[mikasaur]

Hello all.

I'm starting to learn about electrochemistry and I want to make sure I fully understand the concept of electrolysis and electrolytic cells.

For the electrolysis of H₂O into H₂ and O₂ I understand that we have the two half-reactions:

[itex]\ce{2H+(aq) + 2e- -> H2(g)} \;\;\;\; \textrm{E}_{\textrm{red}}^{\circ} = 0.0 \textrm{ V}[/itex]
[itex]\ce{2H2O(l) -> O2(g) + 4H+(aq) + 4e-} \;\;\;\; \textrm{E}_{\textrm{ox}}^{\circ} = -1.23 \textrm{ V}[/itex]

I have this mental model in my head where the reduction reaction at the cathode is being supplied electrons by the power source and the oxidation reaction at the anode is supplying electrons that are "sucked up" by the power source (and transferred to the cathode).

What I don't understand is why a salt like NaCl is required. I know the conductivity of water is poor, but why does it need to conduct electricity? Isn't the circuit completed by the ionic current of the H⁺ ions?

Could it be that the conditions local to each of the electrodes change enough to make the half reactions more and more unfavorable (which NaCl somehow helps with)?

Any help in understanding would be very appreciated! Thank you all!!

[billnotgatez]

I wonder if NaOH would be better than NaCl in understanding this context.

[Borek]

why does it need to conduct electricity? Isn't the circuit completed by the ionic current of the H⁺ ions?

It is. But there are so few of them the resistance is very high (ultrapure water has a specific resistivity of about 18 MΩm). High resistance means low currents and pretty slow electrolysis.

[mikasaur]

Thanks for the answers!

I wonder if NaOH would be better than NaCl in understanding this context.

So with a basic solution there's gonna be a LOT more OH^- than there would otherwise be with pure water.

Looking up the half reactions in a basic solution I see:

[itex]\ce{2H2O(l) + 2e- -> H2(g) + 2OH- (aq)}[/itex]
[itex]\ce{4OH- (aq) -> O2(g) + 2H2O(l) + 4e-}[/itex]

So the high amount of OH^- will allow this to happen way more often than pure water with barely any H⁺. I see that. And I can see how HCl would do something similar. But NaCl doesn't change the pH like these strong base/acids would. I'm still not sure how IT would help as far as providing more H⁺/OH^-.

why does it need to conduct electricity? Isn't the circuit completed by the ionic current of the H⁺ ions?

It is. But there are so few of them the resistance is very high (ultrapure water has a specific resistivity of about 18 MΩm). High resistance means low currents and pretty slow electrolysis.

Right. Maybe I'm misunderstanding exactly why this helps, though. By reducing the resistance in the water, what I imagine happening is that electrons can flow through the water from the cathode to the anode. But if these electrons are just going through the water, they're not actually contributing to the process of electrolysis. That is, every electron leaving the cathode on its way to the anode is one electron that would have otherwise reduced a hydrogen to H₂. And every electron that makes its way to the anode is one fewer electron that oxygen would have given up to oxidize to O₂.

To put it even further, if 100. A of current is being supplied by my power source and 99 A goes through the salt water, that only leaves 1 A of current actually contributing to the electrolysis.

I'm guessing my mental model of electrons going through the water is incorrect.

[AWK]

NaCl change pH of solution during electrolysis. Mainly H₂ and Cl₂ instead of O₂ are formed.

[billnotgatez]

@mikasaur
We have had many threads on this forum discussing electrolysis and electrolysis of brine
There is many entries about both on WIKI

[mikasaur]

@mikasaur
We have had many threads on this forum discussing electrolysis and electrolysis of brine
There is many entries about both on WIKI

Fair enough. I thought I did due diligence by searching Google and Wikipedia, but I could have searched the forums a bit better. Seems there were a few questions asked that were semi-related.

This post is asking exactly what I'm asking. And it seems like the answer is "it's complicated", at least for someone who is still learning the basics. 

Thanks all.

[Corribus]

A really simple explanation for why you need an electrolyte for electrochemical processes is that plain water doesn't have enough conductivity to "complete a circuit". Oxidation and reduction can take place at the respective electrodes but without a medium to help the charges diffuse away from the electrode, charges build up at the electrodes and it takes progressively more energy for more oxidation and reduction events to take place. The electrolyte substantially increases the electron mobility (or the diffusion rate of charges) to prevent this charge buildup. For basically the same reason, a salt bridge is crucial in a simple voltaic cell battery.

[mikasaur]

Thanks, Corribus, that actually makes a lot of sense!

[Borek]

By reducing the resistance in the water, what I imagine happening is that electrons can flow through the water from the cathode to the anode.

It is not electrons that go through the water, but ions.

But if these electrons are just going through the water, they're not actually contributing to the process of electrolysis.

Transfer of electrons happens on the phase boundary (electrode surface) only, and its passage through the phase boundary requires an electrode reaction. Then the charge is transferred through the bulk of the solution by ions. You can think about it as if the electrode and the solution were put in series - current going through each part is exactly the same.

Hence, if you have a high resistivity of the solution, you can't obtain reasonably large currents during electrolysis without applying some pretty high voltages.

[mikasaur]

Thanks, Borek! The idea of connecting the electrodes and solution in series really hammers the idea home. Combine that with Billnotgatez's suggestion to think about NaOH rather than NaCl and I think I have a pretty good understanding.

Enjoy the mole snacks!

[AWK]

In industrial electrolysis (pure H2 production) KOH solution is the best.

[Enthalpy]

To put it even further, if 100. A of current is being supplied by my power source and 99 A goes through the salt water, that only leaves 1 A of current actually contributing to the electrolysis.

The current in the electrolyte moves every ion present, and these reach the electrodes. What happens nexts depends on the nature of each ion. Some may evolve out of the cell when they get neutralized, others react immediately with the electrolyte to make something else.

As an example, Na⁺ being neutralized reacts with water, so you obtain H₂ rather than sodium. The detailed process may even not need neutral Na at any time.

An other example is Cl^- which, unless you take special precautions, makes much NaClO when you electrolyze brine.

And unless your electrodes are very inert chemically, the most common reaction is their destruction by the very reactive species produced there. For instance, Cl₂ molecules appear in steps since Cl^- arrive individually at the electrode, and these steps may include a metal chloride layer that reacts with further Cl atoms to make Cl₂ - but only if the metal chloride doesn't dissolve before.