# How many joules of energy are required to convert 1 liter of water into hydrogen and oxygen, theoretically? [closed]

I've been reading http://en.wikipedia.org/wiki/Electrolysis_of_water and other related questions on physics.stackexchange, but I fear I don't have a decent enough understanding of math and science to understand and decipher into laymen terms.

I basically just want to know how much energy in watts or joules it takes, taking efficiency out of the equation (assuming the perfect, yet realistically possible method / voltage / etc), it takes to convert 1 liter of water into hydrogen and oxygen (electrolysis).

## 1 Answer

If you assume perfect efficiency, then the energy of dissociation of a liter of water is computed as follows:

1 liter of water, molar mass 18g, => 55.6 moles

The energy needed is 237 kJ per mole (from your link - see under "thermodynamics").

237 * 55.6 => 13.2 MJ of energy for a liter of water.

In terms of power, this is 3.67 kW for one hour.

This shows you that the energy density of hydrogen (in terms of energy stored per gram) is very very high: it is why people have from time to time had such high hopes for a "hydrogen economy" which could leverage this energy density (much more than the energy per gram of gasoline). But storing the stuff safely and cheaply is remarkably difficult...

• Thanks so much. So, if one was to consider electrolysis followed by burning the resulting hydrogen and oxygen in a sealed environment as a means to produce fresh water from dirty/salty water (as opposed to simply distilling, etc.), they'd basically need a nuclear power plant to make any real headway :/ – orokusaki Aug 15 '14 at 17:22
• Distillation followed by condensation is much, much more efficient than electrolysis followed by burning - see for example clarowater.an/about.html for a real world example of such an installation. Of course the burning could generate heat which drives an engine which generates electricity which dissociates more water... but the inefficiencies in that system would make it indeed highly impractical. – Floris Aug 15 '14 at 17:24
• Taking the complicated engineering out of the equation, is it at least materially and economically possible to make such a machine that could reuse enough of the energy of burning to bring it up to the level of efficiency (or better) than distillation (which has no energy recapture potential, and also requires energy for condensation)? In other words, is it only impractical because such a machine would be highly complex or is it pretty much unrealistic to ever make something that efficient? – orokusaki Aug 15 '14 at 17:27
• I would say no - distillation can be quite efficient if you set it up right. See wqpmag.com/water-distillation for an example (98% recapture of energy: 0.12 kWh of electricity per gallon of product water). I don't think an electrolysis solution could get close to this, but I haven't done the detailed analysis. – Floris Aug 15 '14 at 17:31
• 237 * 55.6 is actually 13177.2. – Quora Feans Apr 2 '18 at 16:13