How to teach electrolysis at 14–16

The applications of electrolysis are everywhere – from the tin covering of a tin can to the shiny chrome of our kitchen taps. Even helping development of sustainable fuels like hydrogen power. Although the products of electrolysis are in our everyday lives, the content and concepts for a 14–16 year-old are tricky and demanding.

Teaching this topic involves several abstract concepts: ions, particle theory, redox and energy, all sprinkled with new terminology. This presents a challenge and an opportunity for chemistry teachers to help students improve their understanding in this area.

What students need to know

• Electrolysis is the breaking down of an ionic compound with electricity. An electrolyte is the liquid that is electrolysed. This can be either a solution or a molten compound where the ions are free to move.
• An electrode is a solid that conducts electricity into and out of the electrolyte.
• During electrolysis, the cathode is the negative electrode, and the anode is the positive electrode.
• When ions reach an electrode, they are discharged; they lose or gain electrons to become a neutral species.
• At this level, only elements form in electrolysis: metals or hydrogen at the cathode and oxygen or halogens at the anode.

What you need to know

Most post-16 specifications do not look at electrolysis, a process that uses electrical energy, but instead focus on electrochemical reactions to produce electrical energy. This is an important distinction, and one to make clear to your A-level students. It also presents a nomenclature challenge because in electrolysis we say oxidation occurs at the positive anode. However, when we teach electrochemical cells, we say oxidation occurs at the negative anode. Many people find it difficult to remember this tricky distinction. The polarity of the anode differs between electrolysis and electrochemical cells though the process of oxidation always happens at the anode. It is important that we appreciate the connection and the difference between electrolysis and electrochemical cells. 

You can use the electrochemical series to explain why hydrogen forms instead of sodium in the electrolysis of aqueous sodium chloride. This is also an opportunity to explain reactivity in terms of how easily ions accept or lose electrons.

Suggestions for your teaching

In its simplest terms, electrolysis – if you look at its etymology – means breaking down with electricity. This could be a way to introduce the topic: ask students to predict which elements form when zinc chloride is electrolysed.

By attempting to electrolyse solid sodium chloride and then adding water, you can illustrate quite nicely that ions need to be free to move. Using questions and answers will lead students to understand molten compounds as the second way ions are free to move.

To help students appreciate the role of electrons, use active learning. Make one wall of your teaching room a cathode and one the anode, then assign some students to be positive ions and some to be negative ions. Use Post-it notes as electrons. The cathode and negative ions would have lots of Post-it notes on them becasue the negative ions carry electrons and the cathode receives electrons. Then, with some theatre, declare the circuit on. The positive ion students go to the cathode and gain electrons (Post-it notes), and the negative ion students go to the anode and lose their Post-it notes. You, as the teacher, can move Post-it notes from the anode to the cathode to illustrate the need for a power supply. When it comes to writing the half equations, refer to the students who were anions and what happened to their electrons.