Electrolysis
Electrolysis is a chemical reaction that occurs when a direct current of electricity is passed through an electrolyte. This produces a chemical reaction at the electrodes that can be observed as bubbles, deposition of metal or a change in color of the electrolyte.
Liquid Electrolytes
A liquid, which conducts electric current because it has mobile ions, is an electrolyte and can be used for electrolysis. Solids, on the other hand, do not conduct electricity because the ions are in fixed positions and cannot move at ordinary temperatures.
In an electrolyte, the ions of an ionic compound undergo reduction or oxidation as they are moved towards oppositely charged electrodes in a reaction that is reversible, and that can be easily controlled by temperature. For example, sodium chloride, a common salt that is dissolved in water and then reacted with electric current, can be reduced or oxidized to produce chlorine gas (Cl2) and hydrogen gas (H2).
Molten solutions of a wide variety of ionic compounds are also commonly used for electrolysis. Examples include sodium hydroxide and potassium carbonate.
Liquid vs. Solid Electrolytes
Liquids are more suitable for electrolysis than solids, because the ions in a solution can move to oppositely charged electrodes and are able to be reacted with electric current. The ions can be either positively or negatively charged depending on the nature of the ionic compound and its concentration in the solution.
An aqueous solution of silver nitrate, AgNO3, is an ideal candidate for electrolysis because silver is very reactive to the metal ions in it and will dissolve in the electrolyte. When silver is electrolyzed, the ions will migrate from the anode to the cathode and get reduced. The ions will then migrate back from the cathode to the anode and be deposited on it. This process is often referred to as the reverse electrolysis of silver nitrate and can be applied to purification of impure samples of metals such as lead or nickel.
Cations and Anions
The cation of sodium chloride, Na+, is more likely to be reduced than the anion, Cl-. This is due to the higher overpotential for oxidation of Na+ ions than for oxidation of Cl- ions.
Aqueous Solutions and Water Electrolysis
In electrolysis, a solution of water is decomposed to produce hydrogen and oxygen in a proportion of 2 to 1. This ionization of aqueous solutions is a useful disinfectant.
A battery that can generate the reversible potential needed to split the aqueous solution of water is necessary for electrolysis. The cell voltage must be at least 1.7 V to ensure that the reversible potential can be achieved without irreversible processes occurring. This potential is determined by a combination of the electric energy required to reduce or oxidize the cations and the ohmic resistance of the electrodes.
Teaching Electrolysis
To make the process more interesting, teachers should introduce a fun activity to help students understand how the reaction takes place. For instance, they can draw a process diagram or write a chemical equation to explain what is happening during the reaction. They can also show students a video that illustrates the redox reaction taking place at each end of the cell and what products are produced.
