6.2 — Chemical Cell

Simple Chemical Cell

• A simple chemical cell consists of:
  • two different metals immersed in an electrolyte;
  • an external circuit connecting the metals with connecting wires.
• Chemical changes in the cell produce electric current.
• Energy conversion: chemical energy → electrical energy.
• The positions of the two metals in the electrochemical series determine their terminals:
  • The more electropositive metal donates electrons more readily and becomes the negative terminal.
  • The less electropositive metal becomes the positive terminal.
• Electrons flow through the external circuit from the negative terminal to the positive terminal.

Magnesium–Copper Chemical Cell

• Components:
  • Magnesium electrode.
  • Copper electrode.
  • Copper(II) sulphate solution, $\mathrm{CuS}{\mathrm{O}}_4$, as the electrolyte.
  • Voltmeter and connecting wires in the external circuit.
• Magnesium is above copper in the electrochemical series and is more electropositive.

At magnesium—the negative terminal

1. Magnesium donates electrons more readily than copper.
2. Magnesium atoms form magnesium ions and dissolve in the copper(II) sulphate solution.
3. The released electrons flow through the external circuit from magnesium to copper.

At copper—the positive terminal

1. Electrons from magnesium reach the copper electrode.
2. Copper(II) ions receive the electrons instead of hydrogen ions because copper(II) ions are less electropositive than hydrogen ions.
3. Solid copper forms and is deposited on the copper strip.

Energy and electron-flow summary

• Chemical reactions release electrons at magnesium.
• The movement of electrons through the external circuit produces electric current.
• The voltmeter detects the potential difference.
• Overall conversion: chemical energy → electrical energy.

Activity 6.2 — Building a Simple Chemical Cell

• Aim: Build a simple chemical cell.
• Materials: sandpaper, two magnesium ribbons, two copper strips and $1.0\mathrm{mol}\mathrm{d}{\mathrm{m}}^{-3}$ sodium chloride solution, $\mathrm{NaCl}$.
• Apparatus: measuring cylinder, beaker, connecting wires with crocodile clips and voltmeter.

1. Clean two magnesium ribbons and two copper strips with sandpaper.
2. Measure and pour $150\mathrm{c}{\mathrm{m}}^3$ of sodium chloride solution into a clean beaker.
3. Immerse one magnesium ribbon and one copper strip in the solution.
4. Connect both metals to the voltmeter with connecting wires.
5. Turn on the switch; observe and record the voltmeter reading.
6. Repeat using a pair of magnesium ribbons, then a pair of copper strips.

Pair of metals	Voltmeter result and inference
Magnesium–copper	Reading is detected; two different metals produce electrical energy.
Magnesium–magnesium	No reading; identical metals do not form a simple chemical cell.
Copper–copper	No reading; identical metals do not form a simple chemical cell.

Generating Electrical Energy from Various Sources

• The chemical-cell concept can generate electrical energy using electrolytes from:
  • fruits;
  • other plant parts, including vegetables;
  • seawater.
• A functional cell requires:
  • two different metals as electrodes;
  • a source containing an electrolyte;
  • connecting wires forming an external circuit.
• Chemical reactions involving the two different metals and the electrolyte convert chemical energy into electrical energy.

Activity 6.3 — STEM chemical-cell project

1. Work in groups to apply the chemical-cell concept to different sources.
2. Plan and build a simple chemical cell using fruits, other plant parts or seawater.
3. Use two different metals as electrodes and connect them through an external circuit.
4. Demonstrate that the design converts chemical energy into electrical energy.
5. Present the simple chemical-cell design.