What Is Electrolysis?

Electrolysis 

Daniel Belihu

What is Electrolysis?

Electrolysis is the process by which ionic substances are decomposed (broken down) into simpler substances when an electric current is passed through them. It is often used to obtain chlorine, hydrogen, sodium hydroxide and to purify sodium and copper metals.

For a basic understanding of electrolysis, there needs to be an understanding of ionic substances. These are the substances that form when a metal reacts with a non-metal. They contain ions, which are charged particles. An example is Zinc Chloride, which forms when zinc reacts with chlorine. This contains zinc ions (positive) and chlorine ions (negative).

Ions must be free to move in order for electrolysis to work. This occurs when the ionic substance is dissolved in water or molten. This solution then becomes an electrolyte. The ionic substances in a solution break down into elements during electrolysis. Different elements are released depending on the particular ionic substance. This occurs in a redox reaction. Either oxidation (loss of electrons) or reduction (gaining electrons). Reduction occurs at the cathode (negative electrode), where positively charges ions gain electrons. The electrons are then said to be reduced. During oxidation, at anode (positive electrode), negatively charged ions lose electrons. The electrons are then said to be oxidized. [pic 1]

The process of electrolysis is carried out in a type of electrochemical cell known as an electrolytic cell. These cells consist of pair of electrodes connected to a DC electric power supply. When the electrodes are dipped into a molten or dissolved electrolyte the energy of the applied DC electric potential causes an otherwise non-spontaneous redox reaction to occur in a galvanic cell as in the galvanic cell the energy available from a spontaneous redox reaction is used to produce an electric potential. [pic 2]

Components of an Electrolytic cell

•DC power supply (Voltmeter) – this draws electrons away from the electrode, the anode forces them onto the other electrode, the cathode of the electrolytic cell. It is the applied DC voltage that provides the energy needed to cause oxidation to occur at the anode and reduction to occur at the cathode.

•Anode – oxidation occurs here. This is the electrode that is attached to the positive terminal of the DC power supply. It conducts electrons out of the cell towards the positive terminal of the power supply.

•Cathode – Reduction occurs here. This is the electrode attached to the negative terminal of the DC power supply. It conducts electrons from the power supply into the electrolyte

•Electrolyte – electrodes are immersed in a molten electrolyte. Anions in the electrolyte are attached to the anode while cations move to the cathode.

Electrolysis is a very useful chemical technique for the extraction of metals like aluminium and gold, production of substances like chlorine, sodium, sodium hydroxide and for applications such as silver and chrome plating of metals.

Electrolysis of a molten salt

This process uses an electrolytic cell consisting of a pair of inert electrodes (e.g. platinum electrodes) dipped into a molten salt and connected to DC electric power supply.  Applying a DC voltage to the electrodes causes theses to become oppositely charged. If the applied voltage is sufficiently high, then negative ions from the molten salt migrate to the positive electrode where upon contact with the electrodes they lose electrons to become oxidized. Simultaneously, positive ions from the salt migrate to the negative electrode where they gain electrons to become reduced.[pic 3]

Electrolysis of aqueous solutions  

It is a more complex problem to predict the electrode reactions that occur for electrolytic cells containing an aqueous salt or having electrodes that are potentially reactive. In these cells water is often oxidized or reduced or the anode itself may be oxidized.

• If water is oxidized, then a colourless gas is formed at the anode and the solution of the pH falls in the vicinity of the anode.[pic 4]
• If water is reduced, then a colourless gas is formed at the cathode and pH rises in the vicinity of the cathode.
• If a metallic electrode is oxidized, then the anode is seen to dissolve and lose mass.

Reactions occurring during the electrolysis of an aqueous solution can be predicted from the standard potential data. As with a galvanic cell, the most probable electrode reactions in an electrolysis cell will be the ones with most positive voltage. These predictions are limited to situations involving the standard conditions of temperature, pressure and concentration. 

History [pic 5]

Electrolysis was first discovered in the 1800’s. After the invention of the voltaic cell by Alessandro Volta the same year chemists used a battery and placed the poles in a container of water. There they discovered that currents flowed and that hydrogen appeared at the electrodes. They did the same thing with different solutions of solids, and also here they discovered that current flowed and that the parts of the solid appeared at the electrodes.

Two electrode theories emerged. One was based on an idea suggested by Humphrey Davy.

Later, Humphrey Davy’s assistant, Michael Faraday, started doing experiments on electrolysis. He wanted to know if current would flow in a solution even when the poles of the battery was removed and electricity was introduced to the solution through a spark. What he found out was that there was a current in the electrolytic solution even if both or one of the electrical poles were out of the solution.

Faraday’s experiments showed that the solution itself was part of the current in electrolysis and it led him to the ideas of oxidation and reduction. His experiments also made him have the idea for the basic laws of electrolysis.

Modern day uses of Electrolysis

People use electrolysis commonly to improve their personal appearance, but the process is also used in manufacturing. The uses of electrolysis are limited by the chemical bonds that may be constructed or destroyed.[pic 6]

The most common use of electrolysis is hair removal. This type of electrolysis is used for aesthetic purposes. It works by destroying the hair follicle producing the unwanted strand of hair. People often choose electrolysis for hair removal for this reason because the destruction of the hair follicle results in more permanent hair removal. It isn’t the best for removing large amounts of hair, as it requires treating individual follicles.