Sciencemadness Discussion Board

Electrochemistry - (maybe) stupid question

MKSStal - 8-10-2018 at 03:31

Can we make a reaction go by building correct galvanic cell?
For example: iodine and oxalic acid don't react or do it very slowly. Standard potentials suggest they should. So by building a cell of carbon electrode in oxalic acid solution, salt bridge, carbon electrode in water + iodine solution (preperebly triiodide anion) can we achieve this? Oxalic acid should get oxidised to CO2 gas and H+ which should migrate via salt bridge (pr better "acid bridge") to other half-cell forming HI?

happyfooddance - 8-10-2018 at 21:53

Quote: Originally posted by MKSStal  
Can we make a reaction go by building correct galvanic cell?

Yes, they are called batteries: but I suspect that wasn't actually your question...
Quote: Originally posted by MKSStal  
For example: iodine and oxalic acid don't react or do it very slowly. Standard potentials suggest they should.

If I had a dollar for every time... No, honestly I'm not too sure what you are asking/insinuating.
Quote: Originally posted by MKSStal  
So by building a cell of carbon electrode in oxalic acid solution, salt bridge, carbon electrode in water + iodine solution (preperebly triiodide anion) can we achieve this? Oxalic acid should get oxidised to CO2 gas and H+ which should migrate via salt bridge (pr better "acid bridge") to other half-cell forming HI?

Whaaat?! First off, this is not a galvanic cell, you need a current to drive this. Secondly, you might be able to do this but you would need a membrane or something, not a salt bridge. Third, H+ doesn't exist in solution, I think you mean H3O+. There are many holes in this idea I don't even know where to start, such as how do plan to solvate I2 in aqueous solution? Also, you don't even say what your goal is. To make HI? State clearly your goal or purpose, how you plan to achieve it, and by which mechanism you think it will work...

DraconicAcid - 9-10-2018 at 08:17

Quote: Originally posted by happyfooddance  

Whaaat?! First off, this is not a galvanic cell, you need a current to drive this. Secondly, you might be able to do this but you would need a membrane or something, not a salt bridge. Third, H+ doesn't exist in solution, I think you mean H3O+. There are many holes in this idea I don't even know where to start, such as how do plan to solvate I2 in aqueous solution? Also, you don't even say what your goal is. To make HI? State clearly your goal or purpose, how you plan to achieve it, and by which mechanism you think it will work...


Oh, piffle. The fact that H+ is more accurately written as H3O+ is trivial, and a ridiculous objection.

The OP does say how to solvate iodine- as the triiodide ion.

That being said, if iodine and oxalic acid don't react directly (despite the electrode potentials), then it's unlikely that they'll react in a galvanic cell.

happyfooddance - 9-10-2018 at 18:22

Quote: Originally posted by DraconicAcid  

Oh, piffle. The fact that H+ is more accurately written as H3O+ is trivial, and a ridiculous objection.


He is implying that a cation migrates across a salt bridge. I didn't point it out because it is "more accurately written" that way. I pointed it out because if he payed more attention to this one particular behavior ("protons don't exist free in solution") he would understand why protons don't move like he suggested.

In other words, the movement of protons in (aqueous) electrolytic solutions are more governed by their attraction to an electron dense oxygen atom, not their attraction to a current of electrons. Because protons are attached to oxygen in hydronium, or water, or hydroxide, they thus move where the (greater electronegativity) oxygen moves, which is not against the flow of electrons like say, sodium.

DraconicAcid - 9-10-2018 at 20:03

Quote: Originally posted by happyfooddance  

He is implying that a cation migrates across a salt bridge. I didn't point it out because it is "more accurately written" that way. I pointed it out because if he payed more attention to this one particular behavior ("protons don't exist free in solution") he would understand why protons don't move like he suggested.

In other words, the movement of protons in (aqueous) electrolytic solutions are more governed by their attraction to an electron dense oxygen atom, not their attraction to a current of electrons. Because protons are attached to oxygen in hydronium, or water, or hydroxide, they thus move where the (greater electronegativity) oxygen moves, which is not against the flow of electrons like say, sodium.


Um, nope. Protons migrate through a solution much faster than any other ions because they are easily transferred from one water molecule to the other. If you start with
H3O+ H2O H2O H2O H2O H2O H2O

It can rapidly switch to being
H2O H2O H2O H2O H2O H2O H3O+

Without any of the water molecules moving any appreciable distance.

Hydroxide ions will diffuse almost as fast, for the same reason. All the water molecules can pass one H+ to the left, and you suddenly have a hydroxide ion on the right hand side, and the hydroxide that was on the left is now a water molecule.

happyfooddance - 10-10-2018 at 07:10

Diffusion and migration are two different things when it comes to electrolysis. Hydrogen doesn't "migrate" across a salt bridge (salt does). You clearly don't have much experience with electrolysis if you are implying otherwise. Your explanation (while correct) does a good job of explaining why the behavior ionic cations such as sodium are completely different from hydrogen, and why we need salt in a salt bridge.

The whole point of a salt bridge is to avoid polarization of the electrolyte.