Chemistry · Thermodynamics & Thermochemistry

JEE Main 2024 — 5 April, Shift 1 — Question 61

Given below are two statements, one is labelled as Assertion (A) and the other is labelled as Reason (R).

Assertion (A): Enthalpy of neutralisation of strong monobasic acid with strong monoacidic base is always $-57 \mathrm{~kJ} \mathrm{~mol}^{-1}$ .

Reason (R): Enthalpy of neutralisation is the amount of heat liberated when one mole of $\mathrm{H}^{+}$ ions furnished by acid combine with one mole of $\mathrm{OH}^{-}$ ions furnished by base to form one mole of water.

In the light of the above statements, choose the correct answer from the options given below :

Option A:
Both (A) and (R) are true and (R) is the correct explanation of (A).
Correct
Option B:
Both (A) and (R) are true but (R) is not the correct explanation of (A).
Option C:
(A) is true but (R) is false.
Option D:
(A) is false but (R) is true.

Answer: A

Step-by-step solution

Assertion (A) is True: In aqueous solution, both a strong monobasic acid and a strong monoacidic base are completely ionised. Hence, the neutralisation reaction always reduces to $\mathrm{H^+(aq) + OH^-(aq) \rightarrow H_2O(l)}$ . As this reaction is identical in all such cases, the enthalpy of neutralisation is a constant value of $-57\,\mathrm{kJ\,mol^{-1}}$ .

Reason (R) is True: The enthalpy of neutralisation is defined as the amount of heat liberated when one mole of $\mathrm{H^+}$ ions furnished by an acid combines with one mole of $\mathrm{OH^-}$ ions furnished by a base to form one mole of water.

Since neutralisation between a strong acid and a strong base always involves the same reaction between $\mathrm{H^+}$ and $\mathrm{OH^-}$ ions, the heat evolved is the same in all cases. Hence, Reason (R) correctly explains Assertion (A).

Answer key and solution verified before publishing.

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Exam: JEE Main 2024
Paper: 5 April, Shift 1
Subject: Chemistry
Chapter: Thermodynamics & Thermochemistry
Topic: Thermochemistry and Enthalpy Changes

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Answer: A

Step-by-step solution

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