| MadSci Network: Chemistry |
|
Hi! My name is Ting Li and I'm currently a grade 12 student at Sir
Winston
Churchill High School in Canada. Yesterday, during our IB chem class, we
looked
at the electrolysis of aqueous NaCl. Everyone thought that water was
going to
be oxidized and oxygen gas was going to be made at the anode. But it
turns out
that chlorine gas was made instead. Our teacher told us that in the
electrolysis of aqueous chlorine containing ionic compounds, the chlorine
ion
would be oxidized instead of water, and that this was the one exception
that we
need to remember (because according to our reduction potential table,
water is
a stronger reducing agent than the chlorine atom). He told us that no one
really knows why this exception occurs, and so we never found out. So
would you
possibly know the reason? Also, I was wondering what some practical
applications of electrolysis of aqueous salts were. I know that
electrolysis of
aqueous NaCl is used now to chlorinate pools, and you can also use
electrolysis
to plate things with a layer of metal. But, are there any other
applications
besides these?
First part of the question: it's true that the oxidation potential of Cl- to Cl2 is higher than the oxidation potential of H2O to O2; 1.36V vs. 1.23V according to WebElements. In the same way, if you look at the overall reaction (considering both electrodes and the number of electrons passed) you get 25 kJ/mol in favor of oxygen formation over chlorine formation. But... all chemical reactions have some activation energy, an energy barrier that needs to be surmounted before the reaction can happen. (Among other things, this barrier is what makes highly-ordered systems, like living things, possible; see secondlaw.com.) In electrochemistry this is called overvoltage. This means the extra electrical potential necessary to get an electrochemical reaction started. It turns out that oxygen production from water has a high overvoltage, about 700 mV, while the overvoltage for chlorine production is only 50 mV. This difference in activation barriers corresponds to 63 kJ/mol, more than enough to overcome the 25 kJ/mol favorability of oxygen formation over chlorine formation. Thank you to Stephen Lower for his help with this answer.Second part of your question: I don't know of any other applications of the electrolysis of aqueous salts than the ones you mention (chlor-alkali production and electroplating). I imagine that if electrolysis is used for pool chlorination, the pH has to be adjusted afterwards! However, electrochemistry is also used in the production of certain organic chemicals, notably adipic acid which is used in the manufacture of nylon. And, of course, al uminum smelting is electrochemical.
|
Try the links in the MadSci Library for more information on Chemistry.