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JEE Main & Advanced Chemistry Thermodynamics / रासायनिक उष्मागतिकी Question Bank First law of thermodynamics and Hess law

  • question_answer
    \[\Delta {{E}^{o}}\] of combustion of isobutylene is \[-X\,kJ\,mo{{l}^{-1}}\]. The value of \[\Delta {{H}^{o}}\] is                  [DCE 2004]

    A)                 \[=\Delta {{E}^{o}}\]      

    B)                 \[>\Delta {{E}^{o}}\]

    C)                 \[=0\]   

    D)                 \[<\Delta {{E}^{o}}\]

    Correct Answer: D

    Solution :

               \[{{(C{{H}_{3}})}_{2}}C=C{{H}_{2}}(g)+6{{O}_{2}}(g)\to 4C{{O}_{2}}(g)+4{{H}_{2}}O(l)\]                    \[\Delta ng=4-6=-2\] (i.e., negative)                    we know that \[\Delta H=\Delta E+\Delta ngRT\]                    \[=\Delta E-(\Delta ng)RT\] (\[\because \,\Delta ng=-ve)\]                                 \[\therefore \,\,\Delta H<\Delta E\]


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