Galvanic Cell Construction and Notation

Electrodes

Let’s look at one more aspect of construction for galvanic cells - the electrode (wire) that sticks into the beaker. In the cell we discussed here (pictured below), the wires are simply a way to get electrons from one beaker to the other.

Our only requirements in that case are that the wire not corrode, that it not react with the solutions in the beaker and that it conducts well. 

Given those requirements, the ideal choice is platinum. Most diagrams of galvanic cells (that don’t fall into the category described below) will use platinum for the electrodes. Of course, platinum is really expensive, so there are alternatives.

A second choice would be graphite. Graphite is generally non-reactive and conducts well. Of course it is also brittle. In a high school lab, we often use copper, which although it corrodes faster than platinum is a heck of a lot cheaper.

When the electrode matters - a special category of reaction

Let’s look at another cell that uses these two half-reactions:

\(Zn \rightarrow Zn^{+2} + 2~e^{-1}\)

\(Cu^{+2} + 2~e^{-1} \rightarrow Cu\)

Before, when we needed to put all of the parts of each half-reaction into each beaker, we simply added them to the solution like this.

But, now we have a problem. Zn (without a charge) cannot be added to a solution, since it is a solid metal. The same is true for the Cu on the right side of the second half-reaction.

The solution is simple though. We simply make the electrode out of that metal. So the set-up would begin like this

and a galvanic cell made from these reactions would look like this.

So, as a general rule, if a metal (solid, uncharged, and pure) is part of the half-reaction, then make the electrode out of that metal. Otherwise, make it out of Pt (or graphite).

Convention and Galvanic Cell Notation

A word about “convention”

Often in science and mathematics, things are done “by convention.” This means that everyone agrees on an arbitrary choice to make communication easier. For instance, there is no reason that North should be at the top of maps, but virtually all maps are made that way.

When dealing with galvanic cells, the convention is to put the oxidation half-reaction on the left side and the reduction reaction on the right. Hopefully you can see that this is meaningless. If your teacher has a galvanic cell on their front demonstration table, then the reaction that is on your left is on their right. Chemistry doesn’t care.

This convention only really matters when we decide to use a short-cut to communicate about that cell, as discussed below.

Galvanic Cell Notation

• Oxidation is always listed first
• The oxidation and reduction are separated by the symbol || which represents the salt bridge
• Within each half reaction, each species is separated by a comma (,)
• States of matter are separated by a |

So, the galvanic cell at the top would be represented by the notation

\(Pt|Fe^{+2},~Fe^{+3}||MnO_4^{-1},~H^{+1},~Mn^{+2}|Pt\)

and the cell at the bottom would be noted this way:

\(Zn|Zn^{+2}||Cu^{+2}|Cu\)

Now we need to look at Electro-chemical Potential and the effect of switching reactions.