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Author Topic: How much energy does it take to split water?  (Read 18378 times)

Offline industry7

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How much energy does it take to split water?
« on: 21/10/2008 19:28:12 »
I should start off by saying that I completely expect to learn tons if a good discussion develops so I'm not assuming to know everything on this subject, and further I'm willing to admit that what I do understand about this subject may not be completely accurate.  With that out of the way, here's the situation:

So a friend and I have had several conversations recently which touched on subjects like splitting water through electrolysis, hydrogen fuel cells, and using catalysts to reduce the energy needed to make a reaction happen.  In the end we it always becomes the exact same conversation.  Eventually he says that once we can split water efficiently enough, we could use water as a fuel source.  Then I say that's impossible, b/c it would allow you to build a perpetual motion machine.  Now normally he's the one yelling at me about the laws of thermal dynamics, which is why it's been extra difficult for me to deal with his insistence.

At first I tried to explain that the amount of energy needed to split water will always be greater than the amount of energy obtained from recombining them, and therefore water can't be a source of energy.  His response is that there may be some yet undiscovered catalyst which will lower the amount of energy needed to split the water.  He then further elaborated saying that we may some day find a material which you could simply throw water at and through catalytic action, the water spontaneously splits into hydrogen and oxygen, which you could then run through a fuel cell to generate power, thereby utilizing water as a fuel source.  My response was that if splitting water really took as little energy as in the scenario he just described, then our current laws of thermal dynamics would have to be rewritten b/c that situation would allow you to build a perpetual motion machine.  His response to that was that we already have catalysts which greater reduce the energy requirements for breaking hydrogen bonds (using chemicals other than water) so there's no reason that catalysts he described couldn't be made, and that it would not allow perpetual motion machines b/c there would still be inefficiencies in the system that keep the system from being over unity.

At some point I realized that maybe what he meant is that water could be used as a main component in an energy storage medium (a battery), but eventually he went back to saying that water could be a fuel all by itself.  I feel like he must be misunderstanding something, and even worse I'm sure he thinks I'm the one who doesn't understand.  But we're both smart people and this isn't a matter of opinion, it's science and facts.  So first I need some help to understand the subject matter better myself, and then I'll probably need some more help with communicating the main ideas to him.

Overall I've been finding it inexplicably difficult to find basic simple information on this subject.  For example: 2H20 + some amount of energy -> 2H2 + 02.  That's how we split water.  Now my question is, how much energy did that take?  More specifically, how much energy does it take to start the reaction (activation energy/cost/whatever), and how much energy is absorbed when the reaction takes place?  Then I have the same questions for running the reaction in reverse (combining them, or "burning" hydrogen).  Next, does activation energy change with tempuraure/pressure?

Also, what is the efficiency splitting water through basic electrolysis (drop some carbon electrodes hooked up to a car battery into a bucket of tap water)?  And what is the most efficient way we currently have for splitting water and how efficient is that?  Finally, I need to make sure I understand some basics about catalysts.  This is what I "know" about catalysts:
1) A catalyst lowers the amount of energy needed to get a reaction started.
2) Once the reaction starts the catalyst doesn't affect the amount of energy released/absorbed by the reaction.
3) The catalyst isn't actually part of the reaction ie. it isn't used up, and can be used to catalyze more reactions in the future.

Am understanding catalysts correctly, and am I missing any other important facts about how they work?

So obviously I have a lot of questions, and I'm hoping some of you we'll have some answers for me.  To recap, I'm looking for some basics on the energy involved in splitting water and for recombining those parts, a primer of sorts on the function and nature of catalysts, and some help on how to communicate ideas pertaining to the usefulness of water as a fuel source.

Mod edit - Formatted the subject as a question - please do this to help keep the forum tidy and easy to navigate - thanks!
« Last Edit: 28/10/2008 10:54:44 by BenV »


 

Offline Pumblechook

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How much energy does it take to split water?
« Reply #1 on: 05/11/2008 12:29:53 »
The problem with many energy systems is that long chains of conversions can result it an overall poor figure.

Proponents of a particular technology tend to isolate out one conversion and say it is an efficient system.  Electric vehicles being the worst example and you get silly figures such as 150 mpg quoted even though in the vehicles themselves there is no 'g' (gallons).   They compare the calorific value of fuel  with the energy stored in a battery as if the energy in the battery has appeared by magic. 

4 conversion at 90% (E.G burning fuel - generators - power grid - AC to DC - charging process)  =  66%

4 at 80% =  41%   (0.8 x 0.8 x 0.8 x .8)
 

Offline Bored chemist

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How much energy does it take to split water?
« Reply #2 on: 05/11/2008 18:44:44 »
"So a friend and I have had several conversations recently which touched on subjects like splitting water through electrolysis, hydrogen fuel cells, and using catalysts to reduce the energy needed to make a reaction happen.  "
You can do all the clever tricks you like. The best answer you can hope to get for the question in the thread title is "the same energy that you can get by burning the hydrogen; that is 142J/g" and the real answer is that it takes more because some is wasted.
You are right; if we could do better than this we would have a perpetual motion machine.
 

Offline srobert

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How much energy does it take to split water?
« Reply #3 on: 08/11/2008 01:44:32 »
If you could create a process that was 100% efficient then the energy you get out of "burning" the hydrogen (oxidation) is exactly the same as the energy required to split the water in the first place. This is a consequence of the first law of thermodynamics. A catalyst does not change this basic equation, all a catalyst does is reduce the activation energy, and therefore speed up a reaction that was thermodynamically favourable in the first place. Catalysts only ever make a reaction happen quicker and never affect the energy required to be put in to a reaction or given out by a reaction. In every reaction the energy input required to make a reactio  happen is exactly the same as the energy output from the back reaction and vice versa. The direction a reaction happens in depends on the thermodynamics.

Think of it like this.I'm standing in a field which is 100m above sea level with a football. To get to sea level the ball needs to fall through 100m. To the North of me is a hill which is 100m above my current position. In order to get the ball over this hill I need to give it enough energy to get up 100m, but on the other side it falls through 200m to the sea. At the end its still 100m below where it started (+100m - 200m = -100m). To the South of me the hill is only 50m above my current position. Now I only have to put in 50m worth of energy, it then falls to sea level giving out 150m worth of energy, again the sum come to -100m. All a catalyst does is lower the height of the hillbut the summ of all the changes of height always comes to -100m
 

Offline Minstrelle

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How much energy does it take to split water?
« Reply #4 on: 31/12/2010 20:03:02 »
My friend and I attempted to measure how much energy is required to split water, by means of electrolysis.  Needless to say it didn't go well.. it was around 94 times the value per mole of the formation of water.  Just thought it'd be interesting..
 

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How much energy does it take to split water?
« Reply #4 on: 31/12/2010 20:03:02 »

 

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