Question

Assertion (A) Higher order ionisation constant $\left(K_{a_{2}}, K_{a_{3}}\right)$ are smaller than the lower order ionisation constant $\left(K_{a_{1}}\right)$ of a polyprotic acid.
Reason (R) It is more difficult to remove a positively charged proton from a negative ion due to Coulombic forces.

• A. Both $A$ and $R$ are correct; $R$ is the correct explanation of $A$
• B. Both $A$ and $R$ are correct; $R$ is not the correct explanation of $A$
• C. $A$ is correct; $R$ is incorrect
• D. $A$ is incorrect; $R$ is correct

Answer 1

Answer: (C)

Higher order ionisation constants $\left(K_{a_{2}}, K_{a_{3}}\right)$ are smaller than the lower order ionisation constant $\left(K_{a_{1}}\right)$ of a polyprotic acid. The reason for this is that, it is more difficult to remove a positively charged proton from a negative ion due to electrostatic forces.
This can be seen in the case of removing a proton from the uncharged $H _{2} CO _{3}$ as compared from a negatively charged $HCO _{3}^{-}$.
Thus, $A$ is correct but $R$ is incorrect.