March 3, 2015

Homework Help: Electrochemistry

Posted by Sienna on Sunday, January 14, 2007 at 10:44am.

Hi, I'm in my final year of high school doing an electrochemistry unit and there is a concept I just dont really get. I would appreciate all helps please--

Redox reaction half reactions are:

Oxidation: Zn(s) --> Zn 2+ (aq) + 2e-
Reduction: Cu2+ (aq) + 2e- ---> Cu(s)

In terms of electrodes, Zn in the anode and Cu is the cathode.

Its in a galvinic cell, Zn releases electrons to become Zn2+, but how come the anode of the galviniv cell is negatively charged? If it releases electrons, shouldn't it be Postivively charged?

Also, in terms of porous barriers to maintain electrical neutrality, why are the negative ions (anions) migrating toways the anode and the positive ions (cations) migrate towards the cathode? Shouldn't the opposites attract?

Thank you muchly for all your help!!

OK, it is a matter of definition: Where did you get the statement "In terms of electrodes, Zn is the anode and Cu is the Cathode"? That statement is correct, but it is based on the flow of current. Forget about electrons. The world long ago in physics and chemistry defined the direction of current flow as the direction of Positive charge carriers (even though in wires they dont exist). So with this DEFINTION of current flow, the definition that the anode is the terminal where current flows in fits: Zn is the place current flows in. If current is flowing inward, then it is OK for electrons to flow the opposite way.
Forget about electrons having anything to do with current flow direction.
If current (positive) is flowing in, then Zn must have a negative charge, or in physics terms, have a lower potential.

Negative ions migrate towards the Zn++ cations, which are leaving the Zn anode. See the picture here:

Another way of saying it, although Bob Pursley was eloquent, is---
By definition, the anode is where oxidation occurs.
Zn==> Zn++ + 2e

Therefore, Zn is losing electrodes and it is the anode by definition.

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