Electrolysis is a topic studied in Unit 3 Area of Study 2 and is a topic that students often confuse given its similarity to the galvanic cells.
One of the key differences between an electrolytic cell and a galvanic cell is the use of a power supply. As you would know in a galvanic cell, no external power supply is needed to get the reaction going, however in an electrolytic cell an external power supply is needed. WHY?
Although both galvanic and electrolytic cell involves redox reactions, the reaction in an electrolytic cell is non-spontaneous, which means it cannot react on its own – it needs some energy to get going. The redox reaction in a galvanic cell is spontaneous and can occur without an external power supply.
Another mistake that students usually make is denoting the polarity of the anode as negative and cathode as positive. This is not the case!
In an electrolytic cell, the polarity of the electrodes is dictated by the power supply i.e. battery.
- The power supply will push electrons to the negative electrode – ions will accept the electrons, and we recognise this as a reduction reaction. Electrode here is the cathode.
- Likewise, the other electrode is where electrons will be withdrawn – loss of electrons from atoms/ions, and we recognise this as an oxidation reaction. Electrode here is the anode and will be the positive electrode.
When solving electrolysis questions, I always get my students to always think about the following:
- Oxidation always occurs at the anode
- Reduction always occurs at the cathode
- The strongest reductant will undergo oxidation at the anode
- The strongest oxidant will undergo reduction at the cathode
- Know where the strongest reductant and oxidant is on the electrochemical series
These considerations will prove valuable to you when you come to doing questions relating to the electrolysis of aqueous solutions e.g. aqueous sodium chloride, when there are multiple reactions involved, and you need to pick one reaction that occurs at each of the electrodes.
Questions for you to consider:
- Why is it that the reactants in an electrolytic cell can be placed in one container, whilst in galvanic cells, we need to separate the reactants into two half-cells?
- What is the energy transformation that occurs in an electrolytic cell and how is different to the galvanic cell?
If you struggle with electrolysis, practice is the key to success. Practice with questions from your textbook and past VCAA exam questions. The more questions you do, the more likely you will understand what is going on.
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