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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: kensher on July 18, 2019, 03:02:48 PM

Title: Electrolysis of sodium chloride -- why chlorine instead of oxygen?
Post by: kensher on July 18, 2019, 03:02:48 PM: Hello!

In my textbook there is this one practice problem which involves the electrolysis of both aqueous sodium chloride and sodium sulfate. This is what the book says about the products being formed at each electrode:

Electrolysis of sodium sulfate
Cathode: H₂(g)
Anode: 0₂(g)

Electrolysis of sodium chloride
Cathode: H₂(g)
Anode: Cl₂(g)

I don't understand why chlorine gas gets formed instead of oxygen gas because the cell potential of oxygen gas forming from water is higher than chlorine gas forming from chloride ions (see picture below).
The cell potential of water oxidating to oxygen gas is -1.23V, while the cell potential of chloride oxidating to chlorine gas is only -1.36V.

(https://i.imgur.com/gEuI5pG.jpg)
(The book is written in Norwegian).

Is this a special case or am I missing something?
Title: Re: Electrolysis of sodium chloride -- why chlorine instead of oxygen?
Post by: Borek on July 18, 2019, 04:15:14 PM: Special case. While you are perfectly right comparing potentials suggests oxygen should be produced, it is kinetically disfavored (reaction is very slow and requires large overpotentials to proceed quickly). In effect on a typical electrode chlorine evolves first.
Title: Re: Electrolysis of sodium chloride -- why chlorine instead of oxygen?
Post by: kensher on July 18, 2019, 04:47:34 PM: Thank you very much for clarifying :)
Title: Re: Electrolysis of sodium chloride -- why chlorine instead of oxygen?
Post by: Corribus on July 18, 2019, 07:39:41 PM: Anything that consumes or produces oxygen gas is typically slow without a large activation energy supplied because oxygen is a ground state triplet.