MadSci Network: Physics
Query:

Re: What would happen if you electrolyse water in AC?

Date: Mon Sep 13 12:32:38 2004
Posted By: Guy Beadie, Staff, Optical sciences, Naval Research Lab
Area of science: Physics
ID: 1092830472.Ph
Message:

Hi, Chris.

What an interesting question.  I like it.  It takes something youíve 
learned about (electrolysis) and adds a twist from a totally different 
topic (alternating current).  Itís thought processes like this that really 
add to your understanding.  Good job!

Answering your question turns out to be the subject of a whole area of 
physical chemistry Ė chemical kinetics.  While a lot of chemistry is 
concerned with steady-state, or equilibrium, conditions, what youíre after 
in this question is how reactions change as their conditions change.

To clarify, you want to know what happens to the normal electrolysis 
reactions when the voltage on the electrodes oscillates periodically from 
plus to minus at some frequency.  As you guessed, it is a real question 
whether water will be able to grab electrons from the electrodes fast 
enough to convert to gas before the voltage flips.

Because Iím not a chemist, Iím not going to be able to give you a 
quantitative answer.  Iíll sketch what could happen, give you my best 
guess as to what might happen, and then give references that you can go to 
for more details.

[For a primer on electrolysis, see the excellent demo site at:
 http://www.chem.uiuc.e
du/clcwebsite/elec.html
]

The simplest thing that could happen is the reaction occurs just as you 
would expect from the voltages on the electrodes, at each instant of 
time.  Thus, half of the time oxygen is produced at one electrode while 
the other half of the time itís produced at the other electrode.  
Meanwhile, hydrogen is being produced at the opposite electrode.  The gas 
formed at the top of each tube would have equal volumes, comprised of two 
parts hydrogen to one part oxygen.  This is what would happen if the 
reactions all occurred much faster than the AC time period.

The converse situation is that your oscillating voltage flips sign so fast 
that the reaction occurs at a rate too slow to observe.

Another thing that may occur is both reactions are fast enough for your 
oscillation, but you deplete the water faster than it can be replenished.  
This is a more-complex situation than the previous case Ė here weíre 
limited by chemical _transport_ rather than the actual rate of reaction.

For example, during one half of the voltage cycle, water is used up to 
produce hydrogen gas and OH ions next to one of the electrodes.  In the 
next half of the cycle, water is used up to produce oxygen gas and H 
ions.  Eventually, youíre going to need more water to get near the 
electrode to continue the electrolysis.  If the necessary water is 
prevented from approaching the electrodes due to all the reaction products 
in the way, then the reaction will grind to a halt.

So, among all these things, what might actually happen?  Iím guessing that 
the reaction is plenty fast, so unless your AC frequency is really high 
you wonít be limited by rates of reaction.  Even better, the gases 
produced during the reaction will cause a lot of turbulence near the 
electrodes as they bubble up to the top.  This means that there will be 
efficient mixing of water and reactants nearby the electrodes, and a 
natural means of getting the reaction products out of the way (not to 
mention the fact that the H ions and OH ions will quickly re-form new 
water).

Therefore, I believe that electrolysis will proceed normally, but because 
of the alternating voltages youíll have oxygen and hydrogen being produced 
on both electrodes.

For further reference, you can see that this idea forms the basis for 
measuring reaction kinetics as well as transport properties.  Thereís an 
excellent overall reference for chemical kinetics and electrochemistry at:
 http://www.cheng.cam.ac.uk/research/groups/electroch
em/JAVA/electro
chemistry/ELEC/contents.html


Within this site, they outline a method for examining chemical reactions 
based exactly on the kind of AC oxidation/reduction reaction you asked 
about.  The method is called _cyclic_voltammetry_, and is treated in their 
chapter at:
 http://www.cheng.cam.ac.uk/research/groups/e
lectrochem/JAVA/electro
chemistry/ELEC/l4html/cv.html#cyclic



A less-detailed, slide presentation on different types of voltammetry 
(including cyclic voltammetry) can be found at:
 http://ull.c
hemistry.uakron.edu/analytical/Voltammetry/

For basic electrolysis questions, including the sticky point of what kinds 
of electrodes you need to avoid the complication of competing reactions, 
see:
 http:
//www.madsci.org/posts/archives/may98/893445065.Ch.r.html

 http://antoine.frostburg.edu/chem/senese/101/redox/faq/safe-
water-electrolysis.shtml








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