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Question

Bond dissociation enthalpies of H2(g) and N2(g) are 436.0 kJmol1 and 941.3 kJmol1 respectively. Enthalpy of formation of NH3(g) is 46.0 kJmol1. What is the enthalpy of atomization ofNH3.

A
490.2 kJ/mol
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B
490.2 kJ/mol
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C
390.2 kJ/mol
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D
390.2 kJ/mol
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Solution

The correct option is C 390.2 kJ/mol
We are given,
B.E. of H2=436.0 kJ/mol
B.E. of N2=941.3 kJ/mol

Reaction taking place is:
12N2+32H2NH3

ΔHo(NH3)=B.E.ofreactantsB.E.ofproducts
=12(B.E. of N2 bond)+32(B.E. of H2 bond)3(B.E. of NH bond
putting all the values,
46=12(941.3)+32(436.0)3(B.E. of NH bond

solving this, we get;
B.E. of NH bond=390.2 kJ/mol
​​​​​​​

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