Chemistry & Electricity Worksheet for 15-Year-Olds — Lemon Battery, Daniell Cell & Corrosion

Instructions

Read each question carefully and answer based on the experiments you conducted with your MEL Science kits. Think about the underlying chemical principles at work. Use complete sentences for your explanations.

Section 1: Chemistry & Electricity

Part A: The Lemon Battery

1. The lemon juice plays a crucial role in allowing the battery to work. What is the scientific term for a substance like lemon juice that contains ions and can conduct electricity?
   _________________________


2. In your lemon battery, you used two different metals (e.g., magnesium/zinc and copper). Electrons flow from the more reactive metal to the less reactive metal.
   • a) The electrode where oxidation occurs (loss of electrons) is called the anode. Which metal was the anode?
   • b) The electrode where reduction occurs (gain of electrons) is called the cathode. Which metal was the cathode?


3. Explain why a single piece of copper pushed into a lemon would not generate electricity to power the LED.
   ____________________________________________________________________________________
   ____________________________________________________________________________________

Part B: The Daniel Galvanic Cell

4. A galvanic cell converts chemical energy into electrical energy through spontaneous redox reactions. The Daniel cell you built likely used zinc and copper.
   • a) At the zinc electrode (in zinc sulfate), the zinc atoms lose two electrons. Write the chemical half-reaction for this process. Is this oxidation or reduction?
     Half-Reaction: _________________________
     Process: _________________________
   
   
   • b) At the copper electrode (in copper(II) sulfate), the copper ions (Cu²⁺) in the solution gain two electrons. Write the chemical half-reaction for this process. Is this oxidation or reduction?
     Half-Reaction: _________________________
     Process: _________________________


5. Over a long period of time, what change in mass would you expect to observe in the zinc strip and the copper strip? Explain your reasoning.
   Zinc Strip: _______________________________________________________________________
   Copper Strip: _____________________________________________________________________

Section 2: The Chemistry of Corrosion

Part A: Rust Protection

6. Rusting is the common term for the corrosion of iron. It is an oxidation process. What is the primary chemical compound that makes up rust? (Give the chemical name).
   _________________________


7. In one experiment, you may have wrapped an iron nail with a magnesium strip. This protects the iron from rusting, even if scratched. This method is called "sacrificial protection." Why is magnesium able to "sacrifice" itself to protect the iron? (Hint: Think about which metal is more reactive or more easily oxidized).
   ____________________________________________________________________________________
   ____________________________________________________________________________________

Part B: Electricity vs. Iron

8. When you passed an electric current through a solution containing iron nails, you created an electrolytic cell, which is the opposite of a galvanic cell.
   • a) What is the main energy conversion that happens in an electrolytic cell?
     _________________________ energy is converted to _________________________ energy.
   
   
   • b) The indicator phenol red turns pink in basic (alkaline) conditions, while potassium hexacyanoferrate(III) turns blue in the presence of iron(II) ions (Fe²⁺). Based on your likely observations:
     At which electrode (positive or negative) did you see a blue color, indicating the iron nail was corroding (oxidizing)?
     _________________________
     At which electrode (positive or negative) did you see a pink color, indicating the formation of hydroxide ions?
     _________________________

Section 3: Connecting the Concepts

9. All the experiments in these two kits are based on a single class of chemical reactions that involve the transfer of electrons. What is the name for this type of reaction?
   _________________________


10. Compare the lemon battery and the corrosion of an unprotected iron nail. In what way is the process of rusting like a tiny, unwanted galvanic cell?
    ____________________________________________________________________________________
    ____________________________________________________________________________________
    ____________________________________________________________________________________

Answer Key

Section 1: Chemistry & Electricity

1. Electrolyte
2. • a) The more reactive metal (Magnesium or Zinc) was the anode.
   • b) The less reactive metal (Copper) was the cathode.
3. To generate an electrical current, there must be a potential difference, which requires two different metals with different tendencies to lose electrons. A single piece of metal has no potential difference with itself, so no electron flow will occur.
4. • a) Half-Reaction: Zn → Zn²⁺ + 2e^-
     Process: Oxidation
   • b) Half-Reaction: Cu²⁺ + 2e^- → Cu
     Process: Reduction
5. Zinc Strip: The mass would decrease because the zinc metal is being oxidized into zinc ions (Zn²⁺), which dissolve into the solution.
   Copper Strip: The mass would increase because copper ions (Cu²⁺) from the solution are gaining electrons and depositing as solid copper metal onto the strip.

Section 2: The Chemistry of Corrosion

6. Hydrated iron(III) oxide
7. Magnesium is more reactive than iron, meaning it is more easily oxidized (loses electrons more readily). When connected to iron, the magnesium corrodes preferentially, acting as the anode and supplying electrons to the iron, which prevents the iron from being oxidized.
8. • a) Electrical energy is converted to chemical energy.
   • b) The blue color (corrosion) would appear at the positive electrode (anode), where oxidation occurs.
     The pink color would appear at the negative electrode (cathode), where reduction of water produces hydroxide ions.

Section 3: Connecting the Concepts

9. Redox (Oxidation-Reduction) reactions
10. The process of rusting is like an unwanted galvanic cell because different areas on the surface of the iron can act as tiny anodes and cathodes. In the presence of an electrolyte (like water with dissolved salts), iron in one spot (anode) gets oxidized, releasing electrons. These electrons travel through the metal to another spot (cathode) where they reduce oxygen from the air. This flow of electrons and chemical change creates a micro-cell that results in corrosion.