Personally, I find section 9.2 one of the hardest parts of the course to teach. It relates to voltaic cells. I can never remember is it left minus right or vice versa, which side is positive, which is negative? In order to help teach this I put together a useful set of ideas / principles that I now use to help my teaching. It does involve some learning / memorization of facts but once these points are learnt, everything else should become logical and allow students to achieve correct answers in the exams.
So, here we go, this is how I do it:
I always start with a reaction that I (and the students are familiar with) – a straight displacement reaction. If zinc is put into copper sulfate it reacts (to produce copper and zinc sulfate). This is because zinc is more reactive than copper. In this reaction, zinc is oxidized from Zn to Zn2+ + 2e–.
Copper, on the other hand is reduced from Cu2+ to Cu by gaining electrons.
Apply this to an electrochemical cell and the same process occurs with electrons moving from the zinc half-cell to the copper half cell.
The next bits need to be learnt in terms of standard convention. The half cell with the more negative value (the zinc) needs to be put on the left. This is the negative side or the anode. The more positive value (the copper) is on the right (and the cathode or positive side). The actual half cell values can be looked up in the data book (zinc = −0.76v and copper +0.34v).
Electrons will move from negative to positive or anode to cathode.
Once you have this idea in your mind, everything else should make sense and be deduced, not learnt.
This zinc / copper cell will generate electricity so Ecell will be positive. This can be calculated by ‘right – left’ or ‘reduction – oxidation’ or ‘ more positive half cell – more negative half cell’. These are three important ways of figuring the cell potential.
The great thing about this model is that it can be applied to unusual situations with unfamiliar half cell reactions. It doesn’t matter what is used in the cell, the above principles can be applied to achieve correct answers.
How do you approach this subject? How do you teach it? I’d love to hear your ideas so please feel free to post them below.