Question

The amount of heat liberated when one mole of $N{H_4}OH$ reacts with one mole of $HCl$ is:
A. $13.7 \ kcal$
B. More than $13.7 \ kcal$
C. Less than $13.67 \ kcal$
D. Cannot be predicted

Answer 1

Hint: In order of the question, first we should know about the given reactant compound, or know the type of compounds which are given in question. Then, we will write the exact reaction between them and then dissociate them. And, finally conclude the liberated heat during the dissociation of given compounds.

Complete answer:
When one mole of $N{H_4}OH$ reacts with one mole of $HCl$ then the neutralisation reaction is: $N{H_4}OH + HCl \to N{H_4}^ + C{l^ - } + {H_2}O$
In the above reaction: $N{H_4}OH$ is the weak base and $HCl$ is the strong acid.
As we know, if strong acid reacts with strong acid as well, then it will release energy of $ - 13.7 \ kcal$.
When Weak base reacts with the strong acid, then in the dissociation of Weak Base, the heat is required. Due to this, the dissociation of $N{H_4}OH$ occurs and it will split into $N{H_4}^ + + O{H^ - }$.
So, overall heat liberated in the reaction between given Weak Base and Strong Acid is:
$\Delta H = q - 13.7 \ kcal$
According to the above formulae, we came to know that the overall heat liberated is less than $13.7 \ kcal$.

Additional Information: Here, the big question is why is heat released during chemical reactions? So, exothermic reactions give off heat because there is excess energy. The total energy in breaking the bonds is less than the energy released in forming new bonds. The excess energy is then discharged in the form of heat.
Hence, the correct option is (C) Less than $13.67 \ kcal$.

Note: When a weak acid reacts with an equivalent amount of a weak base complete neutralization does not occur. The concentrations of the species in equilibrium with each other will depend on the equilibrium constant, K, for the reaction.