Wednesday, January 14, 2015

Blow the Lid Off of Magnesium Oxide Lab

Magnesium reacting with oxygen in a red-hot crucible
Photo by Julia Paneyko
Every chemistry student makes magnesium oxide in the lab in their introductory course.  My students do it twice in my lab program:  once in the study of chemical reactions (types of chemical reactions lab) and then again in the mole unit.  High school chemistry students around the world are determining the chemical formula of magnesium oxide in this classic experiment.  I use this experiment as my first "mole concept application".

The procedure is very simple:  heat magnesium in a crucible until it is fully reacted with oxygen.  Use the mass data of the magnesium before reaction and product after reaction to calculate the mole ratio of Mg and O in the compound.  This mole ratio is the experimental chemical formula of magnesium oxide.  My "go to" procedure for this lab is found in ChemTopic Labs Volume 7:  Molar Relationships & Stoichiometry by Flinn.  In Flinn's version of the lab, there are detailed instructions to heat the magnesium with the lid on, then open the lid every three minutes, and then heat it some more with the lid at a tilt.  All of this manipulation of the crucible lid invariably results in at least one dropped and broken lid each lab period.  And, the results of the lab are usually not that great, with many groups getting ratios other than the expected 1:1.

My students are heating their crucibles with lids off.
This year I decided to try the lab with no lid on the crucible during the heating process.  It seems only obvious to let the maximum amount of air into the reaction by just leaving it open for the whole reaction.  I was a little nervous that burning magnesium might pop out of the crucible, but that didn't happen all day.  The hardest part of the experiment was adjusting the bunsen burner so that it would get hot enough to start the reaction.  Most of the reactions burned at a steady rate, without a big flare up of bright light.  Once the reactions were completed without any safety issues, my students crunched the numbers.

Here's where the big payoff was obvious.  The lab results were much better this year, with most groups getting the expected 1:1 ratio without the need for sad stories about experimental errors in the conclusion.  Taking the lid off resulted in better results, more interesting observations during the reaction, and better results for the chemical formula for magnesium oxide.

Wednesday, January 7, 2015

Mole Art

My new mole is making friends in class.
As a new teacher I thought that I would never be like my quirky science teacher colleagues, but after nearly twenty years of teaching chemistry, I have developed a weakness for “Mole Art”.  I love the element of fun that mole art brings to my teaching environment.  My students get a little glimpse into my soul when I bring in a new hand-made mole to decorate my classroom. 

Milli Mole
It started about seven years ago when I discovered a stuffed mole pattern in ChemTopic Labs Volume 7:  Molar Relationships & Stoichiometry by Flinn.  The pattern is very easy and quick to make.  I made three immediately: a custom designed mole for every chemistry teacher at my school.  The department chair got a mole in school colors with a “P” on the side for Pomfret.  My other colleague got a mole made out of her college school colors, adorned with flowers so everyone could tell it was a “she-mole”.  I named my first mole Milli, and I even gave her a rabbit fur hat (made out of fur that fell off of our rabbit pelt when I was demonstrating the polarity of water).  I manage to find reasons to have my mole in class throughout the year. I bring her out on the first day that I introduce the mole, and then again when my students learn molarity. 
The image of a one molar solution will always stay fresh in their minds when then think of Milli sitting in a one-liter beaker.  But wait, I can double the concentration by adding one of her mole friends to the beaker.  The site of the two moles crammed into a one-liter beaker always makes me laugh out loud.  My students quickly realize that not only do I like to make things by hand, but I also enjoy bad jokes! 
A one molar solution!

My new Mole Doorstop, the latest addition to the mole art collection.
My latest mole creation is a wonderful mole doorstop, which was inspired by a great book called Faux Taxidermy Knits by Louise Walker.  This fun book is a must read for every knitter.  The mole doorstop was very fun to make; and it knit up in just one day!  When it is finished, the mole is emerging from a mound of dirt as if to say hello to everyone.  The base is filled with rocks to weigh it down so that it will function as a doorstop.  I made the new mole just in time for the start of the mole unit.  On the first day of the mole concept, I split my class into small groups to do the POGIL activity called “Relative Mass and the Mole”.  (POGIL Activities for High School Chemistry by Laura Trout)  One of my students came up to the front of the room and brought the mole back to his table for inspiration.  I saw him with his arm around the mole during class, as if he wanted to include the mole in the group discussion.  When the novelty wears off, I’ll put him to work as a doorstop, but for now he is located on the front table to greet my students when they come to class for more mole calculations. 

A two molar solution!

In addition to the mole doorstop and the fuzzy mole friends I have made, I also have a handsome mole mobile hanging in the room.  I call it the molebile.  Two years ago our Head of School charged us to “make your space your own” at the start of the school year.  He said, “Give your classroom some personality and make it a fun learning environment”.  I knew exactly what I had to do:  make more moles.  For this mole art piece, I used the same stuffed mole pattern from Flinn to make a set of moles in bright colors. 
The original "molebile".
I have bins of fabric scraps for just this kind of spontaneous project.  I found some antiquated glassware in our stock room to serve as the perfect vehicle for the bright moles.  I always enjoy the moment when my students finally understand the molebile, usually a few days (or weeks) after I introduce the concept. 
The second "Molebile" I made for my friend in the lab upstairs.

Mole art is just one of the ways that I try to make chemistry a fun and memorable experience for my students.  I never underestimate the value of a good sense of humor in the science classroom.

The class mole, Bernard Martin, is giving some inspiration during the mole test.

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