If the bond dissociation energies of XY,X2 and Y2 (all diatomic molecules) are in the ratio of 1:1:0.5 and ΔHf for the formation of XY is −200kJmol−1, the bond dissociation energy of X2 will be:
A
100kJmol−1
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B
300kJmol−1
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C
200kJmol−1
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D
800kJmol−1
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Solution
The correct option is D800kJmol−1 X2+Y2⟶XY ΔH=(BE)X−X+(BE)Y−Y−2(BE)X−Y If BE of (X−Y)=a then BE of (X−X)=a andBE of (Y−Y)=a2 ΔHf=(X−Y)=−200kJmol−1 ∴−400=a+a2−2a −400=−a2 or a=+800kJ