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The bond dissociation energies of X2, Y2 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 X2 will be:

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

Correct answer: 200 kJ mol⁻¹

Solution

The enthalpy change for the formation of XY is given as ΔH = -200 kJ mol⁻¹. Using the bond dissociation energy ratio (X2:Y2:XY = 1:0.5:1), let the bond dissociation energy of X2 be x. Then, the bond dissociation energy of Y2 is 0.5x, and that of XY is x. The equation for ΔH is: ΔH = [Bond dissociation energy of reactants] - [Bond dissociation energy of products]. Substituting values: -200 = x + 0.5x - 2x. Solving gives x = 200 kJ mol⁻¹.

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