In a galvanic (voltaic) cell, the anode is taken into consideration negative and also the cathode is thought about positive. This seems reasonable as the anode is the resource of electrons and also cathode is wright here the electrons circulation.

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However before, in an electrolytic cell, the anode is taken to be positive while the cathode is now negative. However, the reaction is still equivalent, by which electrons from the anode circulation to the positive terminal of the battery, and also electrons from the battery circulation to the cathode.

So why does the sign of the cathode and anode switch once considering an electrolytic cell?

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edited Jan 22 "17 at 9:55

Melanie Shebel♦
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The anode is the electrode wbelow the oxidation reaction

eginalign ceRed -> Ox + e-endalign

takes area while the cathode is the electrode wbelow the reduction reaction

eginalign ceOx + e- -> Redendalign

takes location. That"s how cathode and anode are characterized.

Galvanic cell

Now, in a galvanic cell the reactivity proceeds without an exterior potential helping it alengthy. Since at the anode you have actually the oxidation reactivity which produces electrons you get a build-up of negative charge in the course of the reactivity until electrochemical equilibrium is got to. Therefore the anode is negative.

At the cathode, on the other hand, you have the reduction reactivity which consumes electrons (leaving behind positive (metal) ions at the electrode) and also therefore leads to a build-up of positive charge in the course of the reactivity till electrochemical equilibrium is got to. Therefore the cathode is positive.

Electrolytic cell

In an electrolytic cell, you use an external potential to enpressure the reactivity to go in the opposite direction. Now the thinking is reversed. At the negative electrode wbelow you have developed a high electron potential via an outside voltage source electrons are "puburned out" of the electrode, thereby reducing the oxidized species $ceOx$, bereason the electron energy level inside the electrode (Fermi Level) is better than the power level of the LUMO of $ceOx$ and the electrons have the right to reduced their power by occupying this orbital - you have very reenergetic electrons so to speak. So the negative electrode will be the one where the reduction reactivity will take location and hence it"s the cathode.

At the positive electrode where you have produced a low electron potential via an outside voltage resource electrons are "sucked into" the electrode leaving behind the the lessened species $ceRed$ bereason the electron energy level inside the electrode (Fermi Level) is reduced than the energy level of the HOMO of $ceRed$. So the positive electrode will be the one wbelow the oxidation reactivity will take place and also hence it"s the anode.

A tale of electrons and also waterfalls

Because there is some confusion concerning the ethics on which an electrolysis works, I"ll attempt a metaphor to explain it. Electrons circulation from an area of high potential to a region of low potential a lot like water drops dvery own a waterloss or flows dvery own an inclined aircraft. The factor is the same: water and also electrons deserve to lower their power this way. Now the external voltage source acts like two massive rivers connected to waterfalls: one at a high altitude that leads in the direction of a waterautumn - that would be the minus pole - and one at a low altitude that leads away from a waterloss - that would be the plus pole. The electrodes would be prefer the points of the river quickly prior to or after the waterfalls in this picture: the cathode is prefer the edge of a waterfall wright here the water drops dvery own and the anode is prefer the allude wbelow the water drops right into.

Ok, what happens at the electrolysis reaction? At the cathode, you have actually the high altitude case. So the electrons flow to the "edge of their waterfall". They desire to "fall down" because behind them the river is pushing in the direction of the edge exerting some sort of "pressure". But wbelow have the right to they loss down to? The other electrode is separated from them by the solution and also normally a diaphragm. But tright here are $ceOx$ molecules that have actually empty says that lie energetically listed below that of the electrode. Those empty says are prefer small ponds lying at a lower altitude wright here a little bit of the water from the river can autumn right into. So eexceptionally time such an $ceOx$ molecule comes near the electrode an electron takes the opportunity to jump to it and mitigate it to $ceRed$. But that does not intend that the electrode is unexpectedly missing an electron bereason the river is replacing the "puburned out" electron automatically. And the voltage resource (the resource of the river) can"t run dry of electrons because it gets its electrons from the power socket.

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Now the anode: At the anode, you have the low altitude instance. So here the river lies reduced than every little thing else. Now you have the right to imagine the HOMO-claims of the $ceRed$ molecules as tiny barrier lakes lying at a higher altitude than our river. When a $ceRed$ molecule comes close to the electrode it is like someone opening the floodentrances of the obstacle lake"s dam. The electrons flow from the HOMO right into the electrode therefore creating an $ceOx$ molecule. But the electrons do not remain in the electrode, so to stop, they are carried amethod by the river. And since the river is such a large entity (lots of water) and commonly flows into an sea, the bit "water" that is added to it does not adjust the river much. It continues to be the very same, unaltered so that everytime a floodgate gets opened up the water from the barrier lake will certainly drop the same distance.