Reports

Here's a fake abstract that I wrote for my test this week.  I think I have included all the mistakes that my students make in their lab writing.

          The purpose of this lab is to measure pH.  The lab was done with a pH meter, a mortar and pestle, and some hydrochloric acid.  The change in pH was 4.3 for Mylanta Ultimate and 3.9 Mylanta Classic.  The main source of error was human error in the measurements.  This lab was successful.  I learned that when you mix an acid and base together the pH changes.

My challenge to the kids is to revise this abstract into something that is meaningful.  I'll let you know how it goes.

Monday, October 20, 2014

Gravimetric Analysis: Is it worth the wait?

Waiting for the precipitate to settle!
One of the sixteen recommended labs in the AP Chemistry curriculum is a gravimetric analysis (GA) experiment using a metal carbonate to analyze the amount of calcium in hard water.  I read over the lab handout in the kit (Gravimetric Analysis of Calcium and Hard Water- Advanced Inquiry Laboratory Kit) and decided that I needed a lab that would be quicker than the guided inquiry version.  So I went to my "go-to" lab manual (Laboratory Experiments for Advanced Placement Chemistry, Guided-Inquiry Edition - Teacher Edition), which has all the classic AP Chemistry labs in the more traditional format, with detailed procedures and data charts to fill in.   I found the companion GA lab, but this time the metal carbonate is the unknown and the calcium carbonate product is used to determine the identity of the original alkali carbonate.  This version of the GA lab seemed like a nice and quick approach to teach my students the GA technique.

Waiting and more waiting! 
So we started the lab on a Tuesday, when we meet for a single block.  Each group received a sample of an alkali carbonate (either potassium carbonate or sodium carbonate) that they would identify using the GA technique.  The first step in the experiment was to dry the solid in a crucible because the alkali carbonates are hygroscopic.  Yes, you heard me, we dried the solid first.  So on the first day we heated and massed, then heated and massed some more, and then again.  By the end of the period everyone had dried, massed, and then dissolved their unknown alkali metals.  As the bell was ringing each group added the calcium chloride solution and barely had time to notice the precipitate that formed.

These two are very proud of their white precipitate.
On day two, the kids came in to see that their white calcium carbonate precipitate had settled nicely to the bottom of the beaker.  Now they had to decant and filter their solid.  Once again, the progress was slow and frustratingly tedious.  To get good results, we had to use quantitative filter paper, which by the way, filters really slowly.  Oh, did you remember to measure the mass of the filter paper before you started?!  Even AP students forget the basic steps, which is why they need to do these experiments so they can learn how to think like a chemist.  The kids decanted off the supernatant liquid into a waste beaker and started filtering the precipitate.  Everything was going along smoothly, but just very slow!  The bell rang on day two as the students rushed to get their wet filter paper into the drying oven and get out the door for their next class.

Here are the solids drying in the oven.
Day three started with a buzz in the air as the students opened the drying oven to see their products.  The white powdery solids were thoroughly dried and ready for the final mass.  The rush to get the solids out of the oven and onto the balance may have put us into a bit of a frenzy.  That's when tragedy happened:  one of my kids dropped his filter paper on the floor.  The calcium carbonate powder scattered into a fine dust all over the floor.  The boy was devastated.  He was on the verge of tears as he swept up the solid and loaded it into a weighing boat for a last-ditch effort to get an answer to their unknown identification.  His partner was incredibly kind even though he had just lost two days of lab work on the floor.  The excitement was replaced with a hush throughout the room and a "walking on eggshells" feeling.  None of the other groups wanted to lose their product on the floor; they were carrying the filter paper around like it held precious gems.  Day three ended with all the groups huddled together crunching the numbers, trying to figure out which solid they started with on Day One.

Is it over yet?  No!  On day four, the groups turned in their results and their identification of their
An analytical balance would be better, but we got good results.
alkali metal.  Everyone got it right, except for the group who dropped the product on the floor.  The experiment certainly wasn't much quicker than the guided inquiry version in the kit.  Was it worth the time?  My answer is yes.  How can you learn to think like a chemist if you don't do chemistry?  And, the lab accident taught us all to take good care of our experiments.  We all learned an important lesson about patience and careful lab technique along with the technique of GA.

Wednesday, October 15, 2014

Nomenclature: it's all in the cards!

Using the cards and cut outs to write chemical formulas
Nomenclature is a difficult topic to teach new chemistry students because of all the rules.  Is it ionic, molecular, multivalent, polyatomic, or a common name?  Sometimes the hardest part of this whole process is teaching kids to recognize the clues that tell them which rules apply to a a compound.  Nineteen years later, I have finally found a way to teach nomenclature that really works and is fun!  I have a series of card sets that I use to teach kids how to name ionic and binary molecular compounds that makes this lesson so much easier for everyone.

