Q & A: The Lemon Cell

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Most recent answer: 05/13/2008
Q:
Do you have any experiments on lemon or batteries.
- Kristine Nguyen (age 11)
Denver, Co
A:
You are in luck!!!! Here is an experiment that involves a lemon that forms part of a battery.

MATERIALS: one lemon, a copper penny, strip of zinc (you can get this from a hardware store), steel wool, a knife, and a current meter (measures the current flowing in the circuit).

TO DO: Use the steel wool to shine the surfaces of the penny and the zinc strip. File down any sharp edges on the zinc strip. Have an adult use the knife to punch two small slits into the lemon's skin. The slits should be about a 1/2 inch long and should be 1/4 inch apart.

Insert a penny into one of the slits. Insert the zinc strip into the other slit. Make sure the metals don't touch! Touch the leads of the current meter to the exposed metals. Voila!!!! The needle in the current meter moves. Can you explain why?

More information: Here's a bit of an explanation for why this battery works. The two different metals (copper and zinc) don't dissolve equally well in the acidic lemon juice. Positively charged ions (Zn+2) come off the zinc and go into solution, so long as there is a way for the negatively charged electrons left behind to flow out of the zinc metal to the copper. There they pull positive hydrogen ions (H+) out of solution. The electrons' route is provided by the external circuit, which includes the current meter. When two H+'s combine with two electrons from the copper, they form a neutral hydrogen molecule. Both pieces of metal stay nearly neutral, but negative electrons flow through the wire from one to the other and positive ions flow through the lemon from one toward the other.

As the electrical current travels through the external circuit, it makes the meter's needle deflect. It does that via magnetic forces, because the current flows through a little coil in the meter, making a magnetic field.

Something else:::: See if other metals can do the came thing. Try some aluminum for example, replacing the copper or replacing the zinc. Does it do the same thing?????? :) And try using the same metal at both ends. If our explanation is right, that shouldn't give any current.

(published on 10/22/2007)

Follow-Up #1: battery chemistry

Q:
Hi. I have two questions about the lemon battery. I understand from your explanation that the Zn electrode gives up two electrons and becomes Zn2+. These two electrons travel through the wire, through the copper electrode, and meet up with H+ ions in the lemon to form H2 gas. 1) Why doesn't the Zn simply gives its two electrons to the H+ that's near the Zn electrode (i.e. why go all the way through the copper wire)? 2) Does the Zn just give up 2 e- spontaneously or is it "attacked" by the H+? Thanks, Jessie
- Jessie Martin (age 25)
Troy, NY
A:
Nice question.

The electrons can travel very easily through the wire, so they have a chance to join up with H+'s at either electrode. However, the solution near the Zn electrode has Zn++ ions around. It's energetically favorable for the solution to stay nearly neutral electrically. So those Zn++ ions drive away H+ ions and draw in some negative ions, e.g. Cl-. That means there's much more H+ near the Cu electrode, so that's where almost all the 2H+ +2e- -> H2 happens.
I know this explanation sounds a little thrown-together. A more reliable version would be written in terms of chemical potentials of the constituents, but that might require more background.

I'm not sure how to answer your last question. If there's no place for the extra electrons to go, the electrical potential on the Zn will build up and quickly stop the release of Zn++. So the H+ or some similar ingredient is needed. Whether you then say it 'attacked' the Zn is a matter of words.

Mike W.

(published on 05/13/2008)