Name: eileen a otoole
Lemon electrolyte experiment - how does this light a
Thanks to Ron Winther's offline email to me, I think I might
be able to answer this now....here's an excerpt from Ron's note.
"I have a guess about what the "lemon electrolytes" question #571 is
about. I believe it is the same topic as that asked about in #585.
You can make a "lemon battery" with a lemon and electrodes of, e.g.,
copper and zinc. Someone asked about this some time ago; I found a
couple of articles about it in the Journal of Chemical Education.
One said you can get about 1 volt; the other said 0.6 - 0.7V .
The precise value wasn't critical for the experiment of the first
article, but was for the second.This latter article, found in the Feb
92 issue (p. 157-8) offers this explanation for the reaction:
cathode: Zn --> Zn + 2e
anode: 2H + 2e --> H
They claim the copper electrode is only involved in collecting electrons;
it could be replaced with platinum or another inert metal."
I suspect that this is right. So actually, it is the reaction of the
zinc electrode with the acid within the lemon which gives rise to
a voltage. Zinc will reduce H+ to H2, but it won't oxidize copper to Cu+...
zinc is a strong reducing agent. In fact, you could use any metal which has
a large positive reduction potential, including platinum.
I just did a little more reading on this one. In
Ebbing's text, General Chemistry (4th ed.), it is
claimed that when you stick a zinc electrode
and a copper electrode into a lemon and hook the two
electrodes up to a voltmeter, you are actually making
the same kind of battery constructed in 1836 by
John Daniell. The electricity-generating reaction would then be
Zn(s) + Cu2+ (aq) -> Zn2+ (aq) + Cu(s)
perhaps generating some H2(g) from the acid in the lemon
as a side-reaction. T
The main point, either way, is that the electrodes are
actually "generating" the electricity, and the lemons
(whoops, maked that "lemon") is just playing the role
of a semi-conducting medium so that you can have a complete
circuit and measure a voltage. In a Daniell cell, you connectr
two half-cells by a salt bridge...here the lemon plays the
role of the salt bridge.
I realize that these two explanations are NOT the same....
personally, I'd like to do the experiment myself,
substituting Pt wire for the electrodes, and see if you
still get a voltage out, which would settle the issue.
- prof topper
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Update: June 2012