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Name:  eileen a otoole
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: 1999 


Question:
Lemon electrolyte experiment - how does this light a light bulb?



Replies:
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:
                              <--    2+
             cathode:     Zn   -->  Zn   + 2e

                      +       <--
             anode: 2H  + 2e   --> H
                                    2
                                    

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.

-prof topper


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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