Question

Enthalpy of neutralization of a strong acid with a strong base is 13.7 kcal. When an unknown acid (1 eq) is neutralized with (1 eq) strong base, the enthalpy change is 10.7 kcal which of the following statements is/are correct regarding the unknown acid?

• A. Unknown acid is a strong acid
• B. Unknown acid is a weak acid
• C. 3.0 kcal heat utilised to dissociate the unknown acid
• D. 10.7 kcal heat utilised to dissociate unknown acid

Answer 1

Answer: (B), (C)

The correct options are
B Unknown acid is a weak acid
C 3.0 kcal heat utilised to dissociate the unknown acid

Point to remember:
$\Rightarrow$For Strong $Acid+StrongBase$, heat of neutralisation is always equal to $–13.7kcal/mole$ or $–57.1kJ/mole$.
$\Rightarrow$ For any other combination it must be less than $–13.7kcal/mole$ or $–57.1kJ/mole$.
$\Rightarrow$ Rest of energy utilised for ionisation of weak acid or base
So for given Question:
$S.A+S.B\to Salt\Delta H_{neu}=-13.7kcal/mol$
But for unknown acid
$U.A+S.B\to Salt\Delta H_{neu}=-10.7kcal/mol$
$\Delta H_{neutralisation}=-10.7$ which less than ideal value so it means unknown acid is weak acid.
and $3.0kcal/mol$ energy utilised to dissociate this weak acid.