Why do Cathodes carry a positive charge and Anodes carry a negative charge in galvanic cells, but in electrolytic cells, Cathodes have a negative charge and Anodes have a positive charge? I’m a grade 12 chemistry student and I’m having trouble understanding this concept. Could someone please explain the rationale behind this apparent reversal of charges in the two types of cells?
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There’s an issue what being “positive” and “negative” or “cathode” and “anode” even MEANS, as in their definitions.
A cathode is the electrode into which electrons flow via a contact (think wire) to combine with incoming positive ions from an electrolyte. The anode is the electrode electrons leave via the contact and which attracts negative ions from the electrolyte and/or sheds positive ions (like a consumable anode, for example sacrificial zinc or Lithium metal batteries).
“Positive” charge is one that attracts negative charges (electrons or negative ions) and repels positive ions and “negative” charge attracts positive ions and repels negative charges. But this is ELECTROSTATIC charge. It affects all positive and negative charges equally. Important point, because chemistry can override electrostatics.
In a galvanic cell, the chemical reaction at the anode produces electrons spontaneously (oxidation). It generates an excess of free electrons. It wants to get rid off those electrons. A “negative” charge from the perspective of electrons. But it’s also full of positive ions, attracting negative ions in the electrolyte and/or repelling the positive ions it produces (again, a lithium battery for example). From THAT perspective it would be “positive”, but we ignore that perspective because we measure the electric potential that drives electron motion through a wire, because that’s what we use to do work. So despite being overall electrostatically neutral, we call it “negative”.
The chemical reaction at the cathode wants to accept electrons (reduction). In creates a deficit of electrons. A “positive” charge. But again, only from the perspective of electrons. It attracts positive ions in the electrolyte, like a “negative” charge. But we care about the electrons. So the cathode is positive.
In an electrolytic cell, it’s a different matter. Here we are pumping and drawing electrons by force. We apply the positive side of a power source to one electrode. We forcefully rip electrons from it. It now has a deficit of electrons. TRUE electrostatic positive charge. It will attract both negative ions and electrons equally. But because electrons are leaving via the wire (because we force them to) and negative ions are flowing back through the electrolyte, the definition dictates this is the anode. Conversely, we’re pumping electrons into the other electrode. It has an excess. TRUE negative electrostatic charge. It repels all negative charges equally. But because electrons enter it via the wire (because we force them to) and positive ions are attracted from the electrolyte, by definition it’s the cathode.
Important note, this means that in general the electrolytic cathode and galvanic anode are the same physical piece (and vice versa), it’s just that in the electrolytic cell you’re forcing the reaction to run backwards, against how it would naturally want to progress. You charge the battery, you unrust a metal.
TL;DR anode and cathode have specific definitions that don’t care about true electrostatic charges, and chemistry has the power to create them selectively
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