From my understanding, a battery provides a driving force to pull electrons from the anode, thus the anode becomes positively charged.

Suppose the anode is copper, the process should be as follows:

$\ce{Cu->Cu^{2+} +2e-}$

However, the copper ions should be dissolved into the electrolyte after this process, and only neutral copper atoms are present in the electrode, so why would it be positively charged and attract anions?

  • $\begingroup$ Copper may work as an anode if and only if the cathode is made of a metal whose redox potential is higher than $0.34 V$. like metallic silver $\ce{Ag}$ in a solution of $\ce{AgNO3)$ $\endgroup$
    – Maurice
    May 4 '21 at 19:30
  • $\begingroup$ Neutral atoms turn into cations according to the half-reaction you wrote. What is confusing about it? Anions are not a part of the process at all. $\endgroup$ May 5 '21 at 9:01

I frequently get confused by the terms cathode and anode when they are used without specifying where they are being used. Electrochemists have to juggle words that are very similar.

In an active cell, the electrode dissolves and positive CAT-ions leave the AN-ode and leave it negative so it can push an electronic current (electrons go from - to +) thru an external system while the electric current flows the other way (from + to -). But in a passive cell (one that is operated upon by an external electromotive force, it's the reverse: the electrode made negative, called the CAT-hode, attracts the CAT-ions, while the electrode made positive, called the AN-ode, attracts AN-ions.

The solution is to visualize the process pictorially, without words, then apply the words carefully, like labels on a jar of chemicals. I'm going on at length to demonstrate as many of the confusing terms as I can remember.

The question to ask is "What is the first process - what is the initiating agent?" Is it something spontaneous that you are trying to explain, or is it something you are causing and have to describe it?

So your question is: why would it (the copper anode) be positively charged and attract anions? You say "battery provides a driving force to pull electrons from anode". The implication is that the cell is passive; it is operated on by an external voltage. So far, everything makes sense.

In the very next sentence, "Suppose the anode is copper...", you show copper actively (arrow -->) dissolving, which makes it negatively charged. Now copper is low in the electrochemical series, but an aggressive medium can corrode it - let's not get too specific. This makes sense too.

The "paradox" is that the (active) copper anode is now negative while in the preceding situation, the (passive) copper anode is positive. But there is no paradox - you have changed the system from passive to active. The term anode is the same, copper is the same, so your terminology is correct - in each case, separately. But the last paragraph asks 1. why positive, while implying 2. active copper dissolution. There's the problem.

I would say, blame it on the terminology; it's complex, and you have to visualize first and name it afterward, rather than name it and then call up a picture of what's going on.


Try to remember that, in the $\ce{Zn/Cu}$ cell, like in all other electrochemical devices, whatever the working type (cell or electrolysis), the following sentences may be memorized :

  1. the anode is always where oxidation happens (transforming $\ce{M}$ into $\ce{M^{z+}}$ ion), so that the cathode is always where reduction happens.

  2. the zinc electrode is always the negative pole, and the copper plate is always the positive pole. The + sign can be printed on the copper plate, and "minus" on the zinc plate

Depending on the outer connections, each plate can be anode or cathode, and oxidation and reduction can occur on either plates. But remember : Oxidation = Anode.

Some more explanations. When working in electrolysis, $\ce{Zn}$ is still the negative pole, but Zn is reduced from $\ce{Zn^{2+}}$ ion into metallic $\ce{Zn}$. So Zn is working as a cathode. Simultaneously, the copper plate is still the positive pole. But $\ce{Cu}$ is oxidized into metallic copper and copper is the anode.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.