KVPY Sample Paper KVPY Stream-SX Model Paper-22

  • question_answer
    The standard enthalpies of formation of \[C{{O}_{2}}(g)\] \[{{H}_{2}}O(l)\] and glucose(s) at \[25{}^\circ C\] are \[-\,400kJ/mol,\]\[-\,300kJ/mol\] and \[-\,1300kJ/mol,\] respectively. The standard enthalpy of combustion per gram of glucose at \[25{}^\circ C\]is

    A) \[+\,2900\,kJ\]

    B) \[-\,2900\,kJ\]

    C) \[-16.11\,kJ\]

    D) \[+16.11\,kJ\]

    Correct Answer: C

    Solution :

    Given
    \[H\underset{C{{O}_{2}}}{\mathop{{}^\circ }}\,={{\Delta }_{f}}H{}^\circ \left( C{{O}_{2}} \right)=-400kJmo{{l}^{-1}}\]
    \[H\underset{{{H}_{2}}O}{\mathop{{}^\circ }}\,={{\Delta }_{f}}H{}^\circ \left( {{H}_{2}}O \right)=-300kJmo{{l}^{-1}}\]
    \[H\underset{glu\cos e}{\mathop{{}^\circ }}\,={{\Delta }_{f}}H{}^\circ \left( glu\cos e \right)\]
    \[=-1300\ \,kJ\,\,mo{{l}^{-1}}\]
    \[H\underset{{{O}_{2}}}{\mathop{{}^\circ }}\,={{\Delta }_{f}}H{}^\circ \left( {{O}_{2}} \right)=0.00\]
    \[{{C}_{6}}{{H}_{12}}{{O}_{6}}\left( s \right)+6{{O}_{2}}\to 6C{{O}_{2}}\left( g \right)+6{{H}_{2}}O\left( l \right)\]
    \[{{\Delta }_{c}}H{}^\circ \left( glu\operatorname{cose} \right)\]
    \[=6\left[ {{\Delta }_{f}}H{}^\circ \left( C{{O}_{2}} \right)+{{\Delta }_{f}}H{}^\circ \left( {{H}_{2}}O \right) \right]\]
    \[-\left[ {{\Delta }_{f}}H{}^\circ \left( {{C}_{6}}{{H}_{12}}{{O}_{6}} \right)+6{{\Delta }_{f}}H{}^\circ \left( {{O}_{2}} \right) \right]\]
    \[=6\left[ -400-300 \right]-\left[ -1300+6\times 0 \right]\]
    \[=-2900kJmo{{l}^{-1}}\]
    Molar mass of \[{{C}_{6}}{{H}_{12}}{{O}_{6}}=180gmo{{l}^{-1}}\]
    Thus, standard heat of combustion of glucose per gram
    \[=\frac{-2900}{180}=-16.11kJ{{g}^{-1}}\]


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