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JEE Main & Advanced Chemistry Thermodynamics / रासायनिक उष्मागतिकी Question Bank Heat of reaction

  • question_answer
    The heat of combustion of \[C{{H}_{4(g)}},\,{{C}_{(\text{graphite})}}\] and \[{{H}_{2(g)}}\] are \[20\,kcal,\,-40\,kcal\] and \[-10\,kcal\] respectively. The heat of formation of methane is         [EAMCET 1998]

    A)                 \[-4.0\,kcal\]     

    B)                 \[+40\,kcal\]

    C)                 \[-80\,kcal\]      

    D)                 \[+80\,kcal\]

    Correct Answer: C

    Solution :

               Given,            (i)  \[C{{H}_{4}}+2{{O}_{2}}\to C{{O}_{2}}+2{{H}_{2}}O\], \[\Delta H=+20\]kcal                    (ii) \[C+{{O}_{2}}\to C{{O}_{2}}\] , \[\Delta H=-40\,\,kcal.\]            (iii) \[{{H}_{2}}+\frac{1}{2}{{O}_{2}}\to {{H}_{2}}O\], \[\Delta H=-10\,\,kcal.\]            Aim: \[C+2{{H}_{2}}\to C{{H}_{4}}\]                    (ii) + 2 × (iii) ? (i) gives.                                 \[\Delta H=-40+2\,(-10)-(+\,20)=-80\, & \,kcal.\]


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