Question

Assertion: Enthalpy of neutralization of $C{H_3}COOH$ by $NaOH$ is less than that of $HCl$ by $NaOH$ . Because
Reason: Enthalpy of neutralization of $C{H_3}COOH$ is less because of the absorption of heat in the ionization process.
A. Both Reason and Assertion are correct and reason is the correct explanation for assertion.
B. Both Reason and Assertion are correct but Reason is not the correct explanation for Assertion.
C. Assertion is correct but Reason is incorrect.
D. Both Assertion and Reason are incorrect.

Answer 1

Enthalpy is the measure of heat capacity. When acid and base react together, neutralization reactions take place with the formation of salt and water. Acids and bases taking part in such reactions can be either strong or weak.

Complete step by step answer:
Enthalpy of neutralization is defined as the heat evolved or decreased in enthalpy when one gram equivalent of acid is neutralized by one gram equivalent of base in dilute solution.
Strong acid $+$ strong base $=$ salt $+$ water $\Delta H = - 13.7kcal$
It is observed that the enthalpy of neutralization of strong acid and base is always equal to $- 13.7kcal$ no matter how strong the acid or base is. This constant value can be explained with the help of ionization.
${H^ + } + {A^ - } + {B^ + } + O{H^ - } = {B^ + } + {A^ - } + {H_2}O$ $\Delta H = - 13.7kcal$
Cancelling the ions that are common on both the sides in the above equation
${H^ + } + O{H^ - } = {H_2}O$ $\Delta H = - 13.7kcal$
Thus the enthalpy of neutralization of strong acid and base is merely the heat of formation of water from ${H^ + }$ and $O{H^ - }$ ions.
This reaction is a common reaction whenever a strong acid and strong base are mixed.
However when a strong acid and weak base or weak acid and strong base or a weak acid and weak base are mixed in equal amounts the enthalpy change is less than $13.7kcal$ .
The reason for lower value is that part of the heat energy evolved is utilised in the complete ionization of weak acid or a weak base or both. Hence the net heat of neutralization is less than $13.7kcal$ .
The heat of neutralization of $NaOH$ and $HCl$ is $13.7kcal$ since both are strong.
But the heat of neutralization of $C{H_3}COOH$ by $NaOH$ is less than the heat of neutralization of $HCl$ by $NaOH$ . because acetic acid is a weak acid and $NaOH$ is a strong base. So during the neutralization a part of the heat evolved will be utilized for the ionization of acetic acid.
So both reason and assertion is correct and reason is the correct explanation of assertion.

So, the correct answer is “Option A”.

Note: Enthalpy of neutralization is calculated in the laboratory using a polystyrene bottle. Enthalpy of neutralization is the same for the reaction of strong acid and strong base. It does not depend on the nature of strong acid or strong base.