Constructing a simple voltaic cell

Constructing a simple voltaic cell is where the Redox Equilibrium chapter pays off: the electron transfer you saw in displacement and in the “electrons at a distance” experiment now does useful work, driving a current through a wire. In Form 5 it links the electrochemical series directly to a measurable voltage, and it explains how a chemical cell turns chemical energy into electrical energy. Build the cell, compare pairs of metals, then rehearse the Paper 3 answers.

Aim

To construct a simple voltaic cell from two different metals in an electrolyte, to show that it produces electricity, and to relate the terminals and the voltage to the positions of the metals in the electrochemical series.

Apparatus and materials

• Beaker
• Strips of different metals, such as zinc, copper, magnesium and iron
• Electrolyte, such as dilute sulfuric acid or sodium chloride solution
• Voltmeter or galvanometer
• Connecting wires with crocodile clips
• Sandpaper
• Safety goggles

Procedure

1. Clean a strip of zinc and a strip of copper with sandpaper until they are shiny.
2. Pour the electrolyte into the beaker.
3. Dip both metal strips into the electrolyte, keeping them apart so they do not touch.
4. Connect the two strips to the voltmeter with the wires and crocodile clips.
5. Note the voltmeter reading, and record which metal the voltmeter shows as the negative terminal and which as the positive.
6. Repeat steps 1 to 5 with other pairs of metals, for example magnesium and copper, or zinc and iron, cleaning the strips each time.
7. Record the negative terminal and the size of the voltmeter reading for each pair in a table.

Expected observations

• The voltmeter gives a reading, showing that the cell produces electricity.
• The more reactive metal of each pair is shown as the negative terminal; for zinc and copper, zinc is negative and copper is positive.
• When the two metals are further apart in the electrochemical series, such as magnesium and copper, the voltmeter reading is larger; when they are closer together, the reading is smaller.
• Bubbles may appear at the less reactive electrode, and the more reactive electrode may slowly wear away.

Report which metal is the negative terminal and whether the reading is larger or smaller for each pair; do not invent a specific voltage value.

Inference and conclusion

A simple voltaic cell converts chemical energy into electrical energy. The more electropositive (more reactive) metal is the negative terminal, where it is oxidised and loses electrons to form ions. These electrons flow through the external wire to the less electropositive metal, the positive terminal, where reduction takes place. Because oxidation happens at one electrode and reduction at the other, the cell works by a redox reaction. The size of the voltage depends on the difference between the two metals in the electrochemical series: the greater the difference in their electropositivity, the larger the potential difference produced. This is why a magnesium-copper pair gives a larger reading than a zinc-copper pair. If the same metal is used for both electrodes, there is no difference in electropositivity and no voltage is produced.

Science process skills (Paper 3 style)

• Hypothesis. “The greater the difference in the positions of the two metals in the electrochemical series, the higher the voltage of the cell.” State a testable relationship.
• Variables. The manipulated variable is the pair of metals used (their separation in the electrochemical series); the responding variable is the voltmeter reading; the controlled variables are the type and concentration of the electrolyte, the temperature, the size of the electrodes and the distance between them.
• Tabulating data. Use columns for the metal pair, the negative terminal, and whether the reading is larger or smaller, so the pairs can be ranked.
• Making an inference. From zinc being the negative terminal against copper, infer that zinc is more electropositive than copper.
• Operational definition. “The negative terminal of a voltaic cell is the more electropositive metal, from which electrons flow out through the external wire, shown here by the voltmeter marking zinc as negative against copper.”

Safety precautions

• Wear safety goggles, because dilute sulfuric acid is corrosive and harmful if splashed into the eyes.
• Keep the two metal strips apart, so the cell is not short-circuited.
• Handle the sandpaper carefully and away from others, so grit does not enter the eyes.
• Dispose of the electrolyte as instructed rather than down the sink, and wash your hands afterwards.

Common errors

• Letting the two metal strips touch, which short-circuits the cell so the voltmeter reads nothing.
• Not cleaning the metals, so an oxide layer lowers or hides the reading.
• Using the same metal for both electrodes and expecting a voltage; two identical metals give no potential difference.
• Connecting the voltmeter the wrong way round and misreading which terminal is negative.
• Comparing pairs with different electrolytes or concentrations, which makes the voltage comparison unfair.

How we help

In our online one-to-one SPM Chemistry lessons, taught in English, our teachers connect the voltaic cell back to the electrochemical series, so you can predict the negative terminal and the relative voltage for any pair of metals and write the electrode reactions with confidence. Fees start from RM50 per hour, with a paid one-hour trial lesson so you can see the reasoning taught before you commit. Because the voltaic cell is the destination that displacement, electron transfer and the electrochemical series all lead to in SPM Chemistry, mastering it ties the whole Redox Equilibrium chapter together for Paper 2 and Paper 3.