Why do electrons move away from the electrode in a galvanic half-cell?

[Slava]

In a standard galvanic half-cell when Zinc rode is placed into the Zinc sulphate. What is the reaction which makes zinc to dissolve and fell into the solution. Why do electrons move away from the sides of the electrode? Are they repelled by the solution or by other ZN atoms on the electrode which haven't been ionized yet?

 

[Suraj M]

the reaction $$Zn ⇔ Zn ^{2+} + 2e^- $$
is in equilibrium..
when the electrode is connected to to the Cu electrode which is also at a similar equilibrium state, the difference in reduction/oxidation potential causes a shift in equilibrium, as Zn has a lower Reduction potential compared to Cu, and hence gets oxidised, giving electrons which are conducted by the external circuit .

 

[James Pelezo]

See my lecture posted at...
https://www.physicsforums.com/threads/galvanic-cells-and-salt-bridges.812134/

 

[Kekule]

One of the biggest hurdles I've helped students overcome in electrochemistry, is understanding how the electrons actually move. A lot of textbooks and teachers portray that the "electrons" move through a wire connecting the cathode and anode. Students then get this visual of a single electron making its journey from one end of the wire to the other. However, this is not the case.

When Cu and Zn electrodes are attached by a wire, there is a potential difference. One way to think about this is that Copper actually as a higher affinity for electrons than zinc does, so it "pulls" electrons in its direction. However, a single electron does not travel the length of the wire. Instead, a series of electrons "hop" along the wire from atom to atom in the wire. So two electrons from a Zinc atom in the Zn electrode "jump" to a neighboring Zinc atom, and in the process displace another two electrons which "jump" to another Zinc atom, and so forth. Eventually, 2 electrons "jump" from a zinc atom on the electrode to an atom in the wire, which therefore displaces another 2 electrons which "jump" to another atom in the wire. Electrons continually make these "jumps" across the wire until some electrons "jump" from the wire to the Cu electrode. It is here that they undoubtedly find a Cu²⁺ anion to combine with and form another atom of Cu on the surface of the Cu electrode.

So the most important thing here is to note that the exact electron(s) which leave a Zinc atom on the Zn electrode, are not the same exact electron(s) which find themselves attaching to Cu²⁺ anions on the other side. Instead what you have is a bunch of consecutive electron "hops" which looking from the outside appear as a continuous flow of electrons from Zn to Cu.

One way to think about it is having a straw and filling it completely full with beads. Now try to pass a bead trough the straw from one side to the other. When a bead goes in the one side, another bead will pop out of the opposite side. Beads (electrons) were "flowing" through the straw (wire), but the bead that entered the straw is not the same bead that came out of the other side.

 

[Borek]

While in general there is a truth behind what you wrote (I like the straw with beads model) this part:

Instead, a series of electrons "hop" along the wire from atom to atom in the wire.

is quite wrong and instead of helping produces new misconceptions. These electrons are not combined with any particular atom, they are in the conduction band and are free to move wherever they want. They don't jump between atoms - and there is no need to produce such an artificial (and wrong) concept to explain the phenomena. I would just say something between the lines of "electrons in the wire shift in a direction", without getting into misleading details.

 

[Kekule]

While in general there is a truth behind what you wrote (I like the straw with beads model) this part:
is quite wrong and instead of helping produces new misconceptions. These electrons are not combined with any particular atom, they are in the conduction band and are free to move wherever they want. They don't jump between atoms - and there is no need to produce such an artificial (and wrong) concept to explain the phenomena. I would just say something between the lines of "electrons in the wire shift in a direction", without getting into misleading details.

It is clear from my beads and straw model that I know the electrons are not combined with any particular atom. Of course the electrons are free to move among the molecular orbitals in the metallic bonding array. However, electrons are still physical pieces of matter that contain mass and have volume. AND the electrons, while not "attached" or combined to any particular metal atom, still belong to an array of metal atoms. Simply by definition, the atoms must contain the appropriate amount of electrons in order to be neutral. As one electron moves into the array of atoms, another electron must pop out. This can be viewed as a shift of electrons in a certain direction. Perhaps "hop" and "jump" aren't the best words since they convey a different meaning to you, but the principle is still the same.

 

[Borek]

I think you have missed one important thing - this is discussed in the context of the Cu → Cu²⁺ + 2e^- reaction. IMHO you risk your students will assume that "hopping electrons" is exactly this reaction happening in the chain fashion, with atoms becoming ions for a very short moment. It is just a matter of selecting better wording to avoid the risk.