This aim if this blog post is to equip you with an aid memoir – something you can copy, paste, edit and print out to aid your revision. The reasoning behind this post is from my experience as an examiner. In my opinion (and this may not be one shared by the IB) students tend to answer questions on electrochemical cells quite poorly.
This is possibly due to the topic being one of the last topics to be taught – maybe your teacher has rushed it a bit as they are conscious of finishing the course and starting some revision. The other reason is that it is full of opposites – and very easy to get one of them wrong (for example, you may think the positive electrode is the negative one and so on).
When faced with an electrochemical cell that contains unfamiliar regents, try to have in your mind a reference model. Something that works and something that you have confidently learnt. Once you have this fixed model in your mind, you will easily be able to apply it to unfamiliar reagents. The model I use is the Danielle cell, a cell comprised of a half cell of copper metal in aqueous copper ions (eg, copper sulfate) and a half cell of zinc metal in aqueous zinc ions (eg, zinc sulfate).
By Gringer – File: Galvanische Zelle.png, by Tinux, CC BY-SA 3.0
Try to remember these points.
Zinc is more reactive than copper. So the zinc metal will react. This means it will dissolve into the solution to form zinc ions (this involves loss of electrons so it is oxidation).
The most reactive metal is always put on the left hand side when writing the standard cell convention and its ecell value will always be more negative than the electrode on the right (note the terminology – more negative does not actually mean negative, although it is possible that it is).
The left hand electrode is the anode and will also contain the reducing agent.
Electrons will move from the anode to the cathode.
And don’t forget that the half cells need to be joined by a slat bridge. The purpose of this is to allow the ions to move into the respective half cells to balance the charges. It certainly does not let electron move through it.
To calculate Ecell we use Right – Left OR Reduction – Oxidation OR more positive – more negative. Ensure that you know all three calculations.
The opposite arguments can be applied to copper (for example, the more positive half cell goes on the right).
How do you remember electrochemical cells? Do you have any handy tips or revision ideas that you can share with us? If you do, please post them below as I would love to hear about them.