Question

The enthalpy of neutralization of four acids HA, HB, HC and HD with $NaOH$are -13, -12, -11 and -10 $Kcal/mol$. Which salt has the maximum degree of hydrolysis?
A. 1M $NaA$
B. 1M $NaB$
C. 1M $NaC$
D. 1M $NaD$

Answer 1

Hint: Enthalpy is also known by the name heat content which can be described as a process takes place at constant pressure then heat is absorbed or released is equal to the enthalpy change. Enthalpy is derived from the Greek word with the meaning warming.

Complete step by step answer:
- Enthalpy of neutralization can be defined as the process in which the heat is evolved when one gram equivalent of the acid is completely neutralized by a base in dilute solution. It is always negative as heat is released during this reaction. Hydrolysis term is used for the loss of water molecules during a chemical reaction.
- Now we know that enthalpy of neutralization is directly proportional to ${{K}_{a}}$ value.
Now $h=\sqrt{\dfrac{{{K}_{w}}}{{{K}_{a}}.C}}$
With the decrease in enthalpy of neutralization the value of ${{K}_{a}}$ also decreases because these are directly proportional and with the increase in ${{K}_{a}}$ we can consider that $h$ value decreases as it is inversely proportional.
Order of ${{K}_{a}}$ value HA > HB > HC > HD.
Therefore order of hydrolysis $NaD$> $NaC$> $NaB$>$NaA$.
Thus 1M $NaD$ has a maximum degree of hydrolysis, The correct answer is option “D” .

Note: The value of enthalpy of neutralization is always constant for a strong acid and a strong base this can be explained on the basis that all the strong acids and strong bases are completely ionized in dilute solution so they have constant value.