What is the reaction at the anode in this electrolysis experiment?

[Brimley]

Hello everyone, I've been reading up on electrochemistry and trying to make the connection from chemistry theory to what I learned during my time as an undergrad and grad student in electrical engineering and I'm having trouble bridging the gap. I have selected one example from my reference text that I'm hoping someone can better help me explain.

An aqueous solution of CuSO₄ was electrolyzed between inert electrodes for 30 minutes. The only cathode reaction was the deposition of 0.231 g of Cu.
Okay, just interpreting that part, I believe that since the cathode reaction only involved Cu, that the SO₄^2- ion wasn't affected by the electrolysis.

The book then says that the "reaction of the anode" can then be determined, and at the anode, a gaseous product evolves.

1. Can anyone describe what they mean by the reaction at the anode can be determined? What is it? How does one find it?

2. For the gaseous product that evolves at the anode, can we calculate it?
2.a - If so, can we calculate it in terms of its mass, volume, and chemical moles?

Thank you! I really appreciate any and all help! - Brim

 

[Borek]

For the current to flow charge has to cross the phase boundary between electrode and the solution twice - one on the anode, once on the cathode. Charge is conserved, so in both cases exactly the same charge (number of electrons) flows through the phase boundary. For the electrons to cross the phase boundary there must occur some reaction - in your case on the cathode copper is reduced, so something be oxidized on the anode. If it is a gas, and you were electrolyzing copper sulfate dissolved in water, what gas can it be?

 

[Brimley]

Hello Borek, so the aqueous word is really important then when it says an aqueous solution of CuSO₄ !

Okay, so I want to understand what you're saying a little bit better. So essentially if I take a bin, fill it with this solution and, but two electrodes in it connected to the same source (assuming one positive terminal, one negative), that is what completes the circuit path. The electrons from from the anode of one electrode through the solution and then to the cathode of the other electrode. By the time the current recirculates to the cathode, 0.231 g of Cu has been deposited (meaning reduced so that means it becomes more negative right?).

(Hopefully my above paragraph is right, I'm just trying to think this out loud via the internet so people can catch me if I'm thinking about it the wrong way).

I now see why you came to the conclusion of it being a gas because it says a gaseous product. I now know I have to pay more attention to the words gaseous and aqueous! I think it could be multiple gases though, couldn't it? So we have H₂O from the water and we have O₄ from the Sulfate ion, and I thought Hydrogren (H) still counts as a gas even though its listed on the metals side of the periodic table.

How can you tell which one is given off? I assume Oxygen but I'd like to know why that is so.

Before I did any deeper, can you tell me if what I've said so far is right or make corrections? Thank you!

 

[Borek]

What you wrote is a little bit chaotic to me, but at least at first sight nothing cries out loud "I am wrong!"

Do you know what are products of electrolysis of water (just water)?

 

[Brimley]

What you wrote is a little bit chaotic to me, but at least at first sight nothing cries out loud "I am wrong!"

Do you know what are products of electrolysis of water (just water)?

Hello Borek! I really do not understand how the electrolysis of water works quite yet.

I did some reading about it on Wikipedia: http://en.wikipedia.org/wiki/Electrolysis_of_water

Electrolysis of water is the decomposition of water (H₂O) into oxygen (O₂) and hydrogen gas (H₂) due to an electric current being passed through the water.

In my opinion, I think that confirms what I said in my past post that it could be multiple gases and I'm not sure which one to choose!

It also shows this:

Overall reaction: 2 H₂O(l) → 2 H₂(g) + O₂(g)

I do not understand how this works and when I tried to follow the wiki explanation, it was a little over my head.

I really appreciate your help! :-)

 

[Borek]

Without understanding redox reactions taking place (and without understanding half reactions) you won't be able to move much further. However, as you already wrote, you expected hydrogen to behave similarly to metals. Trick is, there is already a metal reacting (being reduced) on the cathode - so it is rather unlikely that hydrogen will evolve at the same time. What does it left you with?