Electrochemistry: electrode “polarity”

[TheAspiringDoc]

Back with another electrochemical question

I didn’t understand lzxnl’s explanation of “what is polarity”

Spoiler

1. What does the polarity of a cell mean?

"Positive" and "negative" are misleading ways of thinking about these cells if you don't know what they actually represent. They're not actually charged; you have the salt bridge to make sure that they are not charged. So what are these things?

The "positive" electrode means the electrode at higher electric potential. NOTE: electric potential is not electric potential energy. Electric potential is potential energy per charge. This means that a positive charge will lose potential energy and thus speed up when moving from higher potential to lower potential, whereas a negative charge will gain potential energy and thus slow down when moving from higher potential to lower potential. This agrees with the common observation that negative charges move from "negative" to "positive", i.e. lower to higher electric potential.

Could someone please explain it in a different way? I get the gist that ‘positive electrode’ doesn’t mean ‘positively charged’, because of the salt bridge, but rather it’s something to do with the charge of the ion within the electrode? Maybe?

Thanks!

[TheBigC]

Back with another electrochemical question

I didn’t understand lzxnl’s explanation of “what is polarity”

Spoiler

Could someone please explain it in a different way? I get the gist that ‘positive electrode’ doesn’t mean ‘positively charged’, because of the salt bridge, but rather it’s something to do with the charge of the ion within the electrode? Maybe?

Thanks!

What's up AspiringDoc.

Firstly, to clear things up:
- Positive electrode does mean positive. It is not an analogy or some sort of cover-up. A simplistic explanation of this is that at the anode (which is negative in galvanic cells), electrons are taken up as species undergo oxidation. The uptake of electrons produces a slight negative charge. Similarly, the cathode (which is positive in galvanic cells), is the site of reduction, such that electrons are taken from the electrode and combined with cations to yield neutral atoms. Ultimately, this loss of electrons is what creates a slight positive charge. This is a very simplistic explanation.

To answer the initial question ('what does the polarity of a cell mean'), the above user's explanation is correct in identifying that galvanic cells do not have an overall charge. They are neutral (thanks to the salt bridge). However, the above user incorrectly insinuated that electrodes lack charge: they have polarities.

[lzxnl]

What's up AspiringDoc.

Firstly, to clear things up:
- Positive electrode does mean positive. It is not an analogy or some sort of cover-up. A simplistic explanation of this is that at the anode (which is negative in galvanic cells), electrons are taken up as species undergo oxidation. The uptake of electrons produces a slight negative charge. Similarly, the cathode (which is positive in galvanic cells), is the site of reduction, such that electrons are taken from the electrode and combined with cations to yield neutral atoms. Ultimately, this loss of electrons is what creates a slight positive charge. This is a very simplistic explanation.

To answer the initial question ('what does the polarity of a cell mean'), the above user's explanation is correct in identifying that galvanic cells do not have an overall charge. They are neutral (thanks to the salt bridge). However, the above user incorrectly insinuated that electrodes lack charge: they have polarities.

I have to side with 4 year ago me on this one.

Electric forces are extremely strong; actual charge imbalances would even out very quickly. It is not physical to describe electrodes as actually having charge imbalances; after all, the flow of current is supposed to mitigate any charge imbalances. Let's look at some numbers.

By Coulomb's law, the electric attraction between a proton and an electron is given by

where k is the Coulomb constant, e is the elementary charge and r is the distance. Taking nuclear radii to be roughly in size and using , in SI units, you'll find that the force is roughly 230 N. Given that charge carriers are electrons, and that the electron mass is , we are looking at otherworldly accelerations. Even if you increase the separation to, say, 1 mm, you have a force of , which is still an acceleration on the order of . And this is all assuming you're missing one electron. I hope you realise just how strong electric forces are and how unrealistic it is to assume any long-term charge imbalance.

You have two electrodes, positive and negative. Suppose you had copper and zinc electrodes. Then, the zinc electrode would be negative as it is lower on the electrochemical series with a lower voltage, and the copper would be positive. The higher reduction potential for copper means that electrons are drawn to this electrode. These electrons are balanced by the oxidation occurring at the zinc electrode. At no stage is there a physical charge imbalance. That is not how electric circuits work.

You may ask, why is there a potential difference? The answer comes from the interaction between the different electronic configurations of copper and zinc, as well as having different nuclei; a proper calculation involves quantum chemistry I have not learned, and probably will not encounter.

Back with another electrochemical question

I didn’t understand lzxnl’s explanation of “what is polarity”

Spoiler

Could someone please explain it in a different way? I get the gist that ‘positive electrode’ doesn’t mean ‘positively charged’, because of the salt bridge, but rather it’s something to do with the charge of the ion within the electrode? Maybe?

Thanks!

So, that's what polarity is. It really is two electrodes having different affinities for electrons due to their electronic structure. Zinc, in my example, is better at getting rid of its electrons than copper, so zinc is the negative electrode ('repels' electrons, source of electrons).

[TheBigC]

I have to side with 4 year ago me on this one.

