Unit 8: Electrochemistry Applications Lesson 6: Electrolysis

Electrolysis Electrolysis is the process of supplying electrical energy to a non-spontaneous chemical reaction so that it can occur. An electrolytic cell is an apparatus in which electrolysis can occur.

Electrolysis of a Molten Binary Salt When a binary salt (a salt consisting of two different elements) is melted, its ions are mobile and can be made to react. Since the reaction is non-spontaneous, no salt bridge is needed to separate the reactants. Electrodes are made of inert materials such as C or Pt.

In the example below, the only reactants present are Na+ and Cl-. Looking at the table, we can find that E°cell is -4.07 V and thus the reaction between Na+ and Cl- is non-spontaneous. Hence, to make this cell operate, at least 4.07 V must be applied.

Practice: Pg. 238 #64 (a and b) 10 min

Brain Break! Spot It!

Electrolysis of an Aqueous Salt Set-up is similar to that with a molten salt (no salt bridge, inert electrodes), but now H2O is a possible reactant, and must be considered! The same rule as with electrochemical cells applies: the species with the greatest tendency to reduce and to oxidize will react preferentially.

The Overpotential Effect When electricity is being applied to a cell, reactions involving water have a different tendency to occur than when reactions are spontaneous. This is called the overpotential effect and is shown on your table. Be sure to take this into account!

Example What is the overall reaction and the minimum voltage that must be applied when a 1 M HCl(aq) solution is electrolyzed using carbon electrodes? 10 min

Practice: Pg. 242 #65 10 min