I love my demo sized ion cut outs.
I start the lesson with the basic definition of ionic compounds and binary molecular compounds.  I use a video for this part; the students watch it as homework.  Identifying Ionic and Molecular Compounds The next day in class I present the kids with a set of cards.  The green cards have the names of a variety of compounds, and the purple cards have the corresponding chemical formulas.  First they sort them into to two piles, and they spread out the set of chemical formulas cards.  I ask my student to sort them into two groups:  ionic compounds and binary molecular compounds (there are only four of the molecular and 12 of the ionic compounds).  Once they have sorted the stack, I tell them to find the correct name to go with each compound.  This is pretty easy for them because all the compounds have different elements or polyatomic ions (pais).  (They have a list of common pais on the back of their periodic table for quick reference.)   Once we have all the names and compounds correctly matched, we look for patterns in the names.  I ask the kids questions like "What do all the molecular compounds have in the names that make them different from the ionics?", or "Where are the metals with roman numeral located on the periodic table?" and the classic, "How do you know that a compound contains a pai?"  The light bulbs start to go off over everyones heads as they sort their pairs into groups and begin to see patterns in the nomenclature rules.  We follow up right away with some white board practice to drive the new ideas home.

An example of a compound from the ion cards and ion cut-outs.
Next we have to tackle writing the chemical formula from the names.  This is another tricky lesson to teach because there are so many pieces of information to evaluate with each compound the kids encounter.  Once again I turn to my cards for help.  This time I start with a set of  ion cards.  I made up cards with a wide range of cations and anions in the set.  To go along with the ions, I also have a set of ion cut-outs that the students can use as a visual aid to help derive the chemical formula of ionic compounds.  I ask my students to make a stack of cations and a stack of anions.  They draw a card from each pile and put them at the top of their white board.  Next they find the shapes that represent these two ions.  Using the shapes, they determine what ratio of cations and anions is necessary for create a neutral compound.  The final step is to write the correct chemical formula.  They also practice naming the compounds as they work through the cards, just to keep that skill fresh in their minds.  I tried to include every kind of ion that they would encounter:  ammonium, multivalent, pais, and main group elements.  Once the make it through the stack, they can reshuffle and have a whole new set of compounds to write.

A heated round of the Nomenclature Card Game.
The final piece of the nomenclature puzzle is writing the chemical formula from the name.  I have one more card game for this part of the lesson.  This time I use a card game that I made up that requires the students to recognize if a name represents part of an ionic compound or a molecular compound.  The cards in the deck have one half of the name of a compound.  For example, a card might read "nitrate" or "dioxide", or "carbon".  The students get five cards in their hand.  On their turn, they flip over the top card in the deck.  Let's say that card says "sodium".  The student has to make a complete (and correct) compound name by using a card in their hand.  Sodium is the first half of an ionic compound, so the student must put down a card with the name of an anion.  You can score a point by making a complete compound name using a card in your hand.  The second point comes from writing the correct chemical formula for the compound you made.  If you get the chemical formula wrong, another player can steal this point if they can write it correctly.  I give the winner of each group a prize from the coveted prize beaker.

Cation Cards
My progression of matching cards, ions cards, and the nomenclature card game have helped to make nomenclature a lively unit in my classroom that I enjoy teaching.

Anion Cards
I have a set of acid cards too to teach the kids how to name acids.  I throw that in the mix too.  The cards are especially helpful with acids because the naming rules are so different than the other compounds the learn.
Names and Chemical Formulas Cards

The acid names and chemical formula cards.

Wednesday, October 1, 2014

What's in Your Brass?

The copper solution from the brass and nitric acid reaction.
My AP Chemistry students just finished a fun spectroscopy lab to determine the amount of copper in a sample of brass shot.  This lab is a new twist on the classic "copper cycle" lab that AP Chemistry students have been doing for years.  The transformation from the classic copper cycle lab into a guided inquiry lab is part of the revised AP Chemistry curriculum which has a strong emphasis on application and inquiry-based problem.  (Experienced AP teachers, please don't start throwing things at your computer right now.  I know how much longer it takes to do these labs.)  We used the lab kit from Flinn Scientific called "Percent Copper in Brass:  AP Chemistry Big Idea 1, Investigation 2", publication #7643.

I added the nitric acid in the fume hood.
So we started with three pieces of brass shot.  We poured concentrated nitric acid (yes, the very nasty stuff) over the brass and watched as brown mustard gas was produced.  I want to take this moment to give a shout out to Tony who bought me a fume hood over the summer!  This lab would not be possible without it.   We lingered at the fume hood as we admired the beauty of this chemical reaction.

Eventually, once we all got our fill of watching copper oxidize, my kids went about the task of preparing standard solutions for a calibration curve that will allow them to determine just how much copper was in their brass sample.  The class worked cooperatively to prepare four solutions with concentrations ranging from 0.05 to 0.40 M of copper(II) nitrate.  Beer's law tells us that there is a linear relationship between the absorbance of a solution and the concentration.  No, there were no alcoholic beverages served in this experiment!   The quantity of the copper in the reaction mixture, and thus the brass shot sample, is calculated from the equation for the line in the calibration curve.  The data for this lab was so wonderful.  We got r-squared values of 0.99 and even a 1!

The reaction between copper and nitric acid is fun to watch.
After the reaction was done (it turns out we had to leave it over night to completely react), the only thing left to do was to dilute their reaction mixtures in the adorable 100-mL volumetric flasks and measure the absorbance of the solution.  So simple!  Three days of lab work led to the conclusion that the brass is around 70% copper, and that spectroscopy really isn't that bad!
One of the calibration curves from the lab, very nice.