Polybasic Acids & Polyacidic Bases | Ionic Equilibrium Class 11

Polybasic Acids & Polyacidic Bases

Stepwise Ionization & Basicity/Acidity | Ionic Equilibrium

1. Definitions

Acids and bases that can donate or accept more than one proton ($H^+$) or hydroxyl ion ($OH^-$) per molecule.

                    Basicity of an Acid: Number of ionizable $H^+$ ions per molecule.

                    Acidity of a Base: Number of ionizable $OH^-$ ions per molecule.

Polybasic Acids: Dibasic ($H_2SO_4$), Tribasic ($H_3PO_4$).
Polyacidic Bases: Diacidic ($Ca(OH)_2$), Triacidic ($Al(OH)_3$).

2. Stepwise Ionization of Polyprotic Acids

Polybasic acids ionize in multiple steps. Let's consider a dibasic acid $H_2A$.

Step 1:

$$ H_2A(aq) \rightleftharpoons H^+(aq) + HA^-(aq) \quad (K_{a1}) $$

Step 2:

$$ HA^-(aq) \rightleftharpoons H^+(aq) + A^{2-}(aq) \quad (K_{a2}) $$

Overall Reaction: $H_2A \rightleftharpoons 2H^+ + A^{2-}$

$$ K_{overall} = K_{a1} \times K_{a2} $$

3. Trends in Ionization Constants

For any polybasic acid:

$$ K_{a1} \gg K_{a2} \gg K_{a3} $$

                    Reason: In the first step, a proton is removed from a neutral molecule. In subsequent steps, a positively charged proton must be removed from a negatively charged ion. The electrostatic attraction makes removal progressively harder.
                

4. Special Case: Phosphorus Oxoacids

Not all hydrogens in a molecule are ionizable. Only H attached to Oxygen (O-H) is acidic. H attached directly to P is not acidic.

Acid	Formula	O-H Bonds	Basicity
Orthophosphoric	$H_3PO_4$	3	3 (Tribasic)
Orthophosphorous	$H_3PO_3$	2	2 (Dibasic)
Hypophosphorous	$H_3PO_2$	1	1 (Monobasic)

5. pH Calculation for Polyprotic Acids

Since $K_{a1} \gg K_{a2}$, the primary source of $H^+$ is the first ionization step.

                    Approximation: For calculation purposes, we often treat the acid as monobasic using only $K_{a1}$.
                    $$ [H^+] \approx \sqrt{K_{a1} \cdot C} $$
                

The concentration of the divalent anion ($A^{2-}$) is usually approximately equal to $K_{a2}$.

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