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NEET CHEMISTRYThermodynamicsHard

Question

The absolute enthalpy of neutralisation of the reaction MgO(s)+2HCl(aq)ightarrowMgCl2(aq)+H2O(l)MgO(s) + 2HCl(aq) ightarrow MgCl_{2}(aq) + H_{2}O(l) will be [CBSE PMT 2005]:

A

Less than –57.33 kJ mol⁻¹

B

–57.33 kJ mol⁻¹

C

Greater than –57.33 kJ mol⁻¹

D

57.33 kJ mol⁻¹

Step-by-Step Solution

The standard enthalpy of neutralisation for the reaction between a strong acid and a strong base in dilute aqueous solution is approximately –57.33 kJ mol⁻¹, representing the formation of one mole of water from H+(aq)H^{+}(aq) and OH−(aq)OH^{-}(aq) ions [Source knowledge]. However, for the reaction involving a solid basic oxide like MgO, we apply Hess's Law using the standard enthalpies of formation (ΔfH∘\Delta_{f}H^{\circ}) found in the sources:

  1. Identify Enthalpy Values (at 298 K): ΔfH∘[Mg2+(aq)]=−466.85 kJ mol−1\Delta_{f}H^{\circ} [Mg^{2+}(aq)] = -466.85 \text{ kJ mol}^{-1} ΔfH∘[H2O(l)]=−285.83 kJ mol−1\Delta_{f}H^{\circ} [H_{2}O(l)] = -285.83 \text{ kJ mol}^{-1} ΔfH∘[MgO(s)]=−601.70 kJ mol−1\Delta_{f}H^{\circ} [MgO(s)] = -601.70 \text{ kJ mol}^{-1} ΔfH∘[H+(aq)]=0 kJ mol−1\Delta_{f}H^{\circ} [H^{+}(aq)] = 0 \text{ kJ mol}^{-1} (by convention)

  2. Calculate Reaction Enthalpy (ΔrH∘\Delta_{r}H^{\circ}): The net ionic equation is MgO(s)+2H+(aq)→Mg2+(aq)+H2O(l)MgO(s) + 2H^{+}(aq) \rightarrow Mg^{2+}(aq) + H_{2}O(l). ΔrH∘=[ΔfH∘(Mg2+,aq)+ΔfH∘(H2O,l)]−[ΔfH∘(MgO,s)+2×ΔfH∘(H+,aq)]\Delta_{r}H^{\circ} = [\Delta_{f}H^{\circ}(Mg^{2+}, aq) + \Delta_{f}H^{\circ}(H_{2}O, l)] - [\Delta_{f}H^{\circ}(MgO, s) + 2 \times \Delta_{f}H^{\circ}(H^{+}, aq)] ΔrH∘=[(−466.85)+(−285.83)]−[(−601.70)+0]\Delta_{r}H^{\circ} = [(-466.85) + (-285.83)] - [(-601.70) + 0] ΔrH∘=−752.68−(−601.70)=−150.98 kJ mol−1\Delta_{r}H^{\circ} = -752.68 - (-601.70) = -150.98 \text{ kJ mol}^{-1}.

Since –150.98 kJ mol⁻¹ is numerically less than –57.33 kJ mol⁻¹ (more exothermic), the correct option is A.

Exam Context & Concepts Covered

This question aligns with the NEET CHEMISTRY syllabus, specifically targeting concepts from Thermodynamics. Mastering this topic is crucial for scoring well in the upcoming medical entrance examinations. Solving conceptually related problems will help you understand the nuances of these concepts and improve your problem-solving speed.

CHEMISTRYThermodynamicsabsoluteenthalpyneutralisationreactionightarrow

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