Electrochemistry in the Fall!
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| 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.
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| 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!
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| 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.
Lea and Alex's Battery Lab Report
Khia and Aaron's Battery Lab Report
Rebecca and Josh's Battery Lab Report
