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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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
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