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Electrolysis of water

  1. Sep 30, 2006 #1
    Hi, I have an apparatus which I'm using to electrolyse water into a mixture of hydrogen and oxygen.

    I'm attempting to do some calculations based upon measurements I've made. What I want to do is come up with a Joules per Litre figure for energy consumed to evolve a volume of the mixed gas.

    I'd also like to do some theoretical calculations in order to estimate what the expected minimum J/l would be based on known theory of electrolysis of water.

    At present I have both a theoretical and a measured figure, but I'm not too sure of my workings, so I'm posting them here in the hopes that someone will be able to point out if I've got it wrong.

    The theory:
    Assume electrolysis of water requires 237.13 kJ/mol. Assume 1 mol of Hydrogen gas occupies a volume of 22.4 litres at STP. Also assume 1 mol of Oxygen gas occupies the same volume at STP. Assume the electrolysis of 1 mol of water produces 1 mol of hydrogen and 1/2 a mol of oxygen. So, we expect that we'll use 237.13 kJ to produce 33.6 litres of the mixed gas. My calculations show this implies 7057 Joules per litre.

    The experiment:
    The electrolyser draws 3.73 Amps of current at 10.76 Volts, for a power total of 40.1 Watts. While consuming this power it produces 50 ml of gas in 60 seconds. This implies 2406 Joules were used to produce 50 ml of gas. This comes out to 48,120 Joules per litre.

    So, does all that look to be correct? I wouldn't be surprised if I've made some fundamental mistake, especially with the theoretical side, as I'm unsure of the figures I have there anyway. At present it looks like I'm using 6.8 times as much energy as I should have to, which seems a little excessive. I think this implies an efficiency of only 15%?
  2. jcsd
  3. Oct 14, 2006 #2
    The calculations appear to be correct. Mind you I only have up to grade 11 chemistry so I'm assuming everything you've assumed in the first paragraph.

    To make it more efficient have you tried adding a catalyst to the water? I'm guessing the lost joules are from heat generated by the resistance of the water.
  4. Oct 15, 2006 #3
    Not that you are wrong in anyway but I thought that this website would be quiet good: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/electrol.html

    I will let you see if you are right or not.

    The Bob (2004 ©)
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