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Topic: Why do anions flow to anodes?  (Read 2857 times)

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Offline Schwarz107

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Why do anions flow to anodes?
« on: November 10, 2017, 10:11:09 AM »
An anoin is an atom that's negatively charged. It has a "surplus" of electrons. A cation is an atom that's positively charged, it "lacks" an electron.

So I would assume electrons flow from anodes (where there are many anions) to the cathode. Right? Just like in the scheme below.



But the Oxford Dictionary defines an anion like this:

Anion. A negatively charged ion, i.e. one that would be attracted to the anode in electrolysis

The only way I would make sense of it, is if the process where we make a battery is electrolysis. That way we'd fill up the anode by attracting anions to it.

Offline mjc123

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Re: Why do anions flow to anodes?
« Reply #1 on: November 10, 2017, 11:53:22 AM »
You have to be careful. In the system you show, the electrons are not supplied to the anode by the anions - they are spectators. The electrons come from oxidising the zinc metal to zinc cations:
Zn  :rarrow: Zn2+ + 2e-
Electrons flow through the circuit to the cathode, where they reduce copper cations:
Cu2+ + 2e-  :rarrow: Cu
Ions migrate through the salt bridge to maintain electrical neutrality. Thus you will find that the concentration of both anions and cations increases in the anode solution, and decreases in the cathode solution (in this example).
This is an example of a galvanic cell (as in a battery), where a spontaneous chemical reaction creates an emf and causes current to flow through an external circuit. The release of electrons at the anode gives it a negative potential, and the consumption of them at the cathode gives it a positive potential (relative to each other), so electrons flow from anode to cathode. In a galvanic cell the anode is the negative electrode and the cathode is the positive electrode.
In an electrolytic cell, the application of a current from an external power source causes a chemical reaction to occur, e.g. the electrolysis of water to hydrogen and oxygen. In this case electrons are pumped by the power source into the cathode, where they cause a reduction reaction, and pulled from the anode, where an oxidation reaction occurs. Therefore in an electrolytic cell, the anode is the positive electrode (attached to the positive terminal of the battery) and the cathode is the negative electrode.
The constant thing between galvanic and electrolytic cells is that oxidation takes place at the anode; reduction takes place at the cathode.

Offline Schwarz107

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Re: Why do anions flow to anodes?
« Reply #2 on: November 10, 2017, 06:55:54 PM »
Much appreciated, mjc123! I am a complete neophyte, so the battery was perhaps not the best thing for me to bring up. I was trying to unpack this definition of an "anion" from the Oxford Dictionary.

An anion is "A negatively charged ion, i.e. one that would be attracted to the anode in electrolysis"

In human relationships they say equals attract. But in chemistry, the negatively charged anions wouldn't be attracted to other anions, but they would be attracted to cations. Right?

So, it if an "anode" is a place with many anions (not sure if that's correct), then I was surprised anions would be attracted to the anode.
« Last Edit: November 10, 2017, 07:36:52 PM by Schwarz107 »

Offline Borek

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Re: Why do anions flow to anodes?
« Reply #3 on: November 11, 2017, 03:46:47 AM »
if an "anode" is a place with many anions (not sure if that's correct)

Anions that get to the anode get discharged there and they are no longer anions.
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Offline Enthalpy

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Re: Why do anions flow to anodes?
« Reply #4 on: November 11, 2017, 04:46:10 AM »
The drawing describes a battery that provides electricity to the outer circuit.
The Oxford definition relates to electrolysis, where the outer circuit provides electricity to the cell.
The direction of electron flow is opposite, and the direction of ion movement too.

Offline Schwarz107

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Re: Why do anions flow to anodes?
« Reply #5 on: November 11, 2017, 04:49:31 PM »
Thanks, Borek. Then it makes perfect sense.

So negatively charged anoins have "too many" electrons and go to the anode to get discharged.

But the Oxford dictinoary have these definions of anode.

It seems like description 1.1. matches your explanation, but not the other?

Anode
1 The negatively charged electrode by which electrons enter an electrical device.

Anode
1.1 The positively charged electrode of an electrical device, such as a primary cell, that supplies current.
(This seem to match your description perfectly)

@Enthalpy, much appreciated! Very useful!

Offline Schwarz107

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Re: Why do anions flow to anodes?
« Reply #6 on: November 11, 2017, 09:23:39 PM »
The Merriam-Webster definition of anode seems to answer my last question. The anode is a negative terminal in one cell, but a positive in another.

1 :the electrode of an electrochemical cell at which oxidation occurs: such as
  a :the positive terminal of an electrolytic cell
  b :the negative terminal of a galvanic cell
2 :the electron-collecting electrode of an electron tube; broadly :the positive electrode of a diode — compare cathode

I suppose when we say "anode" we most often talk about definition 1a, since I've seen this mnemonic that goes Positive Anode Negative Is Cathode.

Offline Borek

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Re: Why do anions flow to anodes?
« Reply #7 on: November 12, 2017, 03:53:50 AM »
It just repeats what you were told by mjc123, doesn't it?
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Offline Schwarz107

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Re: Why do anions flow to anodes?
« Reply #8 on: November 12, 2017, 04:01:53 PM »
Yes, you're right! I wish I could have read mjc123's whole reply just like that, but as many new—and pertinent—concepts were introduced, I lost the thread. I am getting the very best replies here (thank you all!), and I will read everything everyone writes many times, so it'll stick forever.

Offline Enthalpy

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Re: Why do anions flow to anodes?
« Reply #9 on: November 13, 2017, 10:35:23 AM »
Also beware that "anode" and "cathode" mean different things to chemists (who follow the original definition) and to nearly the rest of the world. We had this discussion in the forum.

For chemists, the cathode changes from one terminal of a secondary battery to the other, depending on if you charge or discharge it.

For many other people (including all battery makers I know!), the cathode is the negative electrode, both at the power source and the consumer.

In electrical engineering, the cathode of a diode is marked by a painted ring and stays the same terminal, whether it's positive or negative, and even if it's a solar cell that provides power.

As chemists seem to be consistent, my suggestion is to follow their habits, at least within the profession.

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