How much electricity is required to create the electrolysis necessary to extract hydrogen from water?

Answer 1

"Electricity" is the flow of charged particles (commonly electrons). So there is not an answer to "how much" electricity is needed, However, I am sure that is not what you mean to say.

In order to split water into Hydrogen gas and Oxygen gas, an electric current can be used.
The rate at which the Hydrogen gas is produced is directly proportional to the electric current (units of amps) used through the electrolytic cell. Theoretically, there is no "minimum" current needed, it is just that higher currents will produce Hydrogen faster (higher rate of production) than lower currents.

There is, however, a minimum voltage (potential different) which must be applied to the electrolytic cell in order to facilitate breaking the bonds between the Hydrogen and Oxygen.
The minimum voltage can be calculated by knowing the reduction potentials of Hydrogen and Oxygen.

The electrolytic reaction which occurs to split water is,
2H2O (l) --electrolysis--> 2H2 (g) + O2 (g)
But you can break this down in to an oxidation and reduction reaction occurring at the anode and cathode of the electrolytic cell,
2 H2O(l) --> O2 (g) + 4 H+ (aq) + 4 electrons
2 H+ (aq) + 2 electrons --> H2 (g)

You can see that for every 1 mole of Hydrogen gas produced, 2 moles of electrons (AKA, 2 Faradays of electrons) must pass through the cell. This is why the current is the determining factor of reaction speed.

Also, remember Ohm's law,
V = RI
Where V is the voltage, R is the resistance, and I is the current.
With a given resistance, a higher voltage will produce a higher current. Also, with a given voltage, a lower resistance allows a higher current.
Sometimes (depending on the electrolyte used) a higher voltage is not desirable, even undesirable in some circumstances, so it is best not to use a higher voltage than what is necessary.

Answer 2

The amount of energy needed is equal to the Gibbs Free Energy of the reaction.

The formula for one mole of H2O is:

ΔG = ΔH - TΔS = 285.83 kJ - 48.7 kJ = 237.1 kJ

To express this as watt/hours, you multiply 237.1 kJ times a conversion factor of 0.2778, which gives 65.87 watt/hours of electricity.

To give you an idea of the inefficiency of the reaction, the same amount of produced gases reacting in an "ideal" fuel cell will result in TΔS 48.7 kJ less energy output. You will lose
almost 20% of the energy input to decompose the water.

To give you an idea of how much electrical current you will need:
2 H+ + 2 e- ---> H2 Eo = 0.00 volt
O2 + 4 H+ + 4 e- ---> 2 H2O Eo = 1.23 volt

For platinum electrodes at a current density of 0.01 amp/cm2, an overvoltage of 0.07 volt is required for evolution of hydrogen, and 0.40 volt for oxygen evolution. So you must apply at least 1.23 + 0.07 + 0.40 = 1.70 volts at 0.01 amp/cm2 to make the reaction take place. For these numbers I am assuming you are using platinum electrodes and 0.5 M H2SO4 as an electrolyte solution. The cm2 unit is the surface area of your electrodes.

See this site for a longer description of the electrical needs:
http://www.madsci.org/posts/archives/nov98/910748435.Ph.r.html

The temperature of the water, the size and quality of your electrode, the purity and molarity of your electrolyte, the voltage of your power source, and the amperage of your power source will all affect the rate and success of your reaction.

Answer 3

not that much
5 volts is enough if you add ions to the water to make it conductive (some salt will work)

I did it before with 120 volts and too much salt. That caused the salt to separate in to Sodium and Chloride. Then all of the Sodium reacted with the water and melted my equipment. The stainless-steel electrode was eroded.

Do not go over 20V and do not put to much salt in the solution. Otherwise you will have undesired results.

IMPORTANT: DO NOT TOUCH THE WATER IT BECOMES SODIUM HYDROXIDE WHEN THE SALT BREAKS DOWN. SODIUM HYDROXIDE HAS HEALTH SIDE EFFECTS. http://en.wikipedia.org/wiki/Sodium_hydroxide MAKE SURE THAT YOU DISPOSE OF ALL OF THE WASTE PROPERLY. DONT BREATHE THE CHLORIDE EITHER.

I put my hands in the water and got pink spots all over my skin. Not very nice.

Answer 4

Depends on the water's purity.

Answer 5

it all depents of the amout of water you have so it is in dircet propotin to mass