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NEET CHEMISTRYThermodynamicsMedium

Question

The bond energy of H—H and Cl—Cl is 430 kJ mol−1430 \text{ kJ mol}^{-1} and 240 kJ mol−1240 \text{ kJ mol}^{-1} respectively and ΔfH\Delta_f H for HCl is −90 kJ mol−1-90 \text{ kJ mol}^{-1}. The bond enthalpy of HCl is:

A

290 kJ mol−1290 \text{ kJ mol}^{-1}

B

380 kJ mol−1380 \text{ kJ mol}^{-1}

C

425 kJ mol−1425 \text{ kJ mol}^{-1}

D

245 kJ mol−1245 \text{ kJ mol}^{-1}

Step-by-Step Solution

The chemical equation for the standard enthalpy of formation of HCl is: 12H2(g)+12Cl2(g)→HCl(g)\frac{1}{2}\text{H}_2\text{(g)} + \frac{1}{2}\text{Cl}_2\text{(g)} \rightarrow \text{HCl(g)}

The enthalpy of formation (ΔfH\Delta_f H) is related to bond enthalpies by the equation: ΔfH=∑B.E.(reactants)−∑B.E.(products)\Delta_f H = \sum \text{B.E.(reactants)} - \sum \text{B.E.(products)} ΔfH=[12B.E.(H−H)+12B.E.(Cl−Cl)]−B.E.(H−Cl)\Delta_f H = \left[ \frac{1}{2}\text{B.E.}(\text{H}-\text{H}) + \frac{1}{2}\text{B.E.}(\text{Cl}-\text{Cl}) \right] - \text{B.E.}(\text{H}-\text{Cl})

Given: ΔfH=−90 kJ mol−1\Delta_f H = -90 \text{ kJ mol}^{-1} B.E.(H−H)=430 kJ mol−1\text{B.E.}(\text{H}-\text{H}) = 430 \text{ kJ mol}^{-1} B.E.(Cl−Cl)=240 kJ mol−1\text{B.E.}(\text{Cl}-\text{Cl}) = 240 \text{ kJ mol}^{-1}

Substituting the values: −90=[12(430)+12(240)]−B.E.(H−Cl)-90 = \left[ \frac{1}{2}(430) + \frac{1}{2}(240) \right] - \text{B.E.}(\text{H}-\text{Cl}) −90=(215+120)−B.E.(H−Cl)-90 = (215 + 120) - \text{B.E.}(\text{H}-\text{Cl}) −90=335−B.E.(H−Cl)-90 = 335 - \text{B.E.}(\text{H}-\text{Cl}) B.E.(H−Cl)=335+90=425 kJ mol−1\text{B.E.}(\text{H}-\text{Cl}) = 335 + 90 = 425 \text{ kJ mol}^{-1}

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.

CHEMISTRYThermodynamicsenergyrespectivelydeltafenthalpy

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