IONIC THEORY
To account for the phenomena of electrolysis
the Ionic Theory was put forward by Arrhenius in 1880. The theory states that
electrolytes are made up of ions, which are built up in certain patterns called
crystal lattice. When these substances dissolve in water, the structure is
destroyed and the ions are set free to move .Concentrated mineral acids such as
sulphuric acid, hydrochloric acid and nitric acid do not contain ions but they
consist of molecules. However, when they are diluted, the molecular structure
is destroyed and ions are formed.
Electrolytes and
Non-electrolytes
Distinguish electrolytes from non-electrolytes
The main purpose of this chapter is to investigate the effects which electricity has on a range of substances, and to develop a thorough explanation of those effects in terms of our present knowledge of atomic structure. Before we begin, it is important that we familiarize ourselves with different terms that we are going to use to explain different phenomena. It is crucial that the definitions and meanings of these terms be understood at the outset in order that concepts defined in this chapter to be easily and clearly apprehended. These terms are given hereunder:
Electrolysis: decomposition of a compound in solution or molten state by passing electricity through it.
Conductor: a solid substance that allows electricity to pass through it. All metals are included in this class.
Non-conductor or insulator: a solid substance that does not
allow electricity to flow through it.
All non-metals fall in this class.
Electrolyte: a substance which, when dissolved or molten, conducts electricity and is decomposed by it.
Non-electrolyte: a compound which cannot conduct electricity, be it in molten or solution state.
Electrode: a graphite or metal pole (rod) or plate through which the electric current enters or leaves the electrolyte.
Cathode: a negative electrode which leads electrons into the electrolyte
Anode: a positive electrode which leads electrons out of the electrolyte
Ion: a positively or negatively charged atom or radical (group of atoms).
Cation: a positive ion which moves to the cathode during electrolysis.
Anion: a negative ion which moves to the anode during electrolysis
Electrolytes and non-electrolytes
Liquids such as ethanol, paraffin, petrol and
methyl-benzene do not conduct electricity. The bonding in these compounds is
covalent. These substances consist of molecules. There are no free electrons or
charged particles to flow through them. Solutions of covalent compounds, for
example sugar solution, do not conduct electricity.
These compounds are non-electrolytes.
Non-electrolytes exist only in the form of molecules and are incapable of
ionization.
Ionic compounds contain charged particles (ions), but in solid state, the ions are firmly held in place and they are not free to move. An ionic solid does not conduct electricity. However, the ions present can become free to move if the solid is melted or dissolved in water. Then they can conduct electricity. For example, solid sodium chloride cannot conduct electricity but when melted or dissolved in water, the ions, Na+ and Cl- are set free. Then these ions are free to move in solution and hence conduct electricity. These compounds are called electrolytes.
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