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11th Class Physics Thermodynamical Processes Question Bank Critical Thinking

  • question_answer
    Given the bond energies \[N\equiv N,\,\,H-H\] and \[N-H\] bonds are \[945,\,436\] and \[391\,kJ\,mol{{e}^{-1}}\] respectively, the enthalpy of the following reaction \[{{N}_{2}}(g)+3{{H}_{2}}(g)\to 2N{{H}_{3}}(g)\] is [EAMCET 1992; JIPMER 1997]

    A)                 \[-93\,kJ\]          

    B)                 \[102\,kJ\]

    C)                 \[90\,kJ\]            

    D)                 \[105\,kJ\]

    Correct Answer: A

    Solution :

           \[\underset{\begin{matrix}    {}  \\    \text{Energy absorbed}  \\ \end{matrix}}{\mathop{\underset{945\,\,\,\,\,\,\,\,\,\,\,\,\,+\,\,3\,\,\,\,\,\times \,\,\,436\,}{\mathop{N\equiv N+3H-H}}\,}}\,\,\,\xrightarrow{{}}\underset{\begin{matrix}    {}  \\    \text{Energy released}  \\ \end{matrix}}{\mathop{\underset{2\times (3\times 391)=2346}{\mathop{\overset{H}{\mathop{\overset{|}{\mathop{\underset{H}{\mathop{\underset{\,|}{\mathop{2N}}\,}}\,-}}\,}}\,H}}\,}}\,\]                    Net. energy released = 2346 ? 2253 = 93 kJ                                  i.e. ΔH = ? 93 kJ .


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