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

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