[cliverlong]

Hello,

   I have read the following in a couple of textbooks and exam answers. The statement seems entirely bogus to me. Can someone please comment?

The question is when sodium chloride solution  is electrolysed why does the solution around the cathode (note cathode) become alkaline?

The argument given that water disassociates to form H₃O⁺ and OH^- ions. The H₃O⁺ is discharged at the cathode to hydrogen gas and water is liberated. Ok, fine.

Now why do the answers say the area round the cathode becomes alkali? Surely the OH^- ions are repelled by the cathode and move to the anode, hence making the solution around the anode alkali ?? The chloride Cl^- is preferentially discharged to chlorine gas at the anode rather than the OH^-. Of course, my argument produces a different problem, that is the OH^- builds up around the anode and “shields” the anode, limiting its ability to discharge the chloride ions.

Can someone explain?


Thanks

Clive

[jansenwrasse]

LEO the RED CAT says GRR.

Red cat = REDuction occurs at the CAThode

Leo = loss of electrons is oxidize
GRR.  Gaining electrons is reduction

So H₃O⁺ is missing 1 electron (thus the + charge) so it can gain its electron at the cathode

There is alot more to it than this but for high school this should pass the test

[dantron]

When the water splits into H₃O⁺ and OH^- at the cathode, the H₃O⁺ is reduced to H₂ (also producing water) as you said.  So OH^- remains at the cathode.  Sure, over time, the OH^- will diffuse towards the anode but diffusion is a relatively slow process and the concentration of OH^- will be much higher at the cathode because of this.