Introduction to Electrolysis

Electrolysis is a process by which electrical currents can be passed through solutions. Pure water is a non-conductor of electricity. But if a salt such as NaCl is dissolved in it, the solution will start conducting if two electrodes are placed in it along with a battery that provides the initial current. In the present chapter we will see what is electrolysis and how it is useful in our day to day life.

What we will study in this chapter
1. Process of electrolysis
2. Conductor versus electrolyte
3. Electrolysis of water
4. Definition of standard solutions
5. Faraday’s Law of Electrolysis
6. Application of electrolysis

1. Process of electrolysis
When an electric current is passed through a chemical compound, some compounds are able to conduct electricity. The compound dissociates into ions under the influence of the electric current. The electrical current initiates a chemical reaction or a break up.

To see how electricity is conducted through a solid compound, a molten compound and an aqueous solution of the compound, do the following. Take two metal rods, a 6V battery, a bulb or an ammeter, wires. Take sodium chloride in a petri dish. Keep a burner and water in a beaker also. Connect the circuit as shown in the figure below. Bulb or an ammeter will visually indicate the passage of current.

You notice the following :

No current flows through the circuit when the sodium chloride is in a solid form.

When the salt is molten, current flows.

When the salt is in an aqueous solution, the current flows and the bulb glows brightly. Also bubbles are seen at the electrodes, indicating that some chemical reaction (or dissociation of the compound) is taking place.

In the above experiment, instead of salt solution, try other solutions like sugar solution, glycerin, alcohol, dilute sulphuric acid, copper sulphate solution, acetic acid.

You will notice the following :

Current flows through dilute sulphuric acid and copper sulphate solution.

A weak current flows through acetic acid.

No current flows through sugar solution, glycerin, alcohol.

The above experiments show that there is a relationship between the passage of electricity and the chemical reactions taking place due to it. All compounds do not conduct electricity; and the state of the compound, whether molten or aqueous, is an important factor.

Some definitions regarding electrolysis
1.Electrolysis : It is a chemical process where a substance in its molten state or in an aqueous solution is decomposed by the passage of electric current.

2.Electrolyte : A compound that allows electric current to pass through itself, when either in a molten state or in an aqueous solution, is called an electrolyte. In the above experiment, solutions of sodium chloride, copper sulphate, dilute sulphuric acids, acetic acid are electrolytes. Strong electrolytes are those that allow large electric currents to be passed through them. Solutions of sodium chloride, copper sulphate, dilute sulphuric acid are examples of strong electrolytes. Weak electrolytes are those compounds which are poor conductors of electricity when they are in a molten state or in an aqueous solution. The solution of acetic acid in the experiment above, showed that it is a weak electrolyte.

3. Non-electrolyte : A compound which does not allow electric current to pass through itself in any state, molten or aqueous, is called a non-electrolyte. In the above experiments we have seen that sugar solution, glycerin, alcohol, are non-electrolytes.

4.Electrodes : The strips of metals inserted in the electrolytes for conduction of electricity are called electrodes. The metal electrode connected to the positive terminal of the battery is called the anode (+). The metal electrode connected to the negative terminal of the battery is called the cathode (-).

5.Electrolytic cell : The complete set-up for electrolysis is called the electrolytic cell. This consists of the vessel containing the electrolyte, anode, cathode, battery and wires. Electrolytic cell is also known as a voltameter, since it generates voltage (or current) at its two terminals.

Mechanism of electrolysis
In the example of NaCl above, we saw that NaCl does not conduct electricity as it is. NaCl is an ionic compound and both the Na+ and the Cl- ions are strongly attracted to each other by electrostatic attraction. The strength of the electrical current is unable to break the ionic bond. Not only breaking of the ionic bond is needed, the flow of charges also has to take place. This does not happen in solid NaCl. Thus solid NaCl is not an electrolyte.

On the other hand, in case of molten NaCl, the bond length between the Na+ and the Cl- ions has loosened. The bond is weakened. Hence the ions can become mobile and conduct electricity.

In an aqueous solution of NaCl, water molecules separate the Na+ and the Cl- ions. This makes them very mobile. The mobility is enhanced when two electrodes in the form of anode (+) and cathode (-) are inserted in the salt solution. The Na+ ions get attracted toward the cathode and the Cl- ions get attracted toward the anode. The aqueous solution of NaCl is therefore a good electrolyte.

We can conclude from the above discussions that the movement of ions is responsible for the flow of current in an electrolytic cell.

Steps occurring during the passage of electricity in an electrolytic cell with NaCl aqueous solution :


1. Dissociation of NaCl : NaCl Na + + Cl-
Cation Anion


2. Reaction at the cathode : Na + + 1e- Na (neutralization)


Reaction at the anode : Cl- 1e- + Cl (neutralization)

Cl + Cl Cl2



To summarize the process of electrolysis, we can say the following

Electrolytes dissociate to form negatively charged anions and positively charged cations.

The ions conduct electricity through the electrolyte.

Cations are attracted towards the negative electrode. They take the excess electrons from the electrode and neutralize themselves.

Anions are attracted towards the positive electrode. They give up the excess electrons from the electrode and neutralize themselves. The electrolyte dissociates and the constituent elements of the salt are liberated at the electrodes.