KVPY SX SB Exam 20141 Question Paper

Question : 105 of 120

Marks: +1, -0

Given that the bond energies of: N≡N is 946 kJ mol

^{-1}

, H -H is 435 kJ mol

^{-1}

, N -N is 159 kJ mol

^{-1}

, and N - H is 389 kJ mol

^{-1}

, the heat of formation of hydrazine in the gas phase in kJ mol

^{-1}

Solution:

N_2 + 2H_2 → 1 N_2H_4 ; ∆H_\f

∆H_\f = 1 × E_{N≡N} + 2 E_{H−H }

-4 E_{N−H} -1 E_{N−N}

[(1 × 946) + ( 2 × 435) -4 × (389)

-1 × (159)] kJ

= 101 kJ/mol

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