Question

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 kJ mol^–1
Reason (R): Enthalpy of neutralisation is the amount of heat liberated when one mole of H⁺ ions furnished by acid combine with one mole of ^–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.

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

Answer 1

The Correct Option is B

Enthalpy of neutralisation of a strong acid (SA) and strong base (SB) is always −57 kJ/mol because a strong monoacid gives one mole of H⁺ and a strong mono-base gives one mole of OH⁻, which combine to form one mole of water. Both the assertion and reason are true, and (R) correctly explains (A).

Answer 2

Step 1: Recall the concept of enthalpy of neutralisation
The enthalpy of neutralisation is defined as the amount of heat liberated when one mole of hydrogen ions (\(H^+\)) from an acid reacts with one mole of hydroxide ions (\(OH^-\)) from a base to form one mole of water under dilute conditions:
\[ H^+ (aq) + OH^- (aq) \rightarrow H_2O(l) \] For strong acids and strong bases, this process always involves the same ionic reaction, regardless of the acid or base used.

Step 2: Evaluate the Assertion (A)
Assertion (A): “Enthalpy of neutralisation of a strong monobasic acid with a strong monoacidic base is always –57 kJ mol⁻¹.”
This is true because both the acid and base are completely dissociated in aqueous solution, and the same reaction between \(H^+\) and \(OH^-\) occurs in every case.

Step 3: Evaluate the Reason (R)
Reason (R): “Enthalpy of neutralisation is the amount of heat liberated when one mole of \(H^+\) ions furnished by acid combine with one mole of \(OH^-\) ions furnished by base to form one mole of water.”
This statement is also true and correctly explains why the value is constant for all strong acid–strong base reactions.

Step 4: Conclusion
Since both (A) and (R) are true, and the Reason (R) correctly explains the Assertion (A), the correct choice is:

Both (A) and (R) are true and (R) is the correct explanation of (A)