Question

Which of the following is the correct option regarding $K_{a_1}$ for the first dissociation and $K_{a_2}$ for the second dissociation of a polyprotic acid ?

• A. $K_{a_1}$ has less value than $K_{a_2}$
• B. $K_{a_1}$ has more value than $K_{a_2}$
• C. $K_{a_1}$ has value equal to $K_{a_2}$
• D. None of the above

Answer 1

The correct option is B $K_{a_1}$ has more value than $K_{a_2}$

$K_{a_1}$ is the first dissociation constant. It describes how first $H^{+}$ dissociates from neutral polyprotic acid.
$K_{a_1}$ is the second dissociation constant. It describes how second $H^{+}$ dissociates from negatively charged polyprotic acid.
Neutral molecule has higher tendency to loose a proton than anionic species because negatively charged molecule resists to give a proton. Thus, $K_{a_1}$ value is more than $K_{a_2}$ value for polyprotic acid.