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The bond enthalpies of H₂, X₂ and HX are given in the ratio 2: 1: 2. If the enthalpy of formation of HX is −50 kJ mol⁻¹, what is the bond enthalpy of X₂?

1. 100 kJ mol⁻¹
2. 300 kJ mol⁻¹
3. 200 kJ mol⁻¹
4. 400 kJ mol⁻¹

Correct answer: 100 kJ mol⁻¹

Solution

With D(H2):D(X2):D(HX) = 2:1:2 = 2a:a:2a, dHf(HX) = a + a/2 - 2a = -a/2 = -50, so a = 100. The X2 bond enthalpy is 100 kJ/mol, not 200.

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