Based on the values of bond energies given, ΔfHo of N2H4(g) is:
Given: N−N=159kJ mol−1;H−H=436kJ mol−1 N≡N=941kJ mol−1;N−H=398kJ mol−1
A
700kJ mol−1
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B
62kJ mol−1
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C
−98kJ mol−1
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D
−711kJ mol−1
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Solution
The correct option is B62kJ mol−1 N2(g)+2H2(g)→N2H4(g) ΔfH(N2H4(g)) =Sum of bond energies of bonds being broken - sum of the bond energies of the bonds being formed=((B.E.)N≡N+2×(B.E.)H−H)−((B.E.)N−N+4×(B.E.)N−H)=(941+2×436)−(159+4×398) =1813−1751=62kJ mol−1