Monday, December 8, 2014

Building Batteries: Electrochemistry in the Fall Term!

Electrochemistry in the Fall!

My students loved building batteries.
Every teenager in my class has an electronic device that is powered by a battery.  You would have difficulty finding a more relevant chemistry topic to the typical teenager!  This year I decided to move electrochemistry to the fall term because it is the perfect follow up to the Six Types of Chemical Reactions Unit (ChemEd Workshop). With the Chemical Reactions unit still fresh in their minds, I presented electrochemistry as a “spotlight” on the single displacement reaction.  This two-week unit was the perfect opportunity to introduce the activity series of metals, oxidation and reduction, and standard reduction potentials in an engaging and relevant series of hands-on activities.

The activity series of metals was the bridge between the single displacement reaction and the galvanic cell.  The students conducted a micro scale study of the reactivity of common metals.   (Flinn ChemTopic Labs:  Oxidation and Reduction We set up a series of reactions between metals and salt solutions, with hydrochloric acid included, to create an activity. The working knowledge of the relative reactivity of these common metals was the backbone of the galvanic cells that they will construct in the lab later in the week. 

The magnesium-copper cell in the Hot Dog Clock demonstration  (Flinn ChemFax Publication No. 91335)     was the first galvanic cell they observed.   This cell is easy to construct “real time” and provides enough voltage to replace the AA battery in a clock, I used my chemistry clock, of course. I have the students write the half reactions on their white boards during the demonstration, identifying them as oxidation and reduction.  I reinforced the concept that electrons flow from the more active to less active metal, which is a direct application of their observations of the activity series of metals (both magnesium and copper were included in the activity series lab).  At the end of this demonstration the students have gained experience with the construction and chemistry of a simple galvanic cell.

Success!  Light is on!!!

Armed with a basic knowledge of how a battery works, I then gave the kids a design challenge with very few instructions.  I provided them with all the parts of the classic Daniel Cell:  copper and zinc electrodes, 1 M solutions of copper sulfate and zinc sulfate, and a 1 M solution of sodium nitrate for the salt bridge.  They used 24-well plates to construct a single Daniel cell, the salt bridge is a small piece of a cotton swab, alligator clips, and a multimeter.  Their goal was to construct a battery that will generate enough voltage to light a 2.6 V LED.  The students used a voltmeter to record the voltage of the cells as they construct them, and to monitor the voltage when more cells were added to the system.  This fun activity was complete when they took a photo of the lit LED connected to their battery.  The students really enjoyed this hands-on activity because they don’t know how many cells were required and they had to tinker with the system to get it to work.  You could hear cheers from lab teams when they got the LED to go on. A minimum of three cells in series was required to light the LED.  This year, I had one group of boys who took this challenge to the next level by constructing a battery that produced 10 volts!
10 Volts!

The final piece of this fun electrochemistry unit was another micro scale lab to explore the voltage of galvanic cells constructed from other metals. (Flinn reference here) We used silver, aluminum, iron, magnesium, copper, and zinc to create a galvanic cell “snow flake”.  The students constructed the snowflake out of filter paper, with a different metal electrode in each point.  The students soaked the tip of the filter paper by each electrode with a matching metal solution.  They soaked the center of the snowflake with sodium nitrate.  Using a voltmeter, they measured the cell potential for all the possible combinations.  The students used the sign of the voltage in each combination to determine the anode and cathode for each cell.  They learn how to calculate the theoretical cell potential of a standard cell for each galvanic cell they tested.  The desired outcome of this experiment is to get the students to make the connection between the standard reduction potential of each metal and the voltage obtained in a galvanic cell.

Exploring galvanic cells with my AP Chem students.
It was a lucky accident that my AP Chemistry class was studying electrochemistry at about the same   Most of my AP Students completed this electrochemistry unit in the spring last year, so they only needed a refresher course to get them up to speed on galvanic cells.  For this advanced group, I set out a series of metal electrodes and solutions and instructed them to construct galvanic cells and study their cell potential.  Each group took a different approach to these opne-ended instructions.  One groups decided to create series and parallel circuits using the same type of cell.  Another group decided to try as many combinations they could construct form the materials.  And a third group made four different cells, then connected them all in series to see how much loss was in the system.  All of the AP students experimented enough with these supplies to allow for a deeper understanding of the electrochemical circuit and the chemistry happening in each cell.
time as my Honors class.

