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5. Thermodynamics and thermochemistry — Chemistry STD 11 Science — Question

Gujarat BoardEnglish MediumSTD 11 ScienceChemistry5. Thermodynamics and thermochemistry1 Mark
MCQ
Enthalpy of combustion of $CH_4, C_2H_6$ and $C_3H_8$ are $-210.8, -368.4$ and $-526.2\, k\, cal\, mol^{-1}$ respectively. Enthalpy of combustion of hexane can be predicted as........$k\, cal\, mol^{-1}$
  • A
    $-840$
  • B
    $-684$
  • ✓
    $-1000$
  • D
    none of these
✓

Answer

Correct option: C.
$-1000$
c
$\Delta {H_c}({C_2}{H_6}) - \Delta {H_c}(C{H_4})$

          $ =  - 368.2 - ( - 210.8) =  - 157.4$

$\Delta {H_c}({C_3}{H_8}) - \Delta {H_c}({C_2}{H_6})$

          $ =  - 526.2 - ( - 368.4) =  - 157.8$

Thus, average

$\Delta {H_c}( - C{H_2}) = \frac{{157.4 + 157.8}}{2}$

$ =  - 157.6\,k\,cal\,mo{l^{ - 1}}$

Then,

$\Delta {H_c}({C_6}{H_{14}}) = \Delta {H_c}({C_3}{H_8}) + 3\Delta {H_c}( - C{H_2})$

$ =  - 526.2 + 3( - 157.6) =  - 999\,k\,cal\,mo{l^{ - 1}}$

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