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Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: blakloks on April 11, 2005, 11:46:30 PM

Title: electrolysis- selective discharge
Post by: blakloks on April 11, 2005, 11:46:30 PM

hey
i am having major problems with my school project where i am supposed to construct an experiment to teach kids about selective discharge.

first i tried varying the electrolyte but it doesnt really show anything. does anyone know of any experiments that highlight selective discharge?

thanks.

Title: Re:electrolysis- selective discharge
Post by: savoy7 on April 17, 2005, 06:28:34 AM

Some Ideas(just initial):  
a demo of burning gases release from electrolysis could be done

pure water should produce H+ and OH- with the OH- becoming oxygen gas
presence both gases could be shown with a glowing ember

now to show the selective discharge - SO₄^-2 is the ion I'm thinking about
either a Copper(II)Sulfate solution or a dilute sulfuric acid soln would do the trick

the Cu⁺² would go to the cathode (in the copper(II) sulfate soln) coating the pt electrode, but the sulfate ion will not go to the anode - OH- will be produced and 4OH- = O₂ + 2 H₂O + 4 e-

the same is true with the dilute acid except H+ would be at the cathode

the presence of oxygen can be shown with a glowing ember

NOTE:  potential problem - what about the free OH- in the soln, won't both soln produce oxygen gas even if sulfate wasn't selective?


Please remember, I'm just thinking through this problem and trying to come up with some ideas.

Title: Re:electrolysis- selective discharge
Post by: savoy7 on April 17, 2005, 06:41:47 AM

just some added thoughts on the last post

discharge usually goes by the most conc ion present - SO₄^2- is more conc than the hydroxide ion and show go the anode, but doesn't

therefore the demo could work

Title: Re:electrolysis- selective discharge
Post by: Borek on April 17, 2005, 09:15:16 AM

Quote from: savoy7 on April 17, 2005, 06:41:47 AM

discharge usually goes by the most conc ion present

Not exactly. First thing to consider are half-potentials. Half potential can be modified by the concentrations (Nernst equation) and thats where concentrations come into play. But that's not a starting point.

Title: Re:electrolysis- selective discharge
Post by: Sebastian on May 16, 2005, 03:50:17 AM

Hi,
Do you know some one, something about the electroreduction of tungsten (W) from tungstophosphoric acid (H3PW12O40). What happend? I tried and I obtained the blue of tungsten (the colour of solution turned into blue). What is this? Help me!
Sebastian.