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,
Acids which contain ionizable hydrogen are called protic acids. These are classified as:
Types of protic acids
These acids contain only one ionizable hydrogen in its molecule. It is denoted by HA.Example: HCl, CH3COOH, HNO3 etc.
The diprotic acids contain two ionizable hydrogen in its molecule. They are denoted by H2A.Example: H2SO4, (COOH)2, H2CO3 etc.
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.