Chemical Forums
Specialty Chemistry Forums => Citizen Chemist => Topic started by: P-man on December 18, 2005, 09:38:40 AM
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Can the process of electrolysis (of water) be sped up by a catalyst? Could the reaction be made more efficient by one?
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http://en.wikipedia.org/wiki/Catalyst (http://en.wikipedia.org/wiki/Catalyst)
Water electrolysis is just a chemical reaction, so everything in the article above will hold.
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What can I use?
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yeah the use of a catalyst should make the process more efficient, it will speed it up and not get used, not sure about this but i heard somewhere that it takes about 44 Kj to break one more of H2O with a catalyst compared to 66 Kj.
anything that will not get used up during the process but speed it up is a catalyst. you could several kinds of metals, you could even some dilute amounts of acid to water to speed up the process, atleast that is what my teacher told me.
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Adding acid makes it faster because there are more ions, and therefore more current flows through the water. I'm not sure if that counts as a catalyst though..
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probably not, but as you said it will make the reaction faster.
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If you can recover the acid after the electrolysis has taken place, then yes, it is a catalyst. A catalyst is defined as a substance which speeds up the rate of a reaction without being consumed in the overall reaction itself.
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The acid looks interesting, as long as it doesn't affect the gases in any way. They need to be as pure as possible.
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Is there a book called CRC that has tables of various electrolysis catalysts activities?
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The acid looks interesting, as long as it doesn't affect the gases in any way. They need to be as pure as possible.
this is were it depends on what you will be using to carry the electrolysis process. some catalysts are not good to work around impure substances because they will be dissolved in your product, and that is the reason that platinum is so widely used as a catalyst, it tends to leave the impurities behind in the electrolysis process/
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If you can recover the acid after the electrolysis has taken place, then yes, it is a catalyst. A catalyst is defined as a substance which speeds up the rate of a reaction without being consumed in the overall reaction itself.
I do not fully agree with the acid being a catalyst. A real catalyst has a uniform effect, with net zero-consumption locally at any location in the reaction mix.
With electrolysis this is not the case. Suppose we have electrolysis of water with dilute HNO3. At the anode we have the reaction:
2H2O - 4e --> 4H(+) + O2
At the cathode we have the reaction:
4H2O + 4e --> 4OH(-) + 2H2
So, the net reaction indeed is decomposition of water to H2 and O2, the acid (or even more generally, the inert electrolyte) being the compound, which assures that the reaction proceeds at acceptable rate.
However, locally, there is definite change of the liquid. Near the anode, the liquid becomes more acidic and near the cathode, the liquid becomes less acidic. This effect can be demonstrated nicely if a solution of NaNO3 is used for electrolysis with an indicator in solution, which is colorless at pH near 7, red at pH above 11 and blue at pH below 3 (a mix of phenolphtalein and bromothymolblue is suitable for this). Graphite rods need to be used as electrode. At the anode, the liquid becomes acidic and turns blue and at the cathode the liquid becomes pink/red. When the liquid is shaken again, the liquid becomes colorless again, because of precise neutralization of the OH(-) and H(+) ions. With suitable membranes between anode and cathode area, the liquids start to differ more and more in the different compartments.
So, you see that the electrolysis really changes the properties of the liquid and electrolyte. Only if they are mixed well these changes counterbalance precisely. Jdurg, would you really call this a catalyst?
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Yet another thing:
User 3.3.141592653 mentioned that without catalyst more energy is needed than with a catalyst. A catalyst does NOT change the amount of energy, released or taken by a reaction, it only changes the pathway of the reaction. Sometimes, the use of a catalyst makes a reaction appear more energetically, but this only is apparent.
As an analogy I use a fully charged battery. You can discharge it slowly, using a LED, you can also discharge it quickly, using a heavy bulb. With the heavy bulb, it looks more energetic, but the time that this energy can be released is shorter than the time that the LED can glow from the same battery. The peak power was larger, but the total energy was the same.
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I do not fully agree with the acid being a catalyst. A real catalyst has a uniform effect, with net zero-consumption locally at any location in the reaction mix.
However, that is not the definition of a catalyst. Look at the production of Nitric Acid. At one point, ammonia is passed over a platinum gauze catalyst. The reaction, however, only occurs locally at the point where the ammonia and oxygen contact the platinum gauze. Everywhere else the reaction is not sped up at all, yet the gauze is considered to be a catalyst.
