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NEET Sample Paper NEET Sample Test Paper-91

  • question_answer
    The standard enthalpy of formation \[({{\Delta }_{f}}H{}^\circ \,{{\,}_{298}})\] for methane, \[C{{H}_{4}}\] is \[-74.9\,kJ\,mo{{l}^{-1}}.\] In order to calculate the average energy given out in the formation of a \[C-H\] bond from this it is necessary to know which one of the following?

    A) The dissociation energy of the hydrogen molecule,\[{{H}_{2}}.\]

    B) The first four ionisation energies of carbon.

    C) The dissociation energy of \[{{H}_{2}}\] and enthalpy of sublimation of carbon (graphite).

    D) The first four ionisation energies of carbon and electron affinity of hydrogen.

    Correct Answer: C

    Solution :

    [c]  To calculate average enthalpy of \[C-H\] bond in methane following informations are needed (i) dissociation energy of \[{{H}_{2}}\]i.e. \[\frac{1}{2}{{H}_{2}}(g)\xrightarrow{{}}H(g);\]\[\Delta H=x\] (suppose) (ii)  Sublimation energy of \[C(\text{graphite})\] to \[C(g)\] \[C(\text{graphite})\xrightarrow{{}}C(g);\] \[\Delta H=y\] (Suppose) Given \[Ag\,\,\text{left}=\text{ }1.081.08=0\] \[C{{H}_{4}}(g);\]\[\Delta H=75\,kJ\,\,mo{{l}^{-1}}\]


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