No. of replaceable H+ counts for the basicity of an acid
Acids having 2 ionisable protons per molecule are known as di basic acids
Example of Dibasic acids is Sulphuric acid ($H_2SO_4$), Carbonic Acid ($H_2CO_3$)
Ionization of Dibasic acids
We can represent the ionization reactions for a dibasic acid $H_2X$ as
$H_2X(aq) \rightleftharpoons H^+(aq) + HX^– (aq)$
$HX^– (aq) \rightleftharpoons H^+(aq) + X^{2–}(aq)$
Ionization constants respectively will be
$K_{a1}= \frac {[H+][HX–]}{ [H2X]} $
$K_{a2}= \frac {[H+][X^{2-}]} { [HX-]}$
Here $K_{a1} > K_{a2}$
It can be seen that higher order ionization constants Ka2 are smaller than the lower order ionization constant (Ka1) of a dibasuc acid as it is more difficult to remove a positively charged proton from a negative ion due to electrostatic forces
Example
$H_2SO_4(aq) \rightleftharpoons H^+(aq) + HSO_4^– (aq)$
$HSO_4^– (aq) \rightleftharpoons H^+(aq) + SO_4^{2–}(aq)$
$H_2CO_3(aq) \rightleftharpoons H^+(aq) + HCO_3^– (aq)$
$HCO_3^– (aq) \rightleftharpoons H^+(aq) + CO_3^{2–}(aq)$
Polybasic Acids
Acids having more than 2 ionisable protons are known as Polybasic acids.
It is also known as polyprotic acids.
Example of Polybasic acids are phosphoric acid($H_3PO_4$) ,Oxalic Acid ($H_2C_2O_4$) , Arsenic Acid($H_3AsO_4$)
Ionisation constant of Polybasic Acids
We can represent the ionization reactions for a polybasic acid $H_2X$ as
$H_3PO_4(aq) \rightleftharpoons H^+(aq) + H_2PO_4^– (aq)$
$H_2PO_4^– (aq) \rightleftharpoons H^+(aq) + HPO_4^{2–}(aq)$
$HPO_4^{2–} (aq) \rightleftharpoons H^+(aq) + PO_4^{3–}(aq)$
Ionization constants respectively will be
$K_{a1}= \frac {[H+][H_2PO_4^–]}{ [H_3PO_4]} $
$K_{a2}= \frac {[H+][HPO_4^{2–}]} { [H_2PO_4^–]}$
$K_{a3}= \frac {[H+][PO_4^{3–}]} { [HPO_4^{2–}]}$
Here $K_{a1} > K_{a2}> K_{a3}$
It can be seen that higher order ionization constants Ka3 are smaller than the lower order ionization constant (Ka1) of a Polybasic Acids as it is more difficult to remove a positively charged proton from a negative ion due to electrostatic forces
Strength of Acidic Character
(a) On moving down the graph; bond HA dissociation enthalpy decreases as Size increase. So Acidity increases.
HF << HCl << HBr << HI
(b) On moving left to right in a period acidic character increases as electronegativity of A increase
F > O > N > C
$CH_4$ < $NH_3$ < $H_2O$ < HF
Since, Florine is highly electro negative
Therefore H – F is most polar and easily gives H+
