Electrolysis I

Relationship between Faraday's Constant and Avagadro's Constant

F: Faraday's Constant = 96500C
L: Avagadro's Constant = 6e23
e: charge of an electron = 1.6e-19

Basically, Faraday's Constant is a quantity that tells us how much charge does a mole of electrons possess. Recall that one mole is 6e23 which is also the Avagadro's number. Thus the relation F=Le

Prediction of the substance liberated during electrolysis
The ion that is discharged at the electrodes during electrolysis is affected by 3 main factors. They are the state of electrolyte (molten or aqueous), position in the redox series and concentration of the ion in the electrolyte.

State of electrolyte
If the electrolyte is aqueous, hydrogen ions (protons) and hydroxide ions are present. Due to the fact that hydrogen ions and hydroxide ions are positioned quite low in the redox series, they are usually preferentially discharged. If the electrolyte is in molten form, hydrogen ions and hydroxide ions are not present.

To illustrate this, let us consider the different products produced during the electrolysis of molten NaCl and aqueous NaCl.

During the electrolysis of molten NaCl, molten Na metal and chlorine gas is produced. Na+ is attracted to the negative electrode (cathode) and chloride ions is attracted to the positive electrode (anode). At the cathode Na+ ion gain an electron to form the molten Na metal. At the anode, Cl- loses an electron to form chlorine gas.

On the other hand, during the electrolysis of aqueous NaCl, hydrogen gas and oxygen gas
are produced. Both hydrogen ions and sodium ions are attracted to the cathode, however hydrogen ion is preferentially discharged. Hydrogen ion gain an electron to form hydrogen gas. Both hydroxide ions and chloride ions are attracted to the anode. Hydroxide ions are preferentially discharged at the anode due to its lower position in the redox series.