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AIIMS AIIMS Solved Paper-2011

  • question_answer
    If the bond dissociation energies of XY, \[{{X}_{2}}\] and \[{{Y}_{2}}\] (all diatomic molecules) are in the ratio of \[1:1:0.5\]and \[\Delta {{H}_{f}}\] for the formation of XY is \[-200\,\,kJ\,\,\,\,mo{{l}^{-1}}.\]. The bond dissociation energy of \[{{X}_{2}}\] will be

    A)  \[400\text{ }kJ\text{ }mo{{l}^{-1~}}\]

    B)         \[300\text{ }kJ\text{ }mo{{l}^{-1~}}\]

    C)         \[200\text{ }kJ\text{ }mo{{l}^{-1~}}\]

    D)         \[800\text{ }kJ\text{ }mo{{l}^{-1~}}\]

    Correct Answer: D

    Solution :

    Formation of XY is shown as \[{{X}_{2}}+{{Y}_{2}}\xrightarrow{{}}2XY\] \[\Delta H={{(BE)}_{X-X}}+{{(BE)}_{Y-Y}}-2{{(BE)}_{X-Y}}\] If       \[(BE)\] of \[X-Y=a\] then   \[(BE)\] or \[(X-X)=a\] and    \[(BE)\] of \[(Y-Y)=\frac{a}{2}\] \[\Delta {{H}_{f}}(X-Y)=-200kJ\] \[\therefore \]  \[-400\] (for 2 moles XY) \[=a+\frac{a}{2}-2a\] \[-400=-\frac{a}{2}\] \[a=+800kJ\] The   bond   dissociation   energy   of \[{{X}_{2}}=800kJ\,mo{{l}^{-1}}\]


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