Thinking about it a bit further, in reality during the electrolysis of water the electrons themselves are a catalyst. They catalyze the reaction of 2H2O => 2H2 + O2. In the end, all of the electrons are recovered and not "used up" by the process.
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Really good point jdurg. That's some smart thinking. I never would have thought as the electrons as being a catalyst in a sense, but your are right.
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However, that is not the definition of a catalyst. Look at the production of Nitric Acid. At one point, ammonia is passed over a platinum gauze catalyst. The reaction, however, only occurs locally at the point where the ammonia and oxygen contact the platinum gauze. Everywhere else the reaction is not sped up at all, yet the gauze is considered to be a catalyst.
Thinking about it a bit further, in reality during the electrolysis of water the electrons themselves are a catalyst. They catalyze the reaction of 2H2O => 2H2 + O2. In the end, all of the electrons are recovered and not "used up" by the process.
I still do not feel comfortable with the definition. I agree with the example you give. I also call his gauze a catalyst. I did not word it precisely enough in my previous post. The reaction at the gauze still is such that no gauze-metal is consumed, while in the electrolysis reaction, H(+) ions are produced on one side, and OH(-) ions are produced on the other side.
The example with the electrons also is not a real good example to my opinion. At one side, electrons are consumed and at the other side, electrons are produced (by the power supply). You can even demonstrate this nicely. Split up your electrolyte in two beakers and between the beakers, insert a bridge, not a salt bridge, but a large electrolytic capacitor, one end dipped in one beaker, and the other end dipped in the other beaker (if you do this experiment in reality, please be careful about polarity of the elco, otherwise it may explode!). You'll see a positive charge buildup in one beaker and a negative charge buildup in the other beaker, accompanied with temporary evolution of gas at the electrodes. How this charge is built up, depends on the electrolyte and it most likely will be in the form of positive ions in the anode beaker and in the form of negative ions in the cathode beaker.
Now, if you remove the power supply, but leave the capacitor in contact with the two beakers, and then close the circuit between cathode and anode, then you'll observe formation of gas in both cells. Ions loose electrons and H2 is formed and O2 is formed. In order to see this, you need a large elco, 100000 uF / 16V would be nice, use a 12 V power supply.
So, electrons really are consumed and produced. A real catalyst is different. Imagine a sphere, of any size, placed at any position in 3D space. Whatever the placement of the sphere and whatever its precise size, inside such a sphere, there is an amount of catalyst. The amount of catalyst inside the sphere, as function of time, ONLY depends on the flux of catalyst through the surface of the sphere. In mathematical terms, the catalyst is conserved locally. Just perform the thought experiment, place your little spheres anywhere and with a normal chemical reaction, you'll quickly see that nowhere in space, catalyst is produced or consumed. Of course, in the chemical reaction mechanism it will be produced or consumed, but the net amount is constant (of course disregarding inward or outward flux, due to convection). In the case of electrolysis, this cannot be said. Electrons are produced somewhere, and consumed at another place.
I think that we agree on the actual mechanisms and the chemistry behind it. It is a matter of wording and definition. I just post this lengty post here, in order to make clear the subtle difference.
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But you see, it is in my opinion that electrolysis involves both the anode and cathode combined. As a result, the combination of both the anode and cathode reaction results in the overall reaction. It's akin to writing out half-reactions instead of full reactions. The main reaction is the full thing, but if you separate it into two half-reactions then you can argue that in reality it's two different things.
I think a close example would be the addition of an acid to a bicarbonate solution. The overall reaction is -HCO3 + H+ => H2O + CO2.
However, you could also write it as two separate reactions:
-HCO3 + H+ => H2CO3
H2CO3 => H2O + CO2.
As is most things in life, it's just a matter of semantics. ;D I look at electrolysis as the combination of the anode and the cathode half-reactions since if you just stick the anode into solution and not the cathode no real reaction would take place.
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http://en.wikipedia.org/wiki/Electrolysis
Distilled water with a few drops of Sulfuric acid was a common method.
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Yes, I thought the acids were good. You see, scientists at the nickel instittute have made a Rainey-nickel catalyst that is soooo muche cheaper and is just about as good with impurities as is platinum. If that catalyst is used inboth the fuel cell and electrolysis, the costs could be lowered greatly.