[T]

Hello,
I am confused about cathodes and anodes from reading a textbook.

"Since reduction of the tin ions, Sn2+, has occurred, the negative electrode is acting as a cathode." This is a quote from the book about electroplating of tin metal on a can.

I don't understand this because shouldn't the negative electrode always be the anode?

Also is the electrode where reduction happens always the cathode, and the electrode where reduction happens always the anode?

Thanks

[thetada]

The cathode is always negative and hence where reduction always occurs. A confusing thing about the vocabulary is that cations are positive whereas the cathode is negative. So "cat" does not denote charge but the ion that matches the electrode. Conversely, the negative anion always goes to the positive anode, where it will be oxidized.

[thetada]

Interesting, both words are from the Greek, anion derived from to go up and cation derived from to go down. Since the anode is oxidizing, the oxidation number of anions will go up. Not sure if that's helpful.

anion (n.) Look up anion at Dictionary.com
"a negatively charged ion, which moves toward the anode (q.v.) during electrolysis," 1834, proposed by the Rev. William Whewell (1794-1866), English polymath, and published by English physicist Michael Faraday, from Greek anion "(thing) going up," neuter past participle of anienai "go up," from ana "up" (see ana-) + ienai "go" (see ion). http://www.etymonline.com/index.php?term=anion

cation (n.) Look up cation at Dictionary.com
1834, from Greek kation "going down," neuter present participle of katienai "to go down," from kata "down" (see cata-) + ienai "to go" (see ion). Proposed by the Rev. William Whewell (1794-1866), English polymath, and published by English physicist Michael Faraday.http://etymonline.com/index.php?term=cation&allowed_in_frame=0

[mjc123]

The key thing to remember is that the cathode is the electrode where reduction occurs, and the anode is the electrode where oxidation occurs.
When I started working with batteries, I was confused that the cathode was the positive electrode, whereas from the electrochemistry I remembered from school and undergraduate days (long, long ago) the cathode was the negative electrode (where e.g. you deposited Cu from CuSO4 solution). It is necessary to appreciate the difference between a galvanic cell and an electrolytic cell.
In a galvanic cell (battery), a spontaneous chemical reaction generates an electric current. The occurrence of the reduction reaction at the cathode consumes electrons from the electrode, giving it a positive potential, and the oxidation releases electrons to the anode. Electrons flow through the external circuit from the anode to the cathode (and conventional current the other way).
In an electrolytic cell, the passage of an electric current causes a non-spontaneous chemical reaction. At the cathode (the electrode connected to the negative terminal of the battery) reduction occurs when electrons are pushed in at a sufficiently negative potential, and likewise oxidation occurs at the positive anode.
So in one the cathode is positive, in the other it's negative, but in both the cathode is where reduction occurs.
(A rechargeable battery cell behaves as a galvanic cell during discharge and an electrolytic cell - where the "cathode" behaves as an anode, i.e. oxidation occurs there - during recharge.)

[Borek]

Another way of remembering it is that the cathode is the electrode through which electrons get into the system (whatever it is).

[Enthalpy]

As the term was coined it related with reduction, hence the battery terminals swapped their names between charge and discharge.

Though, as a battery user for decades, I had never seen that. Even battery suppliers say in their documentation cathode for the negative terminal for all operation modes.

Good to read that someone having worked with batteries confirms that the original definition still holds.

In diodes used for electronics, the cathode is always the same terminal, irrespective of the current direction, direct or reverse (leakage, Zener, photocurrent...). The whole profession is then wrong without a single exception known to me, and conforming to the use permits to be understood there.

Knowing that, I prefer to say "negative electrode" and "positive electrode" where possible.

[T]

Thank you all very much for the responses.

I am still very confused.
 

Another way of remembering it is that the cathode is the electrode through which electrons get into the system (whatever it is).

Electrons will get into the system through oxidation, and since the cathode is the electrode through which electrons get into the system then cathode is where oxidation occurs.

However this conflicts with

The key thing to remember is that the cathode is the electrode where reduction occurs, and the anode is the electrode where oxidation occurs.

Can someone explain to me what I have done wrong?

Also are cations always attracted to cathodes and anions always attracted to anodes?

[Borek]

Electrons will get into the system through oxidation, and since the cathode is the electrode through which electrons get into the system then cathode is where oxidation occurs.

No. Adding electrons means reduction:

Cu²⁺ + 2e^-  Cu

[T]

I see, thanks.

Also what does the negative electrode mean? Is it where the electrons gets created or where the electron goes to? Eg. If a copper half cell and a zinc half cell is connected by a wire, would the negative electrode be the zinc electrode where the zinc ionises or the copper electrode where the Cu²⁺ gets reduced?

[mjc123]

The negative electrode is the one that is at a negative potential relative to the other. Which one this is depends on what type of cell it is - read what I wrote above. Which kind of cell is your zinc/copper cell?

[T]

Thanks mjc 123. What I meant was a electrochemical cell which I think is also a galvanic cell.

I will summarise the general points:

Cathode is where reduction occurs. It is positive in a galvanic cell and negative in a electrolytic cell.

Anode is where oxidation occurs. It is negative in a galvanic cell and positive in a electrolytic cell.

One more question:

The occurrence of the reduction reaction at the cathode consumes electrons from the electrode, giving it a positive potential, and the oxidation releases electrons to the anode. Electrons flow through the external circuit from the anode to the cathode (and conventional current the other way).

This is saying that the cathode is relatively more positive than the anode because it is where the electrons are consumed, I think. In an electrolytic cell the cathode is negative even though it is also where the electrons are consumed. I am pretty sure the answer is in this:

In an electrolytic cell, the passage of an electric current causes a non-spontaneous chemical reaction. At the cathode (the electrode connected to the negative terminal of the battery) reduction occurs when electrons are pushed in at a sufficiently negative potential, and likewise oxidation occurs at the positive anode.

I think mjc123 is saying that the cathode is negative in an electrolytic cell because "electrons are pushed in at a sufficiently negative potential". Could someone explain this?

Thanks for the help.