Given the bond energies N≡N, H−H and N−H bonds are 945, 436 and 391 KJ mole−1 respectively, the enthalpy of the following reaction N2(g)+3H2(g)→2NH3(g) is:
A
-93 KJ
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B
102 KJ
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C
90 KJ
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D
105 KJ
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Solution
The correct option is B -93 KJ It takes 945 kJ of energy to break N≡N.
It takes 436×3=1308 kJ to break 3H2 molecules.
It releases 2×3×391=2346 kJ to form 2NH3 molecules.
Enthalpy of the reaction = Enthalpy of reactants − Enthalpy of products =945+1308−2346=−93 kJ