By Anup Pokhrel
Michael Faraday classified substances into two types on the basis of conduction of electricity.
Substances that conduct electricity in its molten state or solution form (aqueous medium) are known as electrolytes. Ionic compound is known as electrolytes. Ionic compound and very polar covalent compound are electrolytes. Acid, Base, and Salts are the examples of electrolytes. Electrolyte conducts electricity due to the formation of ions produced by disassociation of a substance.
Substances that do not conduct electricity in its molten state or solution form (aqueous medium) are known as non-electrolytes. Non-polar covalent compounds are non-electrolytes. Sugar, glucose, benzene, glycerin etc. are examples of non-electrolyte.
On the basis of state of disassociation (ionization), electrolytes are further classified as:
- Weak electrolytes
- Strong electrolytes
- Strong electrolytes: Electrolytes that ionize almost completely in its solution form are known as strong electrolytes. Examples: NaCl, KOH, H2SO4, HCl, etc.
NaCl(aq) → Na+ + Cl –
As strong electrolytes ionizes to greater extent, it forms more number of ions and conducts electricity into greater extent.
2) Weak electrolytes: Electrolytes that ionizes to a small extent or ionizes incompletely in its solution form is said to be weak electrolytes. For example; H2S, NH4OH, HCN, CH3COOH, etc. Weak electrolytes are feebly ionized and a dynamic equilibrium is established between unionized molecules and ions, this dynamic equilibrium is called ionic equilibrium.
Strong electrolytes and weak electrolytes depend on the nature and the strength of the bond between the atom of the compound.
i) True electrolytes are the electrolytes which exist in ionized form in the pure state. For example; ionic compounds.
ii) Potential electrolytes are the electrolytes which do not exist in the ionic form in a pure state but produces ions when dissolved in suitable solvent. For example: very polar organic compounds
Arrhenius theory of ionization
A Swedish chemist Svante Arrhenius in 1887 AD, put forward a theory regarding ionization of electrolyte in a solution called Arrhenius theory of ionization.
The main points are:
i) When an electrolyte (acid, base or salt) is dissolved in water or any polar solvent, it splits into two oppositely charged particles which are known as ions. The positively charged particles are called cations and negatively charged ions are called anions. For a binary electrolyte,
AB → A+ + B –
ii) Ions of an electrolyte can freely move throughout the bulk of the solution.
iii) An electrolytic solution as a whole is neutral i.e. the total number of cations is equal to the total number of anions.
iv) The electrical conductivity of an electrolyte is due to the migration of cation towards cathode and anion towards the anode.
v) The process of ionization of a weak electrolyte is reversible. Unionized electrolytic molecules are constantly dissociating and formed ions are constantly reuniting. Therefore, an equilibrium is established between unionized molecules and ions. For example:
AB is a weak electrolyte.
The equilibrium constant is represented as
ka =[Applying law of mass action]
where ‘ka‘ is the dissociation or the ionization constant
vi) The process of ionization of a weak electrolyte is incomplete. The only fraction of a total number of electrolytic molecules undergoes ionization. The fraction of a total number of electrolytic molecules presents as free ions are known as the degree of ionization. It is represented by α.
Degree of ionization (α) =
Physical properties like odor, magnetic property, a refractive index of electrolyte are due to the properties of ions present in the electrolytic solution.For example, CuSO4 solution is blue in color due to the presence of Cu++ ions.
LIMITATIONS OF ARRHENIUS THEORY OF IONIZATION
i) Electrolytes ionize and maintain equilibrium not only when dissolved in water but also in the molten state.
ii) This theory is valid only for weak electrolytes.
FACTORS AFFECTING DEGREE OF IONIZATION
The degree of ionization depends on polarity and strength of bond and extent of solvation of ions formed.
i) Nature of electrolyte: The value of a degree of ionization (α) depends on the nature of electrolyte i.e. bond present on the electrolyte. The value of α for strong electrolyte (perfectly ionic compound) is almost equal to 1 (α≅1). For covalent compound, its value is 0 and covalent compounds with ionic character have the value of α between 0 and 1.
ii) Nature of solvent: The value of the degree of ionization for the polar solvent is greater than that for non-polar solvents. It is because of a polarity of solvent assists dissociation of electrolyte in solution.
According to Ostwald’s dilution law,
α = √kdv , where is dilution
i.e. α ∝ √v
It means to say that on increasing the dilution, degree of ionization increases. It is found that an infinite dilution, weak electrolytes ionize almost completely.
The degree of ionization depends on the temperature. When temperature increases, the kinetic energy of molecule also increases which in turn decreases the ionic force of attraction between ions and as a result, more and more ions are formed. Therefore, higher the temperature, higher will be the value of α.
v) Common ion effect
The suppression of ionization of weak electrolytes in the presence of strong electrolytes having one ion common to both is known as common ion effect.
OSTWALD’s DILUTION LAW
According to Arrhenius theory of ionization, a dynamic equilibrium exists in between unionized and ionized ions. Ostwald noted that law of mass action can be applied in ionic equilibrium similar to chemical equilibrium.
Limitations of Ostwald’s dilution law
i) Ostwald’s dilution law cannot be used for strong electrolytes. Since,
For strong electrolytes, the value of α is almost equal to 1 and ( 1-α) becomes 0 As a result, kd becomes infinite (undefined).
(Sthapit and Pradhananga)
Sthapit, Moti Kaji, and Dr.Raja Ram Pradhananga. Foundations Of Chemistry. 5th. Vol. 1. Kathmandu: Supravaha Press, 2010. 3 vols.