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11th Class Physics Thermodynamical Processes Question Bank Critical Thinking

  • question_answer
    If at 298 K the bond energies of \[C-H,\,C-C,\] \[C=C\] and \[H-H\] bonds are respectively 414, 347, 615 and 435 kJ \[mo{{l}^{-1}}\], the value of enthalpy change for the reaction \[{{H}_{2}}C=C{{H}_{2}}(g)+{{H}_{2}}(g)\to {{H}_{3}}C-C{{H}_{3}}(g)\] at 298 K will be   [AIEEE 2003]

    A)                 + 250 kJ

    B)                 ? 250 kJ

    C)                 + 125 kJ

    D)                 ? 125 kJ

    Correct Answer: D

    Solution :

           \[C{{H}_{2}}=C{{H}_{2(g)}}+{{H}_{2(g)}}\to {{H}_{3}}C-C{{H}_{3(g)}}\]            \[414\times 4=1656\]           \[414\times 6=2484\]            \[615\times 1=615\]              \[347\times 1=\,\,347\]            \[435\times 1\,\,\,=\,\underset{\overline{\underline{2706}}}{\mathop{435}}\,\]                           2831                                      \[\Delta H=2706-2831=-125\,kJ\]


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