Electrolytic Cells


The process of driving a chemical reaction through the use of electricity. They occur in electrolytic cells which are opposites of galvanic cells. The anode in an electrolytic cell is positively charged and the cathode is negatively charged.

In contrast to galvanic cells where reactions between a strong oxidant and strong reductant occur, electrolytic cells are reactions that may involve weak oxidants and weak reductants. Even though under normal circumstances, this would lead to a non-spontaneous reaction, electrical energy is being applied to the cell which allows for such reactions to occur. It is important to note that molecules that are stronger oxidants/reductants compared to others are still preferentially reacted before the other species.

Electrolytic cells using molten electrolytes

Having molten electrolytes means having some reactants in a liquid state rather than an aqueous. Being molten means that no water is present unlike aqueous solutions. This can be advantages if water would be reduced/oxidised preferentially to your reactants. However, the disadvantage of using molten electrolytes is the energy expenditure required to convert reactants into a molten state.

Down Cells

Down Cells are an example of a commercial electrolytic cell which utilises molten electrolytes to ultimately produce chlorine gas and Na(l) from NaCl. The two half equations for the reaction that occurs in a down cell are

2Cl^{-}(l)\longrightarrow Cl_{2}(g)+2e^{-} and Na^{+}(l)+e^{-}\longrightarrow Na(l)

It is essential for the reactants to be in a molten state because water is both a stronger oxidant and reductant compared to both the reactants and will thus preferentially react first. There is no contact between products because it will result in a spontaneous reaction reverting the products back to the original reactant.

Hall-Heroult Cell

Hall-Heroult Cell is another commercial cell utilising molten electrolytes but to produce aluminium from aluminium oxide (Al2O3). It I first dissolved in cryolite, a solvent, because Alumina without a solvent melts at 2050degrees and makes it incapable of being used as an electrolyte. The half equations for a Hall-Heroult Cell are:

C(s)+2O^{2-}(cryolite)\longrightarrow CO_{2}(g)+4e^{-} and Al^{3+}(cryolite)+3e^{-}\longrightarrow Al(l)

Just like with other electrolytic cells which require molten electrolytes, no water is present as it will be oxidised/reduced before the reactants do.

See Also