Describe the transfer of charge during electrolysis to include: (a) the movement of electrons in the external circuit (b) the loss or gain of electrons at the electrodes (c) the movement of ions in the electrolyte

Electrochemistry – Electrolysis

Objective

Describe the transfer of charge during electrolysis, covering:

  1. Movement of electrons in the external circuit

  2. Loss or gain of electrons at the electrodes

  3. Movement of ions in the electrolyte

1. Movement of electrons in the external circuit ⚡️

Think of the external circuit like a river that carries water (electrons) from the negative terminal (source) to the positive terminal (load). The electrons flow through the wire because the electric field pushes them, just as gravity pulls water downstream.

In electrolysis, the power supply (battery or DC source) forces electrons to move:

  • From the cathode (negative electrode) into the external circuit.

  • Through the wire, then back to the anode (positive electrode).

Because electrons are tiny, we can’t see them, but their flow is what powers the chemical changes.

2. Loss or gain of electrons at the electrodes 🧪

At each electrode, electrons either leave (oxidation) or arrive (reduction). This is where the real chemistry happens.

  1. Oxidation (anode): Electrons are lost by species in the electrolyte.

    \$\text{M}^{n+} + n\,e^- \rightarrow \text{M} \quad \text{(reduction)}\$

    \$\text{M} \rightarrow \text{M}^{n+} + n\,e^- \quad \text{(oxidation)}\$

  2. Reduction (cathode): Electrons are gained by species in the electrolyte.

    \$\text{M}^{n+} + n\,e^- \rightarrow \text{M}\$

Example: In a copper electrolysis cell with CuSO₄ solution, at the cathode we have:

\$\text{Cu}^{2+} + 2\,e^- \rightarrow \text{Cu (solid)}\$

At the anode (if it's a copper electrode) we have:

\$\text{Cu (solid)} \rightarrow \text{Cu}^{2+} + 2\,e^-\$

3. Movement of ions in the electrolyte 🧫

The electrolyte is like a traffic lane for ions. When electrons are added or removed at the electrodes, ions move to maintain charge balance.

  • When electrons are gained at the cathode, positive ions (cations) move toward it to supply the charge.

  • When electrons are lost at the anode, negative ions (anions) move toward it to replace the lost electrons.

In the copper cell example, the Cu²⁺ ions drift toward the cathode, while SO₄²⁻ anions drift toward the anode to keep the solution electrically neutral.

Key Points Summary

ElectrodeReactionElectron Flow
Cathode (−)\$\text{Cu}^{2+} + 2\,e^- \rightarrow \text{Cu}\$Electrons enter the electrode from the external circuit.
Anode (+)\$\text{Cu} \rightarrow \text{Cu}^{2+} + 2\,e^-\$Electrons leave the electrode to the external circuit.

Exam Tip Box 📚

When answering questions about electrolysis:

  1. Identify the cathode and anode based on the type of electrode (metal, inert).

  2. Write the half‑reactions and show electron transfer.

  3. Explain how ions move in the electrolyte to maintain charge balance.

  4. Use the analogy of a river or traffic lane to illustrate electron and ion flow.

Remember: The external circuit always carries electrons from the anode to the cathode.