Lemon Battery - Chemical Reactions - Experimental Results

Experimental Results

This model of the chemical reactions makes several predictions that were examined in experiments published by Jerry Goodisman in 2001. Goodisman notes that numerous recent authors propose chemical reactions for the lemon battery that involve dissolution of the copper electrode into the electrolyte. Goodisman excludes this reaction as being inconsistent with the experiments, and notes that the correct chemistry, which involves the evolution of hydrogen at the copper electrode, has been known for many years. Most of the detailed predictions of the model apply to the battery's voltage that is measured directly by a meter; nothing else is connected to the battery. When the electrolyte was modified by adding zinc sulfate (ZnSO₄), the voltage from the cell was reduced as predicted using the Nernst equation for the model. The Nerst equation essentially says how much the voltage drops as more zinc sulfate is added. The addition of copper sulfate (CuSO₄) did not affect the voltage. This result is consistent with the fact that copper atoms from the electrode are not involved in the chemical reaction model for the cell.

When the battery is hooked up to an external circuit and a significant electrical current is flowing, the zinc electrode loses mass, as predicted by the zinc oxidation reaction above. Similarly, hydrogen gas evolves as bubbles from the copper electrode. Finally, the voltage from the cell depended upon the acidity of the electrolyte, as measured by its pH; decreasing acidity (and increasing pH) causes the voltage to fall. This effect is also predicted by the Nernst equation; the particular acid that was used (citric, hydrochloric, sulfuric, etc.) doesn't affect the voltage except through the pH value.

The Nernst equation prediction failed for strongly acid electrolytes (pH < 3.4), when the zinc electrode dissolves into the electrolyte even when the battery is not providing any current to a circuit. The two oxidation-reduction reactions listed above only occur when electrical charge can be transported through the external circuit. The additional, open-circuit reaction can be observed by the formation of bubbles at the zinc electrode under open-circuit. This effect ultimately limited the voltage of the cells to 1.0 V near room temperature at the highest levels of acidity.