The principles of electrolysis

The Principles Of Electrolysis


Any liquid (in solution or molten state) that contains ions will conduct electricity because the ions are free to move and act as charge carriers. Unlike the conduction by solids, conduction by ionic liquids results in a chemical change of the liquid. That leads to the definition of electrolysis.

What is electrolysis?

There many ways to define electrolysis, for example:

Electrolysis is the chemical change that occurs when an electric current passes through a liquid.


Electrolysis is the breakdown of a compound using electricity.

During electrolysis, electricity is carried through the electrolyte by ions. When a compound that contains ions conducts electricity, it is broken down. That process is called electrolysis, and the compound is called an electrolyte.

What is an electrolyte?

An electrolyte is a compound that conducts electricity by the movement of ions.

During electrolysis, an electrolyte is decomposed by the electric current. The ions in an electrolyte become mobile enough to carry a current when the electrolyte is in molten or aqueous state.

Some electrolytes are better conductors of electricity than others because they completely ionise when in solution. The are called strong electrolytes. Salts, strong acids and strong bases are good examples of strong electrolytes.

Weak electrolytes are those electrolytes that partly ionises when in solution. They are not good conductors of electricity, since they produce fewer ions. Weak acids and weak bases are good examples of weak electrolytes.

Non-electrolytes are compounds that do not ionise at all and hence do not conduct electricity in any state. Non-polar covalent compounds are good examples of non-electrolytes.


Examples of strong electrolytes, weak electrolytes and non-electrolytes

Strong electrolytes Weak electrolytes Non-electrolytes
Sulphuric acid Ethanoic acid Sugar
Hydrochloric acid Carbonic acid Benzene
Sodium hydroxide Aqueous ammonia Alcohol
Most ionic compounds Water Most organic compounds

The ionic theory in electrolysis

According to the ionic theory, electrolytes contain positively charged ions and negatively charged ions. An electrolyte is overally neutral because it contains the same amount of negative ions as positive ions.

Negatively charged ions have an excess of electrons and are known as anions, which is short for anode ions because they are attracted by the anode during electrolysis.

Positively charged ions have a deficiency of electrons and are known as cations, which is short for cathode ions because they are attracted by the cathode during electrolysis.

Examples of anions and cations

Metals and hydrogen form positive ions when bond to non-metals, because they are eletropositive. Non-metals and acid radicals form negative ions because they are electronegative. The charge on the ion is the same as its valency. For example,

Compound Formula Cation Anion
Sodium chloride NaCl Na+ Cl
Sodium hydroxide NaOH Na+ OH
Copper(II) chloride CuCl2 Cu2+ 2Cl
Sulphuric acid H2SO4 2H+ SO42-

Electrodes

During electrolysis, the electric current enters and leaves the electrolyte through conductors called electrodes. Electrodes are usually made of unreactive metals such as platinum or the non-metal carbon (graphite) if the objective is not to contaminate the products. Inert electrodes are electrodes that do not react with the products of electrolysis.

The principles of electrolysis

The positive electrode is called the anode and the negative electrode is called the cathode. During electrolysis the negative electrode attracts cations (positively charged ions), and the anode attracts anions (negatively charged ions).