Electric forces are extremely strong; actual charge imbalances would even out very quickly. It is not physical to describe electrodes as actually having charge imbalances; after all, the flow of current is supposed to mitigate any charge imbalances. Let's look at some numbers.

By Coulomb's law, the electric attraction between a proton and an electron is given by

where k is the Coulomb constant, e is the elementary charge and r is the distance. Taking nuclear radii to be roughly in size and using , in SI units, you'll find that the force is roughly 230 N. Given that charge carriers are electrons, and that the electron mass is [/tex] 9.1\times 10^{-31} kg[/tex], we are looking at otherworldly accelerations. Even if you increase the separation to, say, 1 mm, you have a force of , which is still an acceleration on the order of . And this is all assuming you're missing one electron. I hope you realise just how strong electric forces are and how unrealistic it is to assume any long-term charge imbalance.

You have two electrodes, positive and negative. Suppose you had copper and zinc electrodes. Then, the zinc electrode would be negative as it is lower on the electrochemical series with a lower voltage, and the copper would be positive. The higher reduction potential for copper means that electrons are drawn to this electrode. These electrons are balanced by the oxidation occurring at the zinc electrode. At no stage is there a physical charge imbalance. That is not how electric circuits work.

You may ask, why is there a potential difference? The answer comes from the interaction between the different electronic configurations of copper and zinc, as well as having different nuclei; a proper calculation involves quantum chemistry I have not learned, and probably will not encounter.

So, that's what polarity is. It really is two electrodes having different affinities for electrons due to their electronic structure. Zinc, in my example, is better at getting rid of its electrons than copper, so zinc is the negative electrode ('repels' electrons, source of electrons).

I can see exactly what you are saying here. Great explanation.

As far as my understanding reaches, I have looked at year 12 texts, as well as Blackman's Chemistry (a first year uni chem text). In terms of my explanation, it holds in the context of 1st year uni and VCE. I am far too ignorant to battle with you on this issue, nonetheless, I will let other users decide what they prefer in terms of understanding the polarities of electrodes. Thanks for clarifying this concept lzxnl.

[lzxnl]

I can see exactly what you are saying here. Great explanation.

As far as my understanding reaches, I have looked at year 12 texts, as well as Blackman's Chemistry (a first year uni chem text). In terms of my explanation, it holds in the context of 1st year uni and VCE. I am far too ignorant to battle with you on this issue, nonetheless, I will let other users decide what they prefer in terms of understanding the polarities of electrodes. Thanks for clarifying this concept lzxnl.

It's good to question things. Science is advanced by finding flaws, no matter how slight, in established knowledge.

If you wish to think of electrodes as actually being charged, it may suffice for VCE, as you've mentioned, because the net effect is still that electrons are attracted to the positive electrode and repelled from the negative electrode. However, don't be surprised if you try experiments and cannot detect any charge imbalance, or if you later find something that contradicts that statement.

[TheBigC]

It's good to question things. Science is advanced by finding flaws, no matter how slight, in established knowledge.

If you wish to think of electrodes as actually being charged, it may suffice for VCE, as you've mentioned, because the net effect is still that electrons are attracted to the positive electrode and repelled from the negative electrode. However, don't be surprised if you try experiments and cannot detect any charge imbalance, or if you later find something that contradicts that statement.

To be honest, half of the stuff taught in VCE is surface-level information. I take almost everything with a grain of salt. I, in fact, often question everything (as you say), however, I unfortunately do not have the time to go too deep and as aforementioned, am far too ignorant as of now. I look forward to deepening my knowledge in university, where the real information lies (at least in the later years).

[lzxnl]

To be honest, half of the stuff taught in VCE is surface-level information. I take almost everything with a grain of salt. I, in fact, often question everything (as you say), however, I unfortunately do not have the time to go too deep and as aforementioned, am far too ignorant as of now. I look forward to deepening my knowledge in university, where the real information lies (at least in the later years).

Very true. As a VCE tutor myself, my more interesting students are the ones to which I can freely offload more advanced content. The VCE courses are designed so dryly and incredibly surface-level, which all stems from the quality of teaching nowadays, but that's a discussion I don't want to get into.

It gets better. A LOT better.

[TheBigC]

It has been awhile since any posts have occurred upon this topic, however, I thought that I'd share another method understanding electrode polarity in both galvanic cells and electrolytic cells. (NOTE: At a VCE level)

Firstly, we have some 'givens':
- Electrons always flow from the anode to the cathode
- Electrons always move from the negative terminal of a circuit to the positive terminal.

In galvanic cells, chemical species undergo oxidation at the anode and move throughout the external circuit toward the cathode. As electrons travel from the anode to the cathode, then the anode is designated as the negative electrode and the cathode is designated as the positive electrode (from negative to positive).

In electrolytic cells, an external power supply is connected to cell concerned. The negative terminal is then connected to the negative electrode (previously the anode), and the positive terminal of the power supply to the positive terminal of the cell (previously the cathode). Ultimately, as electrons are driven from the negative terminal of the power supply to the positive terminal of the power supply, then the cathode is negative and the anode is positive.