In a galvanic (voltaic) cell, the anode is considered an adverse and the cathode is thought about positive. This seems reasonable as the anode is the resource of electrons and cathode is whereby the electron flow.

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However, in an electrolytic cell, the anode is taken to be confident while the cathode is now negative. However, the reaction is still similar, through which electrons indigenous the anode circulation to the hopeful terminal the the battery, and also electrons indigenous the battery circulation to the cathode.

So why go the sign of the cathode and anode switch as soon as considering an electrolytic cell?


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edited january 22 "17 at 9:55
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The anode is the electrode where the oxidation reaction

eginalign ceRed -> Ox + e-endalign

takes ar while the cathode is the electrode where the palliation reaction

eginalign ceOx + e- -> Redendalign

takes place. That"s exactly how cathode and anode room defined.

Galvanic cell

Now, in a galvanic cell the reaction proceeds there is no an outside potential helping it along. Because at the anode you have the oxidation reaction i beg your pardon produces electrons you acquire a buildup of negative charge in the food of the reaction until electrochemical equilibrium is reached. For this reason the anode is negative.

At the cathode, on the other hand, you have the reduction reaction which consumes electrons (leaving behind positive (metal) ions at the electrode) and thus leads to a build-up of confident charge in the food of the reaction till electrochemical equilibrium is reached. Hence the cathode is positive.

Electrolytic cell

In an electrolytic cell, you use an exterior potential come enforce the reaction to walk in the contrary direction. Currently the thinking is reversed. At the an unfavorable electrode where you have created a high electron potential via an exterior voltage resource electrons space "pushed out" that the electrode, thereby reducing the oxidized species $ceOx$, due to the fact that the electron power level inside the electrode (Fermi Level) is greater than the power level that the LUMO that $ceOx$ and also the electrons deserve to lower their power by occupying this orbit - girlfriend have an extremely reactive electron so to speak. Therefore the an unfavorable electrode will be the one whereby the palliation reaction will take place and thus it"s the cathode.

At the optimistic electrode where you have produced a short electron potential via an external voltage source electrons room "sucked into" the electrode leaving behind the the reduced species $ceRed$ since the electron energy level inside the electrode (Fermi Level) is lower than the energy level that the HOMO the $ceRed$. So the positive electrode will certainly be the one whereby the oxidation reaction will certainly take place and thus it"s the anode.

A tale of electrons and also waterfalls

Since over there is part confusion worrying the values on i beg your pardon an electrolysis works, I"ll shot a metaphor to define it. Electrons flow from a an ar of high potential to a an ar of short potential lot like water drops down a waterfall or flows under an lean plane. The factor is the same: water and electrons have the right to lower their power this way. Currently the external voltage source acts choose two big rivers connected to waterfalls: one in ~ a high altitude the leads towards a waterfall - that would certainly be the minus pole - and also one at a low altitude that leads far from a waterfall - that would be the to add pole. The electrodes would certainly be favor the clues of the flow shortly prior to or ~ the waterfall in this picture: the cathode is like the leaf of a waterfall whereby the water fall down and also the anode is choose the point where the water fall into.

Ok, what happens at the electrolysis reaction? in ~ the cathode, you have actually the high altitude situation. So the electrons circulation to the "edge of your waterfall". They desire to "fall down" since behind castle the river is pushing in the direction of the edge exerting some sort of "pressure". However where deserve to they fall down to? The other electrode is separated native them by the solution and usually a diaphragm. But there room $ceOx$ molecule that have empty says that lie energetically listed below that that the electrode. Those empty states are like little ponds lying in ~ a lower altitude where a small bit of the water from the flow can loss into. For this reason every time together an $ceOx$ molecule comes close to the electrode one electron takes the opportunity to run to it and also reduce it to $ceRed$. However that does not mean that the electrode is suddenly absent an electron due to the fact that the flow is replacing the "pushed out" electron immediately. And the voltage source (the resource of the river) can"t operation dry that electrons because it it s okay its electrons from the strength socket.

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Now the anode: in ~ the anode, you have the short altitude situation. So below the river lies lower than whatever else. Now you can imagine the HOMO-states that the $ceRed$ molecule as tiny barrier lakes lying at a greater altitude 보다 our river. Once a $ceRed$ molecule comes close to the electrode it is choose someone opening the floodgates of the barrier lake"s dam. The electrons circulation from the HOMO right into the electrode thus creating an $ceOx$ molecule. But the electron don"t continue to be in the electrode, so to speak, castle are carried away through the river. And since the river is such a large entity (lots that water) and usually flows into an ocean, the tiny "water" that is included to it doesn"t adjust the river much. It continues to be the same, unaltered so the everytime a floodgate gets opened the water indigenous the obstacle lake will drop the exact same distance.