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

Gujarat BoardEnglish MediumSTD 11 ScienceChemistry5. Thermodynamics and thermochemistry1 Mark
MCQ
What will be the heat of formation of methane, if the heat of combustion of carbon is $'-x'\, kJ$, heat of formation of water is $'-y'\, kJ$ and heat of combustion of methane is $'z'\, kJ$ ?
  • A
    $(-x -y + z)\, kJ$
  • B
    $(-z -x + 2y)\, kJ$
  • C
    $(-x -2y -z)\, kJ$
  • ✓
    $(-x -2y + z)\, kJ$
✓

Answer

Correct option: D.
$(-x -2y + z)\, kJ$
d
From given data, we have

$C + {O_2} \to C{O_2} - x\,kJ...(i)$

${H_2} + \frac{1}{2}{O_2} \to {H_2}O - y\,kJ...(ii)$

$C{H_4} + 2{O_2} \to C{O_2} + 2{H_2}O + z\,kJ...(iii)$

The required equation is

$C + 2{H_2} \to C{H_4} + Q$

To get the required equation, operate $(i) + 2 \times (ii) - (iii)$

Thus, we get

$C + 2{H_2} \to C{H_4} + [( - x) + ( - 2y) - ( - z)]$

Thus, heat of formation of methane is $(-x-2y+z)\,kJ$

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