Question by annie: Do cations move to the cathode and anions to the anode in all electrochemical cells or just electrolytic?
I know that oxidation occurs at the anode and reduction at the cathode for all types of electrochemical cells, but I’m not sure about the migration of cations and anions?
Best answer:
Answer by TLV ×
A widespread misconception is that anode polarity is always positive (+). This is often incorrectly inferred from the correct fact that in all electrochemical devices negatively charged anions move towards the anode (hence their name) and/or positively charged cations move away from it. In fact anode polarity depends on the device type, and sometimes even in which mode it operates, as per the above electric current direction-based universal definition. Consequently, as can be seen from the following examples, in a device which consumes power the anode is positive, and in a device which provides power the anode is negative
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Hmmmm??? Where are the “following examples”? It is also customary to cite a long passage of text lifted directly from a web page. TLV did not cite the paragraph she copied. This is called plagiarism.
Plagiarism and missing examples aside, there is an easy way to visualize what goes on in aqueous redox reactions.
Two mnemonics I give my students:
1) OILRIG
Oxidation Is Loss, Reduction Is Gain
2) A red cat ate an ox.
RED CAT = REDuction occurs at the CAThode
AN OX = OXidation occurs at the ANode
Regardless of whether the cell is voltaic or electrolytic, the above statements are true. But there are a lot of misconceptions about what takes place in a cell. For one thing, the metal electrodes are not charged. They only have a polarity relative to each other, not to the system at large.
Consider a copper/zinc voltaic cell, with copper metal and copper sulfate, and zinc metal and zinc sulfate, and a potassium sulfate salt bridge. In the zinc half-cell, zinc metal, can be oxidized and lose electrons, which can move through an external circuit, while zinc atoms go to zinc ions in solution. Sulfate ions move from the salt bridge into the zinc half-cell to maintain charge parity (charge balance).
In the copper half-cell copper ions come into contact with the copper electrode. Since the copper electrode is in contact with the zinc electrode, electrons can be transferred through the metal conductors to the copper ions and they will be reduced at the copper electrode and plate out as copper metal. A zinc atom at the zinc electrode will only give up two electrons when a copper ion collides with copper metal and takes two electrons and plates out as copper metal. The electrons lost by the zinc are not the actual electrons gained by the copper, but charge has moved, nonetheless.
The bottom line is that they whole system stays neutral. Charge moves in order to maintain that neutrality. Electrons move from zinc to copper to balance the charge as copper ions are reduced, and zinc metal is oxidized to zinc ions. Sulfate ions move across the salt bridge to balance the charge. The whole system works by maintaining charge neutrality.
The positive and negative business is simply a convention that we apply to the measurement of the difference in potential energy between the two half-cells. That is what pushes the whole process along.
One of the misconceptions is that the copper electrode has a negative charge and “attracts” the copper ions. Not true. The copper ions simple collide with the copper electrode because the copper electrode is big and there are lots of copper ions. Without the zinc metal being attached to the copper metal, there would be little chance that copper ions would be reduced, but because the zinc can be easily oxidized, the copper ions pick up the electrons from the copper electrode and the whole process runs like clockwork until something runs out.