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Regarding consevation of energy on electrolysis

  1. Oct 1, 2008 #1
    Hi there. I'm a Brasilian high school student. Some time ago we studied electrolysis reactions (rather fast and less deep than I expected, but anyway) and one question that popped into my mind some time later, when taking some electricity principles into account, was the following:

    Faraday's law states the amount of product obtained from a electrolysis reaction is directly proportional to the amount of charge that we flow through the system, and thus the amount of work (chemical potential energy that we get on the product) depends only on, and only on the amount of charge flowing trough the system too.

    From the very concept of Volt, we have that V=J/C, which means that, at a potential difference of 1 Volt, for every Coulomb of charge that is forced to oscillate on the system we'll receive one Joule of energy. That means that the work realized by the system does not only depend of the charge flow, but also on the voltage.

    Now, suppose that we have a electrolysis experiment. By lowering the voltage there would be, even if little, some current flowing on the system. Now, imagine that we lower the voltage so much that, the amount of work done by the charge flow according to the concept of voltage is lower than then amount of chemical potential energy that we receive on the product...Hey! that goes straight against the concept of conservation of energy!

    So, either nature has a bug or there's a big, big flaw on what I just wrote. What could it be? Maybe if the voltage is too low the reaction doesn't start? Any comments are welcome!

    Thanks in advance for the help!
     
  2. jcsd
  3. Oct 2, 2008 #2

    Borek

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    Staff: Mentor

    That's for sure. Minimum voltage is given by the Nernst equation (which in turn uses standard electrode potential, unique for each system).
     
  4. Oct 2, 2008 #3

    russ_watters

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    What you are missing is that in a circuit with a set resistance, lowering the voltage also lowers the current. So if you lower the voltage, you lower the current, and you get less hydrogen production.
     
  5. Oct 2, 2008 #4
    Yes, lowering the voltage will result in a lower current (I did not forget Ohms law). However, I'm not interested in the total amounts of current or work on the system, but on the fact that upon lowering the voltage no matter how much there will still be, even if a little, current going through the system. And now we get back to the amount of work done per unit of charge according to the concept of voltage and according to the chemical energy we obtain on the products (thou now I believe it might really be that, if there's not enough voltage, the reaction wont start).
     
    Last edited: Oct 2, 2008
  6. Oct 3, 2008 #5

    Borek

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    Voltage needed is given by the Nernst equation. It may happen that you will need some overpotential for the reaction to proceed, but this will be energy lost (ie not conserved in your product).
     
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