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?
In: 33
Think of it as what work is being done and who’s doing it. Both are electrochemical cells but what’s happening underneath – chemical energy to electrical versus the other way around ( electrical work to perform a chemical reaction) matters defines them better.
Anode and cathode definitions are not who holds what charge but where the oxidation and reduction reactions are taking place.
In the galvanic case chemical reaction occurs spontaneously while in the electrolytic case it is work done against a potential.
I think of it in terms of anode and cathode define the direction electrons are currently flowing (regardless of which direction is thermodynamically favored) while the charges define the direction that electron flow would be spontaneous or TDF (based on the relative reduction potentials of the two half cells). In essence, the charges are defined based on what two half cells are put together, and electrons are flowing in the spontaneous direction if they are flowing from the negative to the positive electrode.
However, I define the direction I want electrons to flow. If I want them to flow from negative to positive (negative electrode is the anode and positive is the cathode), then I’ve made a galvanic cell and the electrons will flow the desired direction all on their own. However, if I want the elections to flow from positive to negative (positive is the anode and negative is the cathode), then I’ve made an electrolytic cell and I’ll have to hook up my cell to an outside power source to force the electrons to flow the direction I want them to.
In galvanic cells, such as batteries, the cathode is the positive terminal, and the anode is the negative terminal. This is because galvanic cells generate electricity through a spontaneous chemical reaction. During this reaction, electrons flow from the anode (negative terminal) to the cathode (positive terminal), creating an electric current.
On the other hand, in electrolytic cells, an external source of electrical energy is used to drive a non-spontaneous chemical reaction. The cathode in an electrolytic cell is connected to the negative terminal of the power supply, and the anode is connected to the positive terminal. The external electrical energy forces the non-spontaneous reaction to occur, and as a result, electrons flow from the power supply to the cathode (negative terminal) and from the anode (positive terminal) back to the power supply.
So, the polarity of the electrodes in galvanic and electrolytic cells is determined by the direction of electron flow. In galvanic cells, the cathode is positive because electrons flow from the anode to the cathode. In electrolytic cells, the cathode is negative because electrons flow from the power supply to the cathode.
Long reply but hopefully worth your time!
In a galvanic cell, the current of electrons is spontaneous since the reduction potential is positive and favored. What happens is that the anode rod, suppose it’s zinc, it will literally break down (the anode becomes smaller) through zinc + 2 e- –> Zn2+ which is oxidation.
The anode is losing its electrons and now has an electron ‘deficit’ compared to when it started. The cathode on the other hand is gaining to those electrons so it’s being reduced. Anode is negative, cathode is positive.
The key here is understanding what positive and negative mean. So let’s backtrack a bit. When you have an element, for example, Chloride (cl), it needs 1 more electron to become like a noble gas so it’s ‘missing one’ so it’s charge is Cl- which is negative (its an anion – anode anion should ring a bell). Positive charge means something has extra (plus) so it needs to get rid of them, like Ca2+.
In a galvanic cell, the charge is based off that elemental basics. The anode loses electrons so it’s negative and it feels like it’s ‘missing’ something. The cathode has gained more electrons so is now positively charged.
Now why is that different in an electrolytic cell? That’s because that cell is non-spontaneous. If a galvanic cell is ‘discharging’ then a electrolytic cell is ‘charging’. The spontaneous reaction is the set-point of what decides whats the anode and what’s the cathode since what’s non-spontaneous is unnatural
So to connect what I described so far, we use the spontaneous way to label things. When you discharge something, it goes from losing its electrons (at the anode) to provide the electric current to the cathode. The anode is oxidized since it lost its electrons and is now negatively charged, the cathode is positively charged since it now has an excess of those electrons. The anode and cathode are in fixed positions since they describe a certain metal in the cell.
To recharge something, we will need to return the electrons to the anode, so the cathode will now oxidize to provide back the electrons to the anode. Oxidation requires loss of electrons, so the cathode then becomes negative, while the anode becomes positive since it gained electrons.
(Optional) – why does zinc for example, is what will be the anode, while copper is likely to be a cathode in the galvanic cell to begin with? That has to do with standard reduction potential (E) that describes the extent of the reactivity of something to gain electrons (be reduced) as compared to hydrogen which is 0V. Zinc is more likely to oxidize since it has a negative E. While copper is more likely to be reduced. So if those 2 happen to meet, they would use this compatibility to both benefit
In a galvanic cell you are connecting two metals with a wire and letting them react with each other, producing electricity. The amount of electricity *produced* depends on which two metals you connect.
In an electrolytic cell you are APPLYING electricity to *force* the reaction to run in the opposite direction. The amount of electricity *required* depends on which two metals you connect.
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
Latest Answers