The question comes to mind of how much energy is stored in water if we are going to use it as a fuel. Looking around there are conflicting amounts provided on the internet.
A litre of water contains the equivalent of 1366 litres of hydrogen – which provides the same energy as 0.4 litres of petrol.
2H20 –> 2H2 + 02
Mr of H20 = 18
No. of moles of H20 = 1000/18 = 55.66666
No. of moles of H2 produced = 55.66666
At room temperature and pressure, the amount of H2 (hydrogen) produced: 55.66666 x 24 = 1333.33 dm^3 where 1 dm^3 is 1 litre of gas = 1000 m^3 of gas
1 liter of water contains (approx) 55.56 moles of water, so 111.11 moles of hydrogen.
Using the ideal gas equation, PV=nRT at 1 atmosphere (sealevel), and room temperature (22C, 295K)
V = 111.11 * 0.08205784 * 295 / 1
Gives 2689.7 liters of hydrogen.
The amount of hydrogen extracted from a gallon of water can be found very easily using the molecular weight of H20 (water), Hydrogen and Oxygen, along with mass conservation. the molecular weight of water is 2 H (molecular weight 1) + 1 Oxygen (Molecular weight 16) for a total of 18. And for every Molecule of water converted, we would get 2 Molecules of Hydrogen.
So, now the question is, how many molecules of water are there in a gallon of water? The density of water is 1g/(cm3) so in 1 gallon of water ( about 3.785 Liters or 3785 cm3) the mass of the water is, 3785g. 1 mole of 6.02×1023 molecules of water is equal has the mass in grams equal to the molecular weight or 18 grams per mole. so 3785 grams corresponds to about 1.265 x 1026 molecules of water.
Now, if every single one of those molecules were converted into Hydrogen we would get twice as much hydrogen as we had of water. or 2.53 x 1026 molecules of hydrogen. however since hydrogen is a diatomic molecule, meaning that the hydrogen that we talk about is H2, we would get 1.265 x 1026 molecules of hydrogen. at 1 atmospheric pressure and 273K, 1 mole of hydrogen fills approximately 22.4L of volume. so 1.265 x 1026 molecules or about 210 moles, would fill 4707 Liters of volume.
It is known that a gram atom is equal to atomic mass of substance; a gram molecule is equal to molecular mass of substance. For example, the gram molecule of hydrogen in the water molecule is equal to two grams; the gram-atom of the oxygen atom is 16 grams. The gram molecule of water is equal to 18 grams. Hydrogen mass in a water molecule is 2 x 100 / 18 = 11.11%; oxygen mass is 16 x 100 / 18 = 88.89 %; this ratio of hydrogen and oxygen is in one liter of water. It means that 111.11 grams of hydrogen and 888.89 grams of oxygen are in 1000 grams of water.
One liter of hydrogen weighs 0.09 g; one liter of oxygen weighs 1.47 g. It means that it is possible to produce 111.11 / 0.09 = 1234.44 liters of hydrogen and 888.89 / 1.47 = 604.69 liters of oxygen from one liter of water. It appears from this that one gram of water contains 1.23 liters of hydrogen. Energy consumption for production of 1000 liters of hydrogen is 4 kWh and for one liter 4 Wh. As it is possible to produce 1.234 liters of hydrogen from one gram of water, 1.234 x 4 = 4.94 Wh is spent for hydrogen production from one gram of water now.
If you mean hydrogen gas, H2, just naturally present in the water, then essentially none.
If you mean how much hydrogen gas could be generated by the electrolysis of 1 gallon of water, that is a stoichiometry problem. The balanced reaction is:
2H2O –> 2H2 + O2
The mass of 1 gallon of water is 3.7854 kilograms, which is 210.3 moles of H2O. From the stoichiometry, there is a 1 to 1 (2 to 2) ratio of water to hydrogen produced, and so 210.3 moles of H2 will be produced. 210.3 moles of H2 weighs 420.6 grams, or just under one pound.
If you mean hydrogen ions, H+, then it depends on the pH of the water. In perfectly pure, neutral water with pH of 7, then the concentration of H+ is 10-7 moles per liter. In one gallon, there are 3.7854 liters. So in one gallon of pure water, there are 3.7854 * 10-7 moles of H+.
Gasoline has around 44 MegaJoules/kg
H2 has around 121 MJ/kg
So, H2 has 2.75x the energy per weight of gasoline
So, if you assume 20 mpg now @ 60 mph, you burn 3 gallons per hr, which is 0.05 gal/min. Gas is about 4 kgs/gal so you burn 0.2 kg/min. Using the energy density difference (2.75) you get 0.073kg of H2 per min. Assuming room pressure and ideal gas law (sadly H2 is not an ideal gas but lets use it anway, i’m lazy) you get 22.7 liters of gas per 2 g of hydrogen. So, after that bunch of math you get 825 liters/min of uncompressed gas.
This amount of gas would be difficult to store, at 2500 psi this is the equalent of 1 gallon of volume, so it would take a big, heavy tank to hold something useful
The other issue with using hydrogen is how to make it in real quantities. The best available methods right now are electrolosis which uses
Theoretically it takes 32.9 kWh/kg of H2 produced
Typical electric costs in the US are $0.15/kwk so it would cost $4.94 per KG to make it using a large commercial system
An actual system is never this efficient, even big systems are 60 kwh per kg, so I’d double that number.
Tags: e=mc2, Fundamentals by Sparky
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