July 03, 2022, 03:48:59 AM
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Topic: Electrolysis of water/aqueous solutions - why are ions needed?  (Read 308 times)

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Offline nick9995

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You want to generate hydrogen and choose electrolysis.  Pure water doesn't conduct very well and my understanding is even with a very large voltage, splitting it into hydrogen and oxygen would be very slow.   

When you add a salt to the water, such as NaCl.  The reaction proceeds much faster at much lower voltage.

2 NaCl + 2 H2O → 2 NaOH + H2 + Cl2

Since the water molecule will break apart with electricity, why does it (mostly) only do so when it has ions in it?

To say it another way:  Why does H2O dissociate easily into H+ and OH- when NaCl and electricity are added but not when only electricity is added?

How does the presence of ions allow current to flow and cause this dissociation to happen?  I get that ions are free to move in solution and move towards their respective anode and cathode.  I don't understand why water, which is not ionized, participates in the reaction at all. 

Thanks in advance!

Offline Corribus

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Re: Electrolysis of water/aqueous solutions - why are ions needed?
« Reply #1 on: May 25, 2022, 10:06:12 AM »
It is a great question. Water is an insulator but like all insulators is subject to breakdown at high enough electric field strengths. I guess, the physics of this process in water is complicated and still studied. Crudely put, I guess the increased conductivity of water when an electrolyte is added reduces the thermodynamic barrier for breakdown near the vicinity of the electrodes and enhances the autoionization rate. Additionally, the electrolytes probably help to neutralize the charge buildup at the anode and cathode - in much the same way that a salt bridge is needed to maintain charge balance in a Galvanic cell.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline Borek

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Re: Electrolysis of water/aqueous solutions - why are ions needed?
« Reply #2 on: May 25, 2022, 04:05:29 PM »
To say it another way:  Why does H2O dissociate easily into H+ and OH- when NaCl and electricity are added but not when only electricity is added?

Water dissociates into OH- and H+ (almost) exactly the same way no matter whether other ions are present, or not. In both cases concentrations of OH- and H+ are defined by the water ion product - and are very, very low, in the 10-7 M range (that is assuming there are no acids/bases added). Such a small amount of ions means very high resistivity (if memory doesn't play tricks on me ultra pure water has a specific resistivity around 18.2MΩ/com).

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How does the presence of ions allow current to flow and cause this dissociation to happen?  I get that ions are free to move in solution and move towards their respective anode and cathode.

What is the electric current? How is it defined?

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I don't understand why water, which is not ionized, participates in the reaction at all.

Why should it not? Nothing unusual in neutral molecules being reduced/oxidized. While reduction and oxidation always mean charge of transfer, it doesn't mean product or reactant have to be charged.
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