SS2 Chemistry Lesson Note on Electrolysis: Ionic Theory

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TOPIC: ELECTROLYSIS: IONIC THEORY

CONTENT:

1. MEANING OF ELECTROLYSIS AND TERMINOLOGIES – ELECTRODES, ELECTROLYTES, ELECTROLYTIC CELL
2. COMPARISON OF ELECTROLYTIC CELLS AND ELECTROCHEMICAL CELLS AND PRINCIPLES OF ELECTROLYSIS
3. FACTORS AFFECTING THE DISCHARGES OF IONS/ PRODUCT OF ELECTROLYSIS,
4. EXAMPLES OF ELECTROLYSIS – ELECTROLYSIS OF ACIDIFIED WATER, COPPER (II) TETRAOXOSULPHATE (VI) AND BRINE.

IONIC THEORY

Ionic theory was used to explain the behaviour of electrolytes when electric current is passed through their solution.

A Swedish chemist, Swante Arrherius (1887), was the first to present the ionic theory to describe electrolysis. The theory proposed that when an electrolyte is melted or dissolved in water, some if not all of the molecules of the substance dissociate into freely-moving charge ions i.e. positive ions (called cations) and negative ions (anions). The process of dissociation into ions is called ionization.

When an electric current is passed through an electrolyte, the free ions lose their random movement. The positive ion becomes attracted to the negative electrode (called the cathode) while the negative ion s moves toward the positive electrode (called anode). It is this ionization and mobility of ions that constitute current flow.

Arrhenius version of the ionic theory has been modified since the X-ray diffraction studies show that salts and strong alkalis consists of oppositely charged ions even in solid state. The modern theory of the proposes that ion in such solid state are pulled away from one another either as a result of heat energy applied when the solid melts or with the help of the solvent molecules when the solid dissolves. Thus,

PERIOD 1: MEANING OF ELECTROLYSIS

Electrolysis is the process whereby chemical decomposition occurs when an electric current is passed through an electrolyte.

TERMINOLOGIES

1. Electrode: Electrodes are conductors in the form of wires rods or plates through which an electric current enters or leaves the electrolyte. We have the cathode and the anode

Note: In an electrolytic cell, the negative electrode is known as the cathode while the positive electrode is known as the anode.

In an electrochemical cell, the negative electrode is the anode while the positive electrode is the cathode.

2. ELECTROLYTES

An electrolyte is a compound, in a molten or aqueous state which can conduct electricity and is decomposed in the process.

Electrolytes conduct electric currents by movement of ions. Examples of electrolytes include NaCl, CuCl₂, C_USO₄, HCl, H₂SO₄ etc.

Electrolytes may be weak or strong:

(a) Strong electrolyte ionizes completely and can conduct large electric current since they are composed entirely of free moving ions. Examples are strong acids , strong bases and salts

(b) Weak electrolytes  are partially ionized and conduct electricity poorly because the contain low concentration of ions. Examples are weak acids and weak bases

3. Non-electrolytes do not conduct electricity since they do not ionize. Examples are urea ethane, benzene, trichloromethane, cane sugar, ether, tetra chloromethane.
4. An electrolytic cell is an assembly of two electrodes in an electrolyte and is used for the electrolysis of a substance.

EVALUATION

Write short note on the following terms:

1. Electrolysis
2. Electrolyte
3. Electrodes
4. Weak electrolyte
5. Strong electrolyte
6. Non-electrolyte

 

PERIOD 2: COMPARISON OF ELECTROLYTIC CELLS AND ELECTROCHEMICAL CELLS AND PRINCIPLES OF ELECTROLYSIS

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