The Electrolysis of Water

December 6, 2002

This is a simple way to demonstrate Faraday's Law and to find Faraday's Constant

• The apparatus contains two inert (Platinum) electrodes in a dilute solution of sulfuric acid
• An electrical power supply is connected to the electrodes
  • gas bubbles form
  • the apparatus collected the gas in calibrated tubes
• At the cathode the reaction is the reduction of H+ from water
  • 2H₂O + 2e^- --> H₂ (g) + 2 OH^-
  • In an acid solution it might be better to think of this as 2H⁺ + 2e- --> H₂ (g)
• At the anode the oxygen of water is oxidized to O₂
  • 2H₂O ---> 4H+ + O₂(g) + 4e

• Faraday's Law provides a link between chemical stoichiometry and electrical measurements
  • it's basically counting electrons , using as a unit moles of electrons
• The amount of electrical charge (Coulombs) is Current (amperes) x Time (seconds)
  • One mole of electrons has a charge of 96,500 Coulombs

Experimental:

• We will generate a table of Volume of gas vs. time
• We will also determine the average electrical current
  • from PV=nRT we will compute the number of moles of H2 and O2
  • from current vs. time we will compute the number of Coulombs
• We then compute Faraday's constant
  • remember that we need 2 Faradays of charge for 1 mole of H2
  • and 4 faradays for 1 mole of Oxygen

• A few enhancements and explanations
  • We will measure the barometric pressure using the lab barometer
    • (this barometer uses pressure in torr)
  • We will subtract the vapor pressure of water before calculating
    • (about ___ torr of the gas pressure is water vapor, not H2 or O2)
    • This is typically about a _______ correction.

• You will probably notice that the oxygen value is always slightly less than half of the hydrogen reading
  • this is because oxygen is slightly soluble in water
  • once the liquid becomes saturated in oxygen, the gas collects
    • the error is less in the later part of the experiment
    • (it would be preferable to use the slope of the Volume vs. time data)
• hydrogen has such a low solubility in water that we do not see a similar effect in the other tube.