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NEET Chemistry Thermodynamics / रासायनिक उष्मागतिकी NEET PYQ-Thermodynamics

  • question_answer
    Heat of combustion \[\Delta {{H}^{o}}\] for \[C(s),\,\,{{H}_{2}}(g)\] and\[C{{H}_{4}}(g)\] are -94, -68 and -213 kcal/mol. Then \[\Delta {{H}^{o}}\] for \[C(s)+2{{H}_{2}}(g)\xrightarrow[{}]{{}}C{{H}_{4}}(g)\] is:  [AIPMT 2002]

    A)        - 17 kcal

    B)       - 111 kcal

    C)       - 170 kcal   

    D)  - 85 kcal

    Correct Answer: A

    Solution :

    For reaction,
                \[C(s)+2{{H}_{2}}(g)\xrightarrow[{}]{{}}C{{H}_{4}}(g),\,\Delta {{H}^{o}}=\,?\]
                \[\Delta {{H}^{o}}=-\,[(\Delta {{H}^{o}}\] of combustion of \[C+2\times \Delta H\] of combustion of \[{{H}_{2}})]\]
                            \[C+{{O}_{2}}\xrightarrow{{}}C{{O}_{2}};\,\,\,\Delta H=-94\,\,kcal\]  ...(i)
    \[2{{H}_{2}}+{{O}_{2}}\to 2{{H}_{2}}O;\Delta H=-68\times 2\,kcal\]…(ii)
                \[C{{H}_{4}}+2{{O}_{2}}\to C{{O}_{2}}+2{{H}_{2}}O;\,\,\Delta H=-213\,kcal\]…(iii)
                            \[=-[(-213)-(-94+2\times -68)]\,kcal/mol\]
                            \[=-[-213+230]=-17\,\,kcal/mol\]


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