MCQ
The standard heat of formation $\left(\Delta_{t} \mathrm{H}_{298}^{0}\right)$ of ethane in $(kJ/mol)$, if the heat of combustion of ethane, hydrogen and graphite are $-1560$, $-393.5$ and $-286 \;\mathrm{kJ} / \mathrm{mol},$ respectively is ........... $\mathrm{kJ/mol}$
  • A
    $-172.5$
  • $-192.5$
  • C
    $-202.5$
  • D
    $-162$

Answer

Correct option: B.
$-192.5$
b
$2 \mathrm{C}(\text { graphite })+3 \mathrm{H}_{2}(\mathrm{g}) \longrightarrow \mathrm{C}_{2} \mathrm{H}_{6}(\mathrm{g})$

$\Delta_{f} \mathrm{H}\left(\mathrm{C}_{2} \mathrm{H}_{6}\right) =2 \Delta \mathrm{H}_{\text {comb }}\left(\mathrm{C}_{\text {parplite }}\right)+3 \Delta \mathrm{H}_{\text {coond }}\left(\mathrm{H}_{2}\right) -\Delta \mathrm{H}_{\text {comb }}\left(\mathrm{C}_{2} \mathrm{H}_{6}\right)$

$=-(286 \times 2)-(393.5 \times 3)-(-1560)$

$=-572-1180.5+1560=-192.5 \mathrm{kJ} / \mathrm{mol} \mathrm{e}$

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