Electrolysis: Variables & Electric Currents

[he]

Can water molecules be split by simply passing an electric current through water or does it need to be done in a specific manner? Assuming the water solution remains constant, will the results vary depending on materials used for electrodes and does the placement of electrodes affect the outcome?

 

[LeonhardEuler]

I have tried this. All you have to do is dip some wires from the terminals of a 9 volt battery in ordinary water and you can observe bubbles forming at the electrodes. One of the terminals will bubble more than the other, presumably becuse one of them is giving off twice as many moles of gas (hydrogen) as the other(oxygen). It does matter what electrodes you use because ordinary metal begins to corrode after a while. I tried using graphite, and that didn't corrode, but also didn't give of as much gas. I also tried to increase the conductivity of the water by adding salt. It bubbled a lot and produced all this yellow stuff in the water. I wouldn't try that if I was you without being careful, because I'm worried that one of the gasses might be chlorine. If you try this on a metal surface like aluminum, the surface corrodes. It's pretty cool, after a couple of seconds you can burn a hole through aluminum foil.

 

[mrjeffy321]

You can split water by electrolysizing it with an electric current. So in other words, pass an electric current through water, and you will break it apart into hydrogen and oxygen gas.
But there is a catch, water does not conduct electricity. Pure water will not conduct, but if you add electrolytes it will. I assume that the "ordinary" water LeonhardEuler referred to is tap water, which does contain additives (like fluorides), and some of these additives will break into ions when dissolved in solution and allow electric current to pass through the water.

In electrolysis, just about everything in/around/making up the "Cell" as it is referred to matters.
The electrolyte matters, some electrolytes will break into ions more readily than others. Some good electrolytes are Sodium Chloride (NaCl), and Sodium/Potassium Hydroxide (NaOH or KOH), these will readily form ions in solution.
The electrodes matter, a lot. Certain electrodes are more suitable to be used as anodes (+) or cathodes (-) than others. For instance, you do NOT want to use iron/steel as an anode because it will be oxidized in the reaction, and turn into rust and dirty up you water (and stink!). It is much better to use something like graphite as the anode (or more ideally platinum $$$$$), since it won't break down so easily. Mind you, even graphite will erode away with time. For the cathode, you have much more freedom of choice since it is protected by a negative pole, but I myself still use carbon since it is cheap and available to me. Even the placement of the electrodes matter, the more distance you put between them, the more resistance there is in the cell, and since Current goes down when resistance goes up (V = RI), you won't be making as much gas with less current.

There is a minimum voltage needed to electrolysize water, I think it is somewhere around 1.2 volts, but I am too lazy to look up the exact number. But keep in mind, the production of the gasses is dependent on the current through the cell, not the voltage.
Also, if you use NaCl as you electrolyte, you technically won’t be making H2 and O2, you'll get H2 and Cl2, since it is easier to reduce chloride to make chlorine gas than it is to reduce oxygen to make oxygen gas. So in the long run you will be turning your NaCl into NaClO3 and make a lot of hydrogen and chlorine gas.

As you might be able to tell, I have a passion for electrolysis and I have lots of first hand experience with this.

 

[GiTS]

how does pressure affect electrolysis? since the products are separated they cannot reform when pressureincreases and the equilibrium shifts.

 

[he]

Thanks for the answers.

I have another question. If electricity was applied in pulses as apposed to a constant flow, would the outcome be any different?

 

[mrjeffy321]

No, that wouldn't effect the end result any. When the current is on, the reaction occurs, when it is turned off, the reaction stops.

 

[mrjeffy321]

how does pressure affect electrolysis? since the products are separated they cannot reform when pressureincreases and the equilibrium shifts.

I don't think pressure would matter very much. it isn't really a reversible reaction normally when it is done. Most of the time (and in pretty much any practical application), the gasses are separated after being formed (ie. collected at each electrode and not allowed to mix), therefore, the two gasses can't recombine again, and the pressure wouldn't effect the water any, all it would do is reduce the volume of the product gasses.

However, if the gasses were to be allowed to mix and the pressure was increased...I am trying to think, would that reverse the reaction and create water again?
I am inclined to say no since to react Oxygen and Hydrogen together to form water, you need to do more than simply mix the two gasses, you need to supply the activation energy (usually heat) and that sets it off. So I don't think pressure would effect the reaction at all.

 

[GiTS]

But that would mean you have more energy than what you started with. If it takes the same energy to make the same amount of H2 and O2 under pressure as it would under normal pressure, you would end up with extra energy in the form of the potential energy of the pressurized gas.

 

[mrjeffy321]

I see your point, and I may be wrong, but I still don’t think it will affect the electrolysis [right away at least].
Take a container of water set up ready for electrolysis, and then enclose it in some other container so all that nothing can escape. Then run the cell.
It will start out at room pressure; there would be a certain amount of water vapor in the air due to the vapor pressure of water at X pressure, and not very much hydrogen. As the cell continues to run, the pressure will increase as the Hydrogen and Chlorine [or Oxygen] is made. As the pressure increases, the amount of water in the air goes down, leaving more room for the produced gasses. Also as the pressure increases, the amount of gas that can be dissolved in water increases, so the chlorine and oxygen will more readily dissolve in the water (hydrogen too to an extent, but I don’t think it is too soluble). But so far, nothing has happened that should effect the production of gas at the electrodes. When the pressure increases even more, the gasses will continue to push down on the water, but water is not [or only very, very, very slightly] compressible, so although the pressure might be very high above the water, down where the electrodes are, there shouldn’t be a noticeable effect.
Eventually the pressure will reach a point where the hydrogen and Chlorine/Oxygen will react, at least I can’t think of a reason why it shouldn’t. Since as a gas is compressed (pressure goes up), it heats up, the increase in pressure and temperature should eventually cause the gasses to react, thus causing a dramatic increase and then decrease in pressure (explosion!).
So with enough pressure, sure just about anything will begin to react, it might be interesting to calculate just how you would go about doing this, assuming what I just said is correct...how much pressure is need to get to a temperature that will cause the reaction. And then again, I might have missed a step in that process that would change the end result, like the electrolysis stopping early.

 

[Serj]

If the products are kept separate (so they can not react ever) then as pressure builds in the system something has to happen to make up for this gain in potential energy. My chemistry teacher said that it would take more energy to split the water but she was not very knowledged in that particular chapter and the way she answered led me to believe it was merely a guess. But I don't see why it would make a difference, I've seen the equations for cell potential and products formed and no equation involves pressure. So I am left to wonder. If the products were kept separate and the water was and gas were in a closed system able to hold infinite pressure. If you ran the cell and the pressure of the products increased astromically. If at the end you hooked up the pressurized gas to a pressure driven generator. Could you possibly get more electricity than you put into the cell?

 

[mrjeffy321]

Could you possibly get more electricity than you put into the cell?

Well, no, that woud mean breaking the law, and we all know if you break the law...you goto jail.

If you look at this site,
http://230nsc1.phy-astr.gsu.edu/hbase/thermo/electrol.html
You will see it briefly mention something about pressure. The cell must supply the energy to break the bonds and split the water, but it mjust also do work to make and expand the gas.
The Work done to expand the gas is given as, W = PΔV, which will give the needed energy to pressurize the hypothetical container spoke of earlier.
So as the pressure increases, the work the cell must do to expand that gas increases aswell. Usually this eergy is provided by the surrounding environment, but as the pressure increases, it will reach a point where the cell cannot provide enough energy to split the water and expand the gas, so production will slow to a stop.