Why is the cathode of a voltaic battery considered a positive terminal?

Question

I'm a little confused regarding why the cathode electrode of a voltaic battery is denoted as being a positive terminal. Does the positive denote holding a positive charge?

Here is what (I think) I know to be true:

1. The anode (where oxidation occurs) is a source of electrons and the cathode (where reduction occurs) is a sink of electrons

2. Charge moves in the direction of the electric field lines

3. The net charge of the voltaic battery system is zero

4. the electrolyte solutions that bathe the anode electrode and cathode electrode are electrically neutral

For electrons to move from the anode electrode to the cathode electrode, there is clearly some force driving them there. The only thing I can think of that mechanistically explains this is that, even in a closed circuit, there must be some sort of negative charge accumulation at the anode electrode (i.e. the anode holds, albeit small, a static negative charge).

Because the electrolyte solutions remain electrically neutral (ensured by a salt bridge), I can't see why positive charge would be present at the cathode electrode. The only thing I could understand is that the cathode electrode does not accumulate as much negative charge as the anode electrode. In such a case, it's not that the cathode electrode is positively charged...it's just that it is less negatively charged than the anode electrode. In this scenario, electrons would certainly flow to the cathode from the anode.

However, in order for this to be true, it must be the case that the electrolyte solutions actually carry a small positive charge (to counterbalance the negative charges that are held at both the anode and cathode electrodes)...which would contradict statement 4. Not sure how to reconcile this...

So, really, the reason we call a cathode in a voltaic battery a "positive terminal" is actually just because it is less negative than the anode (even thought it doesn't actually retain some net positive charge).

Is this correct?

Answer 1

There will be a slight positive charge on the cathode. In a galvanic cell, we can break apart what happens into two half reactions between the oxidized species plus an electron and the reduced species. Each of these is an equilibrium process, where we find some oxidized species and some reduced. In a zinc/copper galvanic cell, you'll see $Zn$ and $Zn^{2+}$, and also $Cu$ and $Cu^{2+}$. The concentrations will be based on the electronegativity of copper and zinc.

As for the actual number of electrons versus positively charged metals, that balance is decided by electrostatics. If the battery is electrostatically neutral, by definition if there is a region with too many electrons, there must be a region with too few (too many holes). However, this is an incredibly slight difference. The ripping off of a piece of near by scotch tape will change the total number of electrons by far more. While a galvanic cell might produce 2V, ripping off tape can easily apply a static charge of hundreds of volts.

What really matters is the difference. Whether there are holes or electrons really just depends on the precise electristatic charge at that moment. As long as there is a difference, current can flow.