Question

Heat of neutralization of \[CsOH\] with all strong acid is $ 13.4Kcalmo{l^{ - 1}} $. The heat released on neutralization of \[CsOH\] with $ HF $ (weak acid) is $ 16.4Kcalmo{l^{ - 1}} $. $ \Delta {H^\circ } $ of ionization of $ HF $ is
A. $ 3.0Kcal $
B. $ - 3.0Kcal $
C. $ 6.0Kcal $
D. $ 0.3Kcal $

Answer 1

Hint: As we all are familiar with, the heat of neutralization is the change in enthalpy that takes place when $ 1g $ equivalent of strong acid is completely neutralized by $ 1g $ equivalent of strong base.

Complete answer:
As we know that the heat of neutralization is the change in enthalpy that takes place when $ 1g $ equivalent of strong acid is completely neutralized by $ 1g $ equivalent of strong base or we can say that it is the amount of energy released when $ 1 $ mole of water is formed from its ions is its heat of neutralization. It is an exothermic process and the enthalpy change is always negative and the amount of energy released is equal to $ 13.7Kcal $ per gram equivalent.
So, from the given question, the heat of neutralization of \[CsOH\] with strong acids is given as: $ CsOH + {H^ + } \to C{s^ + } + {H_2}O;\;\Delta {H_1} = - 13.4Kcal $

Similarly, heat of neutralization of \[CsOH\] with weak acid is given as:
 $ CsOH + HF \to {F^ - } + {H_2}O;\;\Delta {H_2} = - 16.4Kcal $
So, the heat of ionization of $ HF $ will be given as:
 $
\Rightarrow \Delta {H_3} = \Delta {H_2} - \Delta {H_1} \\
\Rightarrow \Delta {H_3} = - 16.4 - ( - 13.4) \\
\Rightarrow \Delta {H_3} = - 3.0Kcal
 $
Therefore, the correct answer is (B).

Note:
Some important points to remember:
1. When either acid, base or both are weak then the heat of neutralization depends on pH and is less than $ 13.7Kcal $ because some amount of heat is used in complete dissociation of weak acid or weak base and this amount of energy is called heat of dissociation.
2. Heat of neutralization = Heat of dissociation $ - 13.7Kcal $.
3. Heat of neutralization thus is affected by the quantity of acid and base, basicity of acid and base and the strength of acid and base.
4. The heat of neutralization is always constant for any strong and strong base as they completely ionize in dilute solution and change in enthalpy is always negative.
5. Change in enthalpy takes place when one mole of a compound is formed from its element which is present in gaseous state.