1. 200 kJ mol ⁻¹ 2. 800 kJ mol ⁻¹ 3. 100 kJ mol ⁻¹ 4. 400 kJ mol ⁻¹

Answer: 2. 800 kJ mol ⁻¹ Let the bond energies be X-X = a , Y-Y = 0.5a , X-Y = a . For 1/2X₂+1/2Y₂→XY : Δ H=1/2 a+1/2(0.5a)-a=-0.25a . -0.25a=-200⇒ a=800\ mathrmkJ mol⁻¹ . So the X2 bond dissociation energy is 800 kJ/mol.

Topic: Hess's law and enthalpies of reactions (bond dissociation, combustion, formation etc.)· Chemical Thermodynamics

The bond dissociation energies of X_2, Y_2 and XY are in the ratio of 1 : 0.5 : 1. ΔH for the formation of XY is -200 kJ mol^-1. The bond dissociation energy of X_2 will be

1. 200 kJ mol\(^{-1}\)

2. 800 kJ mol\(^{-1}\)

3. 100 kJ mol\(^{-1}\)

4. 400 kJ mol\(^{-1}\)

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Answer: 2. 800 kJ mol\(^{-1}\)

Let the bond energies be X-X = \(a\), Y-Y = \(0.5a\), X-Y = \(a\).

For \(\tfrac12\mathrm{X_2}+\tfrac12\mathrm{Y_2}\to\mathrm{XY}\): \(\Delta H=\tfrac12 a+\tfrac12(0.5a)-a=-0.25a\).

\(-0.25a=-200\Rightarrow a=800\ \mathrm{kJ\,mol^{-1}}\). So the X2 bond dissociation energy is 800 kJ/mol.

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The bond dissociation energies of X_2, Y_2 and XY are in the ratio of 1 : 0.5 : 1. ΔH for the formation of XY is -200 kJ mol^-1. The bond dissociation energy of X_2 will be

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