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

  • question_answer
    For the reaction, \[{{A}_{(s)}}+2{{B}_{(g)}}\to 4{{C}_{(s)}}+{{D}_{(l)}}\]. \[\Delta H\] and \[\Delta U\] are related as ???      [Orissa JEE 2004]

    A)                 \[\Delta H=\Delta U\]   

    B)                 \[\Delta H=\Delta U+3RT\]

    C)                 \[\Delta H=\Delta U+RT\]            

    D)                 \[\Delta H=\Delta U-3RT\]

    Correct Answer: D

    Solution :

               \[A(s)+3B(g)\to 4C(s)+D(l)\]                    \[\Delta ng=0-3=-3\];  \[\Delta H=\Delta E+\Delta ngRT\]\[[\because \,\,\Delta E=\Delta U]\]                    \[\therefore \Delta H=\Delta U+(-3)\,(RT)\]                 \[\Delta H=\Delta U-3\,(RT)\];  \[\Delta H=\Delta U-3RT\]


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