Electrons Do Not Flow Through the Electrolyte

What students typically write

“Electrons flow from the anode through the solution to the cathode.” Or: “The current is carried by electrons moving through the electrolyte.”

This is one of the most commonly reported errors in examiner reports for the electrochemistry topic. Students apply their understanding of electrical circuits in physics (where electrons flow through wires) and extend it incorrectly to the solution.

Why it is wrong

In a metallic conductor (wire, electrode), current is carried by the movement of delocalised electrons. But in an electrolyte, there are no free electrons. The liquid contains ions, and it is these ions that carry the charge.

During electrolysis, the circuit is completed as follows:

1. Electrons flow through the external wire from the power supply to the cathode.
2. At the cathode, electrons are gained by positive ions (cations) in the solution. This is reduction.
3. Inside the electrolyte, positive ions move toward the cathode and negative ions move toward the anode. This ionic movement carries the current through the liquid.
4. At the anode, negative ions (anions) lose electrons. This is oxidation.
5. The electrons released at the anode flow through the external wire back to the power supply.

At no point do free electrons travel through the liquid. The handover happens at the electrode surfaces.

What the mark scheme actually wants

A typical question: “Explain how the electric current is carried in the external circuit and in the electrolyte.” [2]

Marking points:

1. Electrons flow through the external circuit / wires (1 mark)
2. Ions move through the electrolyte / ions carry the current in the solution (1 mark)

An answer that says “electrons flow through the electrolyte” directly contradicts marking point 2 and scores 0 for that part.

Worked example: wrong vs right

Question: During the electrolysis of molten lead(II) bromide, describe how charge is transferred through the circuit. [3]

Wrong answer: “Electrons flow from the battery through the wire to the cathode, then through the molten liquid to the anode, and back to the battery.” This scores 1/3 at most (for the wire part). The claim that electrons pass through the molten liquid is wrong.

Right answer: “Electrons flow through the external wires and electrodes (1). In the molten electrolyte, Pb2+ ions move toward the cathode and Br- ions move toward the anode (1). At the cathode, Pb2+ ions gain electrons (reduction); at the anode, Br- ions lose electrons (oxidation) (1).” This scores 3/3. Each part of the circuit is described with the correct charge carrier.

How to avoid this mistake

Draw a simple circuit diagram and label the charge carrier in each section:

• Wires and electrodes: electrons
• Electrolyte: ions

At each electrode, there is a handover. Electrons arrive at the cathode and are picked up by cations. Electrons are released by anions at the anode and leave through the wire. The electrolyte is the gap in the middle where only ions move.

If your answer mentions electrons inside the solution, cross it out and replace with “ions move through the electrolyte”.

For the full treatment of electrolysis including half-equations, see electrolysis and electrolysis of aqueous solutions.