Question

For a polybasic acid, the dissociation constants have different value for each steps, e.g.,
$H_{3}A\rightleftharpoons H^{+}+H_2{A}^{-};K=K_{a_1}$
$H_2{A}^{-}\rightleftharpoons H^{+}+H{A}^{2-};K=K_{a_2}$
$H{A}^{2-}\rightleftharpoons H^{+}+A^{3-};K=K_{a_3}$
What is the observed trend of dissociation constants in succesive stage?

• A. $K_{a_1}>K_{a_2}>K_{a_3}$
• B. $K_{a_1}=K_{a_2}=K_{a_3}$
• C. $K_{a_1}<K_{a_2}<K_{a_3}$
• D. $K_{a_1}=K_{a_2}+K_{a_3}$

Answer 1

The correct option is A $K_{a_1}>K_{a_2}>K_{a_3}$

The dgree of ionization refers to the stregnth of an acid or base.
Equilibrium constant for the ionization of an acid refers to the acid ionization constant $K_a$.

$H_{3}A\rightleftharpoons H^{+}+H_2{A}^{-};K=K_{a_1}$
$H_2{A}^{-}\rightleftharpoons H^{+}+H{A}^{2-};K=K_{a_2}$
$H{A}^{2-}\rightleftharpoons H^{+}+A^{3-};K=K_{a_3}$
An acid is said to be strong , if its conjugate base is very stable.
Alternatively, if the acid is strong then acid conjugate base should be weak.
$A^{3-}$ is a strong base due to its high negative charge and the basic order is $A^{3-}>H{A}^{2-}>H_2{A}^{-}$
$\therefore$ Strength of acids: $H{A}^{2-}<H_2{A}^{-}<H_{3}A$
Thus the value of ionisation constant is $K_{a_1}>K_{a_2}>K_{a_3}$