What Happens at the Anode During Electrolysis of Sodium Sulphate and Why?

[Krushnaraj Pandya]

Homework Statement

I want to know what happens at the anode and why it happens during the electrolysis of sodium sulphate.
2. The attempt at a solution
Na+ and H+ move towards cathode, H+ is discharged due to Electrode potential values. What happens to the SO42- ions and how is O2 produced at anode?

 

[Borek]

Hint: just like nothing happens to Na⁺, nothing happens to SO₄^2-.

What else is present in the solution?

 

[Krushnaraj Pandya]

Hint: just like nothing happens to Na⁺, nothing happens to SO₄^2-.

What else is present in the solution?

OH- is present, but how does OH- convert to O2?

 

[Borek]

Try to guess the reaction, there are no many possibilities.

 

[Krushnaraj Pandya]

Try to guess the reaction, there are no many possibilities.

I'm guessing OH- breaks into O2- and H+ and then O2- loses an electron

 

[Borek]

Don't go for a mechanism, just write and balance the half reaction.

 

[Krushnaraj Pandya]

Don't go for a mechanism, just write and balance the half reaction.

O2- + e gives O2

 

[Krushnaraj Pandya]

correct?

 

[Borek]

No, you have to start with what is present in the solution.

 

[Krushnaraj Pandya]

3OH- + 3e gives O2 + H2O ??
Its seems counter-intuitive to write either SO42- and OH- that is present directly as O2

 

[Borek]

3OH- + 3e gives O2 + H2O ??

Can you balance it? If not, it can't be a right reaction equation.

But in general you are on the right track - start with OH^-, put O₂ on the right side and see what you can do to balance the equation. You can use any ions and substances that are already present in the solution.

 

[Krushnaraj Pandya]

2H2O gives 4H+ + O2 + 4e- ?

 

[Krushnaraj Pandya]

How do I guess nothing happens to SO42- in an exam? or predict O2 evolves from H2O?

 

[Borek]

2H2O gives 4H+ + O2 + 4e- ?

Will do, although commonly we rather assume it is OH^- that is being oxidized (it nicely cancels out with H⁺ reduced on the other electrode).

How do I guess nothing happens to SO42- in an exam? or predict O2 evolves from H2O?

These are things you just have to remember. SO₄^2- is quite stable in the solution and doesn't easily get involved in the redox reactions, electrolysis in water very often ends up with some oxygen and hydrogen being produced.

 

[Krushnaraj Pandya]

Will do, although commonly we rather assume it is OH^- that is being oxidized (it nicely cancels out with H⁺ reduced on the other electrode).
These are things you just have to remember. SO₄^2- is quite stable in the solution and doesn't easily get involved in the redox reactions, electrolysis in water very often ends up with some oxygen and hydrogen being produced.

Oh, alright. Thank you very much for your help :D
P.S-I couldn't help but notice that PF has a great physics and mathematics community but a relatively inactive one in chemistry. E.g. you end up resolving most of my problems and some related to organic chemistry aren't resolved at all. Is there something like chemistryforums.com where I can look for help in case you aren't present here for a while?
Thank you

 

[Borek]

Is there something like chemistryforums.com where I can look for help in case you aren't present here for a while?

Make it chemical*

 

[Krushnaraj Pandya]

Make it chemical*

Alright, Thank you :D

 

[epenguin]

How do I guess nothing happens to SO42- in an exam? or predict O2 evolves from H2O?

Will do, although commonly we rather assume it is OH^- that is being oxidized (it nicely cancels out with H⁺ reduced on the other electrode).
These are things you just have to remember. SO₄^2- is quite stable in the solution and doesn't easily get involved in the redox reactions, electrolysis in water very often ends up with some oxygen and hydrogen being produced.

Pity if you have never seen it demonstrated experimentally, if you had you would remember. I can remember it was one of the very first demonstrations in chemistry lessons in my school a long long time ago, supposed to demonstrate that water is made out of a combination of hydrogen and oxygen. School teaching laboratories used to have a special apparatus for the purpose of showing this. If you are learning chemistry without a school laboratory, which I sense is increasingly common, this gross lack is these days compensated by YouTube, where you can actually see quite a lot of chemistry. For example here:



A nice thing pointed out is how the volume of hydrogen is twice that of the oxygen - a very significant fact as you might know and might be worth following up immediately looking up the Gay-Lussac law and the kinetic theory of gases.I don't remember Mr Bayford pointing this out the first time I saw it, but that might be my fault. I think he did show the nature of the gases, collecting them in test tube and the one causing a glowing splint to light up brightly, the other making a pop when combusted.

Many YouTube sites illustrate standard visible chemistry, a number on electrolysis of water – I just took a quick look
Here is a nice one.


You'll notice in all cases although they say they are electrolysing water they throw other stuff in. Like Mr Bayford those years ago poured some sulphuric acid in explaining that water was such a bad conductor that this was needed to make the current flow.I thought at the time that deprived the experiment of rigour, and he was just electrolysing sulphuric acid. But at least I remember it!

 

[Krushnaraj Pandya]

Pity if you have never seen it demonstrated experimentally, if you had you would remember. I can remember it was one of the very first demonstrations in chemistry lessons in my school a long long time ago, supposed to demonstrate that water is made out of a combination of hydrogen and oxygen. School teaching laboratories used to have a special apparatus for the purpose of showing this. If you are learning chemistry without a school laboratory, which I sense is increasingly common, this gross lack is these days compensated by YouTube, where you can actually see quite a lot of chemistry. For example here:



A nice thing pointed out is how the volume of hydrogen is twice that of the oxygen - a very significant fact as you might know and might be worth following up immediately looking up the Gay-Lussac law and the kinetic theory of gases.I don't remember Mr Bayford pointing this out the first time I saw it, but that might be my fault. I think he did show the nature of the gases, collecting them in test tube and the one causing a glowing splint to light up brightly, the other making a pop when combusted.

Many YouTube sites illustrate standard visible chemistry, a number on electrolysis of water – I just took a quick look
Here is a nice one.


You'll notice in all cases although they say they are electrolysing water they throw other stuff in. Like Mr Bayford those years ago poured some sulphuric acid in explaining that water was such a bad conductor that this was needed to make the current flow.I thought at the time that deprived the experiment of rigour, and he was just electrolysing sulphuric acid. But at least I remember it!

The trouble is quite the opposite, we have so many experiments here (typical Indian high-school) that it's hard to remember any of the experiments you do in the lab. I'll certainly look into it though, thanks :D

 

[epenguin]

The trouble is quite the opposite, we have so many experiments here (typical Indian high-school) that it's hard to remember any of the experiments you do in the lab. I'll certainly look into it though, thanks :D

I'm relieved to hear it, and if there is a happy medium, at least your course is better than learning just from books. Many things you can learn later in life, but if you miss the sights and sounds of chemistry at school, it is almost never made up for later. I'm not saying Borek wrong by the way, But these demonstrations and revisiting your lab notes can give you a lot of hooks to latch memory on to.