Electrolytes and Electrolysis

In any chemical reaction, the existing chemical bonds are broken and new chemical bonds are formed. Hence, all chemical reactions are fundamentally electrical in nature since electrons are involved in some way or the other in all types·of chemical bonding. Many chemical reactions utilize electrical energy, whereas others can be used to produce electrical energy. As electrical energy involves the flow of electrons, these reactions are concerned with the transfer of electrons from one substance to the other.
Examples of Electrolytes
Strong electrolyte Weak electrolyte Non-electrolyte
Sea water Tap water Chemically pute water
Hydrochloric acid Carbonic acid Alcohol
Sulphuric acid Acetic acid Kerosene
Aqueous copper sulphate Ammonium hydroxide Aqueous sugar solution
Molten lead bromide Citric acid Carbon disulphide
Aqueous sodium chloride Oxalic acid -
Nitric acid - -
Aqueous potassium hydroxide - -
"Electrolysis is the electrolytic dissociation and decomposition of an electrolyte (e!ectrovalent substance), by the passage of a direct current or electricity through its aqueous or molten form
Electrolyte
A compound (mostly an ionic compound) that conducts electricity in molten (fused) or aqueous (solution) state, and which simultaneously undergoes decomposition with the passage of electric current through it is known as an electrolyte.
Electrodes
The two metallic conductors in the form of rods dipped in the electrolyte and connected to the two terminals of the battery are called electrodes.
The anode is the electrode connected to the positive terminal of the battery.
The cathode is the electrode connected to the negative terminal of the battery.
Anions
The negatively charged atoms or group of atoms, which under the influence of an electric field migrate towards the anode (or positive electrode) are called anIons.
Cations
The positively charged atoms or group of atoms, which under the influence of an electric field migrate towards the cathode (or negative electrode) are called cations.
Electrolytic cell
The container or a vessel consisting of the cathode, anode and the electrolyte is called an electrolytic cell. It is also known as a voltameter.
Battery or cell
For a current to flow through a conductor, a difference in charge should be maintained between the two ends of the conductor. To ensure this, a cell or a battery is used. The cell or a battery converts its chemical energy into electrical energy thus maintaining a constant difference of charge between any two points.
Simple Voltaic Cell
In a simple voltaic cell a zinc plate and a copper plate are immersed in a very dilute solution of sulphuric acid, taken in a glass vessel. These two metal plates are called electrodes and the solution is called the electrolyte. The copper plate and the zinc plate are externally connected to a bulb as shown in the figure
Zinc loses electrons more readily than copper, as it is more electropositive than copper. Each zinc atom loses two free electrons and hence gains two units of positive charge.
Zn ------ Z2+ + 2e -
As copper is lower in the metal activity series, it does not undergo a similar reaction and hence remains a region of deficiency of electrons. So, when the zmc plate and the copper plate are connected by means of a metal wire, free electrons from the zinc plate flow to the copper plate and the bulb connected externally starts glowing.
Some of these electrons get added to hydrogen ions of the sulphuric acid of the electrolyte, to form free hydrogen.
It may be noted that in a voltaic cell, that the direction the flow of electrons is not affected if the position of the two metals is reversed.
As zinc metal is oxidised to zinc ions, oxidation occurs here. The electrons given up by zinc leave the cell from this electrode and travel the external circuit to copper electrode. Here, the electrons enter the cell and bring about the reduction of the hydrogen ions of the sulphuric acid. Thus an electric current is delivered as a result of a spontaneous oxidation reduction reaction.
Applications of Electrolysis
Electrolysis has various applications. Some of them are mentioned below:
(a) Electroplating or coating of metals.
(b) Electrorefining or purification of metals.
(c) Electrometallurgy or extraction of metals from metallic ores.
Electroplating
Electroplating is a process of depositing a thin layer of a fine and superior metal (like chromium, zinc, nickel, gold etc.) over the article of a baser and cheaper metal (like iron, copper, brass), with the help of electric current.
Uses
Electroplating is very useful because ofthe following reasons:
- Surface protection e.g. nickel plating of iron to prevent corrosion.
- Makes the article attractive e.g., electroplating of silver or gold on brass etc.
- Repair of finer machine parts.
Process
T4e process of electroplating involves the following steps:
- Before electroplating the metal surface is cleaned thoroughly. Firstly, an alkaline solution is used to remove grease and then it is treated with acid to remove any oxide layer. It is then washed with water.
- The article to be electroplated is made cathode since metallic ions are positi ve and thus get deposited on the cathode.
- The anode is made of pure metal, which is to be coated on the article.
- The electrolyte is the salt of the metal to be coated on the article.
- A direct (D.C.) current is passed through the electrolyte. The anode dissolves, depositing the metal ions from the solution on the article in the form of a metallic coating. The passage of low current is continued for a long time to ensure an even coating.
- To obtain a thin, coherent and bright deposit, the conditions oflow current density, optimum temperature and low metal ion concentration are found to be helpful.
- The choice of electrolyte for use in the electroplating bath is very important. A good electrolyte should have the following characteristics.
- It should be highly soluble in water.
- Its solution should be a reasonably good conductor of electricity.
- Its solution should be stable towards oxidation, reduction and hydrolysis .
- It should be reasonably priced.