Polyprotioc Acid And Bases

POLYPROTIC ACIDS AND BASES Bronsted acids, which can donate more than one proton, are called polyprotic acids or polybasic acids. For example, oxalic acid and sulphuric acid have two ionisable protons per molecule of the acid, which can be donated. Then these are called dibasic acids. Phosphoric acid H3PO4 that can donate three protons is called tri basic or tri protic acid .The dissociation reaction for dibasic can be represented as

The corresponding dissociation constants are:

Ka1 and Ka2 are called the first and second ionization constants respectively of the acid H2X. Similarly, for tribasic acids like H3PO4 we will have three ionization constants.

For example, carbonic acid, H2CO3 has the following equilibrium:

The conjugate base of H2CO3 the first equilibrium, HCO3 – acts as acid and for the second equilibrium forming in turn its own conjugate base, CO3. The equation for the overall reaction is:

This equation is the sum of the two individual equations. It is known that the overall equilibrium constant is the product of the equilibrium constants for each contributing reaction:

It is also observed that higher order ionization constants are smaller than the lower order ionization constants. K a1 > K a2 > K a3 ----

Thus, the acidic strength of polyprotic acid decreases, as protons are lost. Similarly a polyacidic base is a species which can accept more than proton e,g, CO32- and oxalate anion C2O42- are diacidic because they can accept two protons and phosphate anion PO43- is triacidic because it can accept three protons. They have the corresponding ionization constants as, K b1 , K b2 , K b3 ---Common ion effect in the ionization of acids and bases The ionization of an acid gives hydrogen or hydronium ion and corresponding anion. If anyone of the products is added to the acid solution, then the ionization of the acid moves towards the backward direction according to Le Chatelier’s principle. As a result, the ionization of acid suppresses and the acid is ionized to lesser extent e.g. acetic acid (CH3COOH) dissociates as:

If hydrogen ions are added to the solution from any other source, it will combine with acetate ions in the solution and form unionized acetic acid. With this the degree of ionization of acetic acid will decreases. So this will effect when any ion is added to the equilibrium mixture from any external source. This is known as Common ion effect.