Ionization in electrolytes
Strong electrolytes are almost completely dissociated / ionized into the constituent ions in aqueous solution. Even at higher concentrations, very small amount of any strong electrolyte is present in the unionized form. Weak electrolytes are however, partially ionized and a dynamic equilibrium exist between the ionized and unionized forms.
In 1887, S.Arrhenius postulated the first theory of electrolytic ionization. The basic postulates of his theory are:- When an electrolyte is dissolved in water, it ionizes to produce ions in the solution. These ions are free to move throughout the bulk of the solution.
- The total number of positive charges is equal to the total number of negative charges in any solution of any electrolyte. The ions tend to recombine to form the unionized electrolyte. Hence, a dynamic equilibrium exists between the ionized and unionized form of the electrolyte. For example, an electrolyte AB ionizes in the solution to form A+ and B-. The equilibrium established in the solution is:
- The properties of an electrolyte in the solution are due to the ions it gives in the solution. For example, a solution of copper sulphate is blue due to the presence of Cu2+(aq) ions in it.
- The fraction of the total number of molecules present as free ions in the solution is known as the degree of ionization (a). This is defined as,
The degree of ionization at any concentration (C) of AB in moles per litre is denoted as aC.
Degree of ionization
The equilibrium concentration of various species at equilibrium can be represented in terms of degree of ionization as:
Then according to law of mass action,
where 'V' is the volume of the solution in litres containing one mole of the solute. From the above equation it follows that if the concentration decreases, the degree of ionization (a) must increase. Thus, at constant temperature, an electrolyte in solution gets ionized to a larger extent with increasing dilution. This is called Ostwald dilution law. This law is not applicable to solutions of strong electrolytes as they are completely dissociated and there is no equilibrium between the dissociated and undissociated molecules.
Ionization of an acid (weak)
An acid of the type HA can undergo ionization when dissolved in water as,
If 'n' moles of the acid are dissolved in 'V' units of volume (litres L) and 'a' is the degree of ionization, then the equilibrium amounts of various species and the concentration in moles per litre in the solution are,
where 'C' is the molar concentration of the acid. The ionization constant for the above reaction is given by,
where 'V' is the volume of the solution in litres containing one mole of the acid HA. As the degree of ionization increases with-dilution then, the hydronium ion or hydrogen ion concentration is given by,
Ionization of a base (weak)
The ionization of a weak base is characterized by the equilibrium,
If 'n' moles of the base are dissolved in 'V' units of volume (litres L) and a is the degree of ionization, then the equilibrium amounts of various species and the concentration in moles per litre in the solution are,
where 'C' is the molar concentration of the base. The ionization constant for the above reaction is given by,
If 'a' is small then 1 - a ≈ 1
Hence, Kb = Ca2
where 'V' is the volume of the solution in litres containing one mole of the base MOH. As the degree of ionization increases with dilution then, the hydroxide concentration is given by,
Protic acids
Acids which contain ionizable hydrogen are called protic acids. These are classified as:
Types of protic acids
Monoprotic acids
These acids contain only one ionizable hydrogen in its molecule. It is denoted by HA.
Example: HCl, CH3COOH, HNO3 etc.Diprotic acids
The diprotic acids contain two ionizable hydrogen in its molecule. They are denoted by H2A.
Example: H2SO4, (COOH)2, H2CO3 etc.Triprotic acids
These acids contain three ionizable hydrogen in its molecule. It is denoted by H3A.
Example: H3PO4 H3A SO4 etc.In all these acids, the primary ionization constant (K1) is stronger than the secondary (K2), which is much stronger than the tertiary (K3) ionization constant. This is because a proton (H3O+) would be released more readily from an uncharged molecule than from a mononegative ion, and more readily from a mononegative ion than from a binegative ion. Also the hydronium concentration from the first ionization will suppress the subsequent ionizations due to common ion effect.
Some general observations on the behaviour of protic acids in aqueous solution are:- The protic acids which have very high value of the acid dissociation or ionization constant, ionize/dissociate almost completely in aqueous solution at ordinary dilutions. They are called strong acids.
- Acids, which dissociate in aqueous solution to a smaller extent at ordinary dilution, to give low concentration of H+ ions in solution, are called weak acids.
- On dilution, the ionization of an acid increases. So, concentration of H+ ions also increases on dilution. Therefore, strength of the acid increases with dilution. It is for this reason that the acid strengths are compared at equal concentrations.
- For di- and tri- protic acids, first dissociation is stronger than the second, which in turn is much stronger than the third dissociation i.e., K1 » K2 » K3. This is because the removal of a proton from a negatively charged species is more difficult than from a neutral molecule.
Problems
2. Which of the following acids is the strongest and which one is the weakest. Explain.
Solution
Solution
Let 'a' be the degree of ionization. The concentration of various species involved in the equilibrium are as follows:
The equilibrium constant for the ionization of CH3COOH can be written as follows:
CH3COOH is very feebly ionized. So, 'a' may be ignored in comparison to 1. Then,
Solution
Acetic acid ionizes as follows:
