I watched this video in which the host tries to show the principles behind the voltaic pile.

The host first immersed zinc in vinegar and bubbles are observed forming around zinc. I've learned that this is because hydrogen ions in vinegar get electrons from zinc atoms and become gaseous.

Then the host take a conductor and attach at one end the piece of zinc and at the other a piece of copper. Bubbles start to form around copper too! (fig1)

  • Why does this happen?
  • Why do some electrons travel through the conductor instead of being attracted immediately by the hydrogen ions in the vinegar?
  • What if the conductor instead was very long and the pieces of copper and zinc far apart? (fig2)

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  • $\begingroup$ Can't understand downvotes without explanation! $\endgroup$ – Gabriele Scarlatti Dec 2 '17 at 10:51
  • $\begingroup$ You should be so lucky as to have explanations. No I did not -1 but when I get a -1 no explanation is par for the course. $\endgroup$ – Oscar Lanzi Dec 3 '17 at 23:08

When you put a zinc rod in a acid solution containing $\ce{H^+}$ ions, the zinc piece loses $\ce{Zn^{2+}}$ ions to the solution leaving electrons behind. These electrons attract hydrogen atoms to the zinc rod, and an electron transfer occurs, resulting in bubbles of hydrogen forming on the surface of the zinc. However, the evolution of gaseous hydrogen from zinc surface is rather slow, and that is why pure zinc reacts slowly with dilute acids.

The situation changes when a copper,or any other metal less electropositive than hydrogen is attached to zinc, by a wire. The copper can now be considered as an extension of the zinc rod: a place form where hydrogen can take electrons quickly and become gas. As the rate of evolution of hydrogen from the copper piece is faster, hydrogen bubbles out from the copper piece. This does not mean, however, that the zinc stops reacting with acid; it does, but at a very slow rate.

A common mistake is to think that the copper rod participates in the electrochemical reaction taking place. The copper is merely a pathway to release electrons to H+ ions in the solution. The only reaction taking place is

$\ce{Zn + 2H^+ -> Zn^{2+} +H2\uparrow}$

Copper has no place in the reaction.

So, to answer your questions,

Why does this happen?

This happens because copper can do away with the electrons quicker than zinc, and when electrons get a pathway, they will take the pathway.

Why do some electrons travel through the conductor instead of being attracted immediately by the hydrogen ions in the vinegar?

Well, some electrons do attach to H+ ions from vinegar directly, and hydrogen bubbles should form on the zinc plates, although they are less in amount. And I believe my earlier explanation answers this question.

What if the conductor instead was very long and the pieces of copper and zinc far apart?

It does not have anything to do with distance. If you take a large solution, the reaction will certainly occur. On the other hand, if you separate the poles into two solutions the copper rod will not give off electrons as the circuit is not complete. Think about it this way, if the reaction was to occur, the solution with zinc will become progressively positive and the solution with copper will become negative. This can never happen.


(1) As per your comment below, I now attempt to answer the question "What makes copper better than zinc in releasing electrons?"

The answer lies in a factor called overpotential, sometimes referred to as overvoltage. There is one type of overpotential which is important in this case— activation overpotential. This is caused due to the fact that when hydrogen atoms are produced on the surfce of a metal ($\ce{H+ +e-> [H]}$), they take some time before they can combine to form diatomic hydrogen and some more time before the hydrogen molecules can be released as gas. This results in the formation of a layer of hydrogen on the surface of metal effectively reducing the rate of reaction and halting it; this effect is known as polarization.

As you can see from the data chart in the Wikipedia page, Zn electrode has -0.77 V overpotential for evolution of hydrogen while there is no overpotential for copper electrode. This means that copper is not polarized easily.

(2) I mentioned in the beginning of my answer that pure zinc reacts with dilute acid slowly. Commercial zinc, on the other hand, reacts quickly with acids, owing of the same effect of polarization. In commercial zinc, impurities of other metals form cathode and the pure zinc forms anode in the acid medium, essentially producing a galvanic cell, where the anode and cathode are connected. Thus the reaction occurs quickly, without the hindrance of polarization.

(3) You should read this paper : http://pubs.acs.org/doi/abs/10.1021/ed078p516. It has some facts which you may find interesting.

  • $\begingroup$ Hi! This is a perfect answer, it just need to clear me up on some details and it will be accepted. I did not understand why electrons prefer to travel and reach copper, what makes copper better than zinc so that electrons prefer to move to copper even though they can stay in the zinc and still react there. Why the majority of electrons mek this choice? $\endgroup$ – Gabriele Scarlatti Dec 5 '17 at 8:53
  • $\begingroup$ Moreover, so if we take a very big pot of vinegar and a very long and thin wire (kms) the reaction still happen, electrons still prefer to travel for kms to reach copper than react in the zinc? Instead if we use 2 different glasses, shouldn't it happen (the reaction) for a while until everything is electrically balanced and then stop? $\endgroup$ – Gabriele Scarlatti Dec 5 '17 at 8:55
  • $\begingroup$ @GabrieleScarlatti , I will edit the answer. Please wait for some days. I have not had the time to write a detailed explanation. $\endgroup$ – Shoubhik Raj Maiti Dec 6 '17 at 3:25
  • $\begingroup$ @GabrieleScarlatti , How will the reaction happen for a while until everything is electrically balanced. As I mentioned in my answer, if the reaction were to happen for two different solutions, one solution will become positive while the other solution becomes negative. Take an analogy: If you have two objects of different temperatures in contact, what would happen? Should more heat transfer from cold body to hot body, making them progressively hotter and cooler? or should the hotter body give up heat so that both attain equal temperature? $\endgroup$ – Shoubhik Raj Maiti Dec 6 '17 at 4:15
  • $\begingroup$ @GabrieleScarlatti , Edited post to include the answers to your questions $\endgroup$ – Shoubhik Raj Maiti Dec 6 '17 at 5:20

Copper is cathodic when the galvanic circuit is formed. In the acid solution hydrogen ions are reduced at the cathode.

The zinc anode is more complicated. The anodic reaction is zinc oxidation to ions which dissolve. At the same time there is hydrogen evolution, the usual chemical reaction. The chemical reaction and the anodic reaction are competing for the available zinc.

To set up the galvanic cell and make hydrogen evolve from both ends you have to connect the electrodes through the electrolyte (or via a salt bath). Figure 2 does not have a complete circuit, so it gives no reaction except zinc dissolving chemically.

  • $\begingroup$ Hi! Thank you for your help! Btw it's still not clear to me, why electrons decide to travel through the conductor from zinc to copper instead of beaing attracted to the ions immediately, dissolving only the zinc(fig1),and why fig2 is not a complete circuit? What makes it different form circuit 1? $\endgroup$ – Gabriele Scarlatti Dec 4 '17 at 9:40

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