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

Gujarat BoardEnglish MediumSTD 11 ScienceChemistry5. Thermodynamics and thermochemistry1 Mark
MCQ
If $C + {O_2} \to C{O_2} + 94.2\,kcal$ ${H_2} + \frac{1}{2}{O_2} \to {H_2}O + 68.3\,kcal$$C{H_4} + 2{O_2} \to C{O_2} + 2{H_2}O + 210.8\,kcal$ then the possible heat of methane will be......$kcal$
  • A
    $47.3 $
  • ✓
    $20$
  • C
    $45.9 $
  • D
    $-47.3 $
✓

Answer

Correct option: B.
$20$
(b)$C + {O_{2\,}} \to \,\,C{O_2} + 94.2\,\,Kcal.$…..$(i)$
${H_2} + \frac{1}{2}{O_2}\,\, \to \,{H_2}O + 68.3\,\,Kcal.$….$(ii)$
On multiplication of eq. (ii) by 2 and than adding in eq. $(i)$
$C + 2{H_2} + 2{O_2} \to C{O_2} + 2{H_2}O + 230.8\,\,Kcal$…$(iii)$
On subtracting eq. $(iii)$ by following eq.
$C{H_4} + 2{O_2} \to C{O_2} + 2{H_2}O + 210.8\,\,Kcal.$ we get,
$C + 2{H_2} \to C{H_4}\,\,\Delta H = 20\,\,Kcal.$

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