Here is another pedagogical booby-trap: "anode on the left, cathode on the right". The trouble is that, in the real world, roughly half the time it’s the other way round.

I find that the way to handle this is to regard the convention not as a statement but as a question. For example, we are accustomed to write equations such as 2H₂O ® 2H₂ + O₂, even though that is not the spontaneous reaction under standard conditions.

You simply have to write down a particular direction of reaction before you can answer the question, "does it go?" Given the statement of the reaction as a question, you know how to calculate the free energy change (product value minus reactant value). For each direction of reaction, there is a definite sign of free energy change. It tells you whether the reaction is spontaneous. In the above example, the answer (for standard conditions) is No.

Similarly, the cell convention allows you to associate a certain polarity to a cell written in a given way. Note that this also implies a corresponding direction of the current, a way to calculate the conventional cell potential (right side minus left side), and a direction of the cell reaction.

So now it’s quite simple: does everything go that way? Answer any of the polarity questions yes or no, and you’ve also answered the question, "does the cell go in the conventional direction?" and "does the reaction go in the conventional direction?".

The cell potential, determined by the two electrode potentials, answers all of these questions. If the anode is on the left, then the left-hand electrode is the more negative. This means that the potential of the right hand electrode, minus the potential of the left hand electrode, will come out positive. If so, all goes in the conventional direction. If not, all goes the other way.

In the example it’s clear that –0.28 minus –0.34 comes out negative. So the answer to all the questions posed on the diagram is "No", and the spontaneous cell reaction is the opposite of the conventional cell
reaction. It actually goes this way: Co + Cu²⁺ ® Co²⁺ + Cu. Clicking on the question marks reveals the answers to all of the other questions. They are all the same - in this case, No.

The diagram shown poses all of the polarity questions simultaneously for the reaction (i.e. the cell) chosen. You can set up any number of cells. You can show that, in every case, all of the answers are the same (yes or no), depending on whether the conventional cell potential is positive; that is, whether the "cathode" potential is more positive (or less negative) then the "anode" potential.

This SIR allows you to show how the cell convention is simply a convenient procedure for finding out the direction of the cell reaction. It gives the same answer as you would get by simply saying that the spontaneous cell reaction is the one predicted by simple electric principles - electrons naturally flow from the more negative to the more positive electrode.

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