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Acid and Base Electrochemistry
Name: Jamie
Status: other
Grade: other
Location: MI
Country: N/A
Date: April 2006
Question:
I am helping my daughter with a science fair
experiment. She is testing lemons and substances to power a digital
watch. She tested lemons, potatoes, strawberries, kiwi, pears,
cranberry juice, lemon juice, soapy water and baking-soda water.
She used a multimeter to measure the voltage produced by each
"battery" and then tested to see if they would each power the digital watch.
Her hypothesis was that the soapy water and baking -soda water would
not work well to power the watch because they are bases and not
acids. However the highly-saturated baking-soda-water had the
highest reading on the multimeter. (1 volt)
I have not had a chemistry class since high school, and that was
awhile ago-- so I am looking for help in assisting her in
understanding the results.
My thinking is that because both acids and bases contain electrons,
then both have the potential to create electrical energy. In fact,
should not anything made from atoms-- everything-- have the
potential to produce electrical energy? It is a matter of mobilizing
the electrons...
Because the baking-soda solution was so highly concentrated, it
produced the highest voltage.
Replies:
Jamie,
Normally, I hesitate in responding to science fair questions because
I really do think that the importance of such an activity is to
allow the student to explore an idea rather than to get to the same
answers that "experts" have. However, I also appreciate your
involvement in your daughters education, so I will give a few ideas
here to keep you moving forward.
There seems to be a growing misconception that the contents of
fruits, potatoes, and such "powers" electrical circuits. The power
(or potential energy) is really due to the metals that are stuck
into these assorted liquids. One of the metals gets oxidized while
the other gets reduced, and it is the difference in potential energy
between these two metals that provide the measured voltage. It is
the flow of electrons from the oxidized metal to the reduced metal
that provides for the measured current.
Having said that, if you are able to use the same quality metals for
all your experiments (no corrosion, shiny surface, same type of
metal), then you can start to investigate the electron carrying
capacity of the assorted liquids. You are right in thinking that it
is the electron or ion mobility (essentially, the charge carrying
capacity) of the various liquids that will be studied - and it is
now a question of which solution has (a) the most charge carriers,
and (b) the most mobile charge carriers. If we are studying liquids
of similar viscosities, then "b" is mostly negligible and we can
focus on "a". Thus, the type of solution is mostly irrelevant as it
is the density of charge carriers that become important.
However, going back to the second paragraph, since some
oxidation-reduction reactions occur in either acidic or basic media,
this may affect the kind of electrochemistry you are doing. I
suggest a quick look at a Standards Table of Reduction Potentials
(available in the appendix of most Chemistry textbooks) for a good
idea of what reactions are involved.
Hope that helped.
Greg (Roberto Gregorius)
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