Transfer of electrons at a distance

Transfer of electrons at a distance is the experiment that turns redox from a colour change in a test tube into a measurable flow of electrons. In the Form 5 Redox Equilibrium chapter it is the bridge between simple displacement and the voltaic cell, because it separates the oxidising and reducing agents and forces their electrons to travel through a wire. Follow the set-up, then use the Paper 3 section to lock in the direction-of-flow answers.

Aim

To show that a redox reaction can take place when the oxidising agent and the reducing agent are not in direct contact, with electrons transferred through an external circuit, and to identify where oxidation and reduction occur.

Apparatus and materials

• U⁻tube (or a beaker fitted with a porous partition)
• Two carbon (graphite) electrodes
• Connecting wires with crocodile clips
• Galvanometer or microammeter
• Dropper and beakers
• Potassium iodide solution (the reducing agent)
• Acidified potassium manganate(VII) solution or bromine water (the oxidising agent)
• Dilute sulfuric acid
• Safety goggles

Procedure

1. Half fill the U⁻tube with dilute sulfuric acid to act as the connecting electrolyte.
2. Using a dropper, carefully run potassium iodide solution down one arm so that it forms a layer above the acid without mixing.
3. In the same careful way, add the acidified potassium manganate(VII) solution to the other arm.
4. Dip a clean carbon electrode into each arm, one into the potassium iodide and one into the oxidising agent.
5. Connect the two electrodes to the galvanometer with the wires and crocodile clips.
6. Observe the galvanometer needle, and watch for colour changes around each electrode over several minutes.
7. Record the direction of the deflection and the colour changes at each electrode.

Expected observations

• The galvanometer needle deflects, showing that an electric current flows through the external circuit.
• Around the carbon electrode in the potassium iodide solution, the colourless solution slowly turns brown, showing that iodine has formed.
• Around the electrode in the oxidising agent, the purple colour of the potassium manganate(VII) fades (or, if bromine water is used, the brown colour fades).

Describe the deflection and the colour changes you observe; do not invent a current reading or any other measured value.

Inference and conclusion

Electron transfer takes place at a distance through the external wire. At the electrode in the potassium iodide, iodide ions release electrons and are oxidised to iodine, which colours the solution brown; this electrode is the negative terminal. The electrons flow through the wire to the other electrode, where the oxidising agent gains them and is reduced, so its colour fades. The dilute sulfuric acid completes the circuit by allowing ions to move. Because oxidation happens at one electrode and reduction at the other, connected only by a wire and an electrolyte, the experiment proves that a redox reaction is a transfer of electrons and can occur without the reactants touching. Electrons always flow from the reducing agent to the oxidising agent.

Science process skills (Paper 3 style)

• Hypothesis. “When a reducing agent and an oxidising agent are connected by an electrolyte and an external wire, electrons flow through the wire and the galvanometer deflects.” State a testable prediction.
• Variables. The manipulated variable is whether the oxidising agent is present (or its type); the responding variable is the galvanometer deflection together with the colour changes; the controlled variables are the concentration and volume of each solution, the type and size of the electrodes, and the electrolyte used.
• Making an inference. From the brown colour forming in the potassium iodide arm, infer that iodide has been oxidised and that this electrode is the negative terminal.
• Stating direction. Electrons flow through the wire from the potassium iodide electrode to the oxidising-agent electrode, while conventional current flows the opposite way.
• Operational definition. “A reducing agent is a substance that donates electrons, shown here by the electrode where iodine forms and from which electrons leave through the wire.”

Safety precautions

• Wear safety goggles, because dilute sulfuric acid and acidified potassium manganate(VII) are irritant and corrosive to the eyes.
• If bromine water is used, work in a fume cupboard, because bromine vapour is toxic.
• Add the solutions to the U⁻tube slowly and without shaking, so that they do not mix directly and splash.
• Do not let the two carbon electrodes touch, so that the circuit is not short-circuited.

Common errors

• Letting the oxidising and reducing solutions mix directly, so the reaction happens in the tube and no current flows through the wire.
• Allowing the electrodes to touch or the clips to slip, which breaks or short-circuits the circuit and stops the deflection.
• Using a manganate(VII) solution that has not been acidified, so the expected reduction and colour fade do not occur.
• Connecting the galvanometer with poor contacts, so a real deflection is missed and wrongly read as no reaction.
• Confusing the direction of electron flow with the direction of conventional current when writing the conclusion.

How we help

In our online one-to-one SPM Chemistry lessons, taught in English, our teachers use this experiment to fix the single sentence students most often get wrong, that electrons flow from the reducing agent to the oxidising agent, and to link each electrode to its half-equation. Fees begin at RM50 per hour, and a paid one-hour trial lesson lets you see the explanation before deciding. Because this set-up is the direct ancestor of the voltaic cell you meet next in SPM Chemistry, understanding it now makes the electrochemistry that follows much easier to reason through in Paper 2 and Paper 3.