Trying a range of electrodes in series.
The final piece of the electrochemistry exploration this fall was “electrolysis day”.  Sounds a little strange, but I wanted to emphasize the difference between an electrolytic cell and a galvanic cell.  I made an attempt to conduct a silver-plating reactions.  This one needs more work for next fall!  And I did an electrolysis demonstration of tin(II) chloride, which forms both tin and the tin(IV) ion, with a beautiful display of silvery crystals.  Then I finished off with the classis electrolysis demonstration with my Hoffman apparatus   All of these demos need some polishing before I do them again (no pun intended), it was a bit of a rush job to get them together for class.  We ended electrolysis day with some calculations of how much metal is plated if a current is passed through a solutions for a specified amount of time. 

Electrochemistry is an engaging and relevant topic that is a perfect fit for any high school chemistry class.  Moving this unit to the fall term was a successful experiment with positive outcomes for me and for the students.  I enjoyed watching the students dive deeper into chemical reactions.  My students enjoyed constructing batteries “from scratch” and harnessing the power of a chemical reaction.

Here are some examples of my student's lab reports from the battery labs.  These are creative google presentations that include student pictures from the labs and their results.

Lea and Alex's Battery Lab Report

 Khia and Aaron's Battery Lab Report

Rebecca and Josh's Battery Lab Report

Thursday, November 6, 2014

Plant Pigments and pH

I revisited a really fun experiment this fall during my introduction to acids and bases unit.   I tried it out with the library summer science program over the summer.  The community members enjoyed the experiment, and the results were exciting, so I decided to give it a try with my students.  Guest Post from Library Science Program

Our first look at the fascinating world of natural pigments was the classic experiment with red cabbage juice to test the pH of household substances.  The colors from the cabbage juice are just fantastic, red, pink, purple, blue, green, and yellow!  You can make your own cabbage juice at home by simple boiling some chopped up red cabbage in water.  The dark purple liquid is like gold when it comes to universal pH indicators.

During the next lab period we extracted pigments from colored fruits and vegetables.  (By the way, I usually make a big batch of cabbage juice, so this is their first experience with the extraction part of the experiment.)  It's pretty simple to do:  chop the fruit, boil it in just enough water to cover the fruit, and collect the colored liquid.  I brought in grapes, apples, cranberries, beets, red onion, tomato, red pepper, and other colored plants.  Each group extracted a different pigment, and then they shared with each other so the kids could test several different pigments.  The kids tested their pH sensitivity with a range of buffers solutions of pH from 2 to 12.  The results were absolutely wonderful!  The plants gave a beautiful range of colors in the buffer solutions, including pink, blue, green, yellow and red.  None of the pigments were as wonderful as the red cabbage, but we found a couple close seconds.  Here are some of the photos my students collected from the day.  You can look at one of the lab presentations from a group for this lab at this link:  Chase-Donze Pigments Lab

Extracting pigments from beets is very easy!

These boys are boiling their cranberries in water.

Plum pigment gave similar colors to grapes.

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.

Friday, September 26, 2014

Classification of Matter "Sandwich"

Watching the Iron and Sulfur reaction.
Classification of matter (COM) is usually the first "real" chemistry topic we tackle in a new year.  This  year I decided to make a "Lab Sandwich" with my classification of matter unit.   I also like to think of this unit as a pyramid, a swimming reference that seems to fit the symmetry of the unit.

To open the unit, I gave my students a mixture of iron, salt, sand, and pebbles to separate.  The lab groups made a flow chart with a detailed plan for obtaining pure samples of each of the four components.  As an extra challenge, I asked them to collect enough data to calculate the mass percent of the parts of the mixture.  I love starting the kids off with an inquiry lab that they really can't mess up.  This particular mixture demonstrates several different physical properties and gives them a chance to learn some basic lab techniques.  This lab is loosely based on Separation of a Mixture found in Flinn ChemTopic Labs:  Volume 1, Introduction to Chemistry.
The pure samples of iron, rocks, salt, and sand (green and brown sand).

Students working through the "Classification of Matter" POGIL
The meat in my COM sandwich is a POGIL that makes the transition from macroscopic observations of matter to the molecular level of matter.  I love this POGIL because the students get an introduction to chemical formulas and a hint of nomenclature along with a particulate perspective on elements, compounds, and mixtures.  This great activity is found in POGIL, Activities for High School Chemistry by Laura Trout.
Sorting the "samples" into elements, compounds, and mixtures.

Watching the iron-sulfur reaction.
The final layer, the icing on the cake you could say, is the famous Iron and Sulfur lab.  The Iron Sulfur Lab This lab is a classic, and we consider it a rite of passage for our chemistry students at Pomfret School.  In this lab, the students study the mixture of iron and sulfur and then they react the mixture to form the compound iron sulfide.  This is also an exciting lab because it's the first time the kids get to use the Bunsen burners.  The comparison of the physical and chemical properties of the new compound and the mixture of the elements is a wonderful culmination to the COM unit.
My labs are all over instagram!

The smell is all part of the experience.