JEE Main & Advanced Sample Paper JEE Main Sample Paper-47

  • question_answer
    Direction: The bond dissociation energy of a diatomic molecule is also called bond energy. Bond energy is also called, the heat of formation of the bond from the gaseous atoms constituting the bond with reverse sign.
    Example\[H(g)+Cl(g)\xrightarrow{\,}\,H-Cl(g),\]\[\Delta {{H}_{f}}=-431\,\,kJ\,\,mo{{l}^{-1}}\]or bond energy of \[H-Cl=-(\Delta {{H}_{f}})\]\[=-(431)=+431\,kJ\,mo{{l}^{-1}}\] When a compound shows resonance there occurs a fair agreement between the calculated values of heat of formation obtained from bond enthalpies and any other method. However deviation occur incase of compounds having alternate double bonds.
    Example                \[\underset{(g)}{\mathop{{{C}_{6}}{{H}_{6}}}}\,\xrightarrow{\,}\underset{(g)}{\mathop{\,6C}}\,+\underset{(g)}{\mathop{6H}}\,\]
    Resonance energy = experimental heat of formation - calculated heat of formation
    Estimate the average S?F bond energy in \[S{{F}_{6}}\]. The standard heat of formation values of\[S{{F}_{6}}(g),\,\,S(g)\] and \[F(g)\] are -1100, 275 and \[80\,\,kJ\,mo{{l}^{-1}}\] respectively.

    A)  309.17 kJ                   

    B)  206 kJ             

    C)  109.2 kJ                     

    D)  275.8 kJ

    Correct Answer: A

    Solution :

     Given \[S(s)+3{{F}_{2}}(g)\xrightarrow{\,}S{{F}_{6}};\] \[\Delta H=-1100\,kJ\]                                    ?(i) \[\frac{1}{2}{{F}_{2}}(g)\xrightarrow{\,}\,F(g);\]    \[\Delta H=80\,\,kJ\]    ?(ii) \[S(s)\xrightarrow{\,}\,S(g);\,\,\Delta H=+275\,kJ\]                ?(iii) Subtracting Eq. (iii) from Eq. (ii) with 6 gives \[S(g)+3{{F}_{2}}(g)\,\beta \,\xrightarrow{\,}\,6{{F}_{6}};\]\[\Delta H=-1100-275\] \[=-1375\,\,kJ\]            ?(iv) Subtracting Eq. (ii) with 6 gives \[3{{F}_{2}}(g)\xrightarrow{\,}\,6F(g);\,\,\Delta H=6\times 80\,kJ\] \[=480\,kJ\]     ?(v) Subtracting Eq. (v) from Eq. (iv). \[S(g)+6F(g)\xrightarrow{\,}\,S{{F}_{6}};\] \[\Delta H=-1375-480=-1855\,kJ\] or         \[S{{F}_{6}}\xrightarrow{\,}\,S(g)+6F(g);\] \[\Delta H=+1855\,kJ\] Average \[BE=\frac{1855}{6}=309.17\,kJ\]

You need to login to perform this action.
You will be redirected in 3 sec spinner