[bokang]

In text book accounts of a voltaic cell, the author assumes a cycle already in motion, say with zinc oxidizing at the anode (immersed in zinc sulfate) and copper reducing at the cathode (immersed in copper sulfate), which implies a pair of electrons traveling through a wire above the two half-cells, balanced by a sulfate ion traveling through a porous partition or salt bridge in the opposite direction to complete one cycle. My question is: Where is the impetus that "kick starts" or "bootstraps" this kind of cycle into action. Once the whole cycle is in motion, each little piece of the logic seems quite reasonable, but being anthropomorphic for a moment, how do those 2 electrons, for instance, "know" that they are free to travel up into the wire? Or how does the sulfate ion on the copper sulfate side "know" it should go through the porous partition to resolve a charge imbalance (that will in a moment exist) over on the zinc sulfate side? And so on. (This is a very general question, nothing to do with a particular "problem" in a book!)

[DevaDevil]

why does a chemical reaction occur at all?

Energy - simple as that

Since the system is conducting there will be an energy gradient (called Potential) that drives the reaction. This gradient is present as soon as the electrical circuit is completed and drives the reaction.

[bokang]

Thank you for humoring me, with my offbeat question. Your response covers those parts of the cycle occurring in the zinc sulfate solution and in the copper sulfate solution (and probably in the salt bridge that completes the circuit, too, thinking again in terms of the usual text book example). However, I don't think "energy" (or "chemical reactions in general") can explain what is going on in the wire above the two half-cells. It still seems a bit spooky to me that the two electrons "know what their role is" in the overall cycle. Or should one envision only a progression of (double) negative charge traveling along the wire, not literally "those two electrons" traveling through the wire after they are freed up by a particular zinc atom that has turned into a zinc ion and entered the solution? That would make the overall mechanism seem less mysterious to me.

[DevaDevil]

Or should one envision only a progression of (double) negative charge traveling along the wire, not literally "those two electrons" traveling through the wire after they are freed up by a particular zinc atom that has turned into a zinc ion and entered the solution?

Exactly.
What will happen is that on one side 2 electons are used so the charge density at that end of the wire will be low. the entire electron cloud in the wire (remember, in metals the outer valence electrons are "delocalized") will shift slightly towards that lower density.
And of course on the other side the density will be higher, so the cloud shifts away from it.

It is not 2 electrons traveling the whole way; it's charge.