How to Find the Chemical Equation of a Lemon Battery

In a lemon battery, oxidation takes place at the zinc anode, while reduction takes place at the copper cathode.

A lemon battery is a fun experiment you can do at home. It's simple and will teach you some basic chemistry. If you've already made a lemon battery (or are planning to make one), it's a good idea to figure out what kind of reaction is taking place first. Knowing the chemistry involved will help you understand what you see.

Instructions

1. Remember that a lemon cell includes an electrode made from zinc (Zn), an electrode made from copper (Cu) and the citric acid electrolyte. Since the citric acid donates hydrogen ions to the water in the lemon, you can represent the hydrogen ions present in this solution as H+.

2. Notice that in an ordinary battery with zinc and copper electrodes, zinc would give up electrons while copper would gain them, because copper has a higher redox potential (a measure of an element's tendency to gain electrons in an electrochemical reaction). Consequently, the zinc electrode is the anode, because oxidation takes place there, while the copper electrode is the cathode. Here's a hint: to help you remember what anodes and cathodes do, just notice that both anode and oxidation start with vowels, while cathode and reduction start with consonants.

3. Notice also that the lemon battery is a little different from an ordinary zinc-copper battery because there are no copper ions in solution. Something else besides copper is being reduced at the copper electrode. If you've ever done an experiment with zinc and hydrochloric acid, you'll know that when you place zinc in an acidic solution, it generates bubbles of hydrogen gas, because the zinc reduces hydrogen ions in the solution. The same kind of thing is happening with your lemon battery.

4. Use what you know so far to figure out what must be happening. Electrons are flowing from the zinc metal to the copper electrode, creating a negative charge. Hydrogen ions are attracted by the negative charge and take up electrons at the copper electrode to form hydrogen gas. Meanwhile, as zinc metal atoms lose electrons, they become positively charged zinc ions and drift away into the solution inside the lemon. Negatively charged citrate ions flow through the lemon juice towards the zinc electrode. Each atom of zinc loses two electrons, and it takes two electrons plus two hydrogen ions to make a molecule of hydrogen gas, so this is an easy equation to balance.

5. Write out the chemical equation based on what you've figured out. It should look like the following:

   Zn + 2 H+ ---> H2 + Zn+2

   Notice that although you write this as a single equation, the two steps (zinc losing electrons and hydrogen gaining them) are happening at different electrodes. Since the electrons flow through the wire on their way from zinc to hydrogen, you can use that flow of electrons to do work like lighting a bulb.

Tips & Warnings

• Based on the equation you wrote, you now know your lemon battery will release some hydrogen gas. It won't create very much, of course, and this whole reaction is quite safe. Nonetheless, just to be cautious, you should keep your lemon battery away from high heat and open flame.

References

• Journal of Chemical Education: Observations on Lemon Cells
• Mad Science: Food Batteries