JEE Main 2014 · 12 Apr · Shift 1 · Chemistry Q40

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ChemistrySingle correct+4 / -1Question 40 of 90

The standard enthalpy of formation ( $\Delta_fH^\circ_{298}$ ) for methane, $\ce{CH4}$ is $-74.9$ kJ mol $^{-1}$ . In order to calculate the average energy given out in the formation of a C−H bond from this it is necessary to know which one of the following ?

the dissociation energy of the hydrogen molecule,

\ce{H2}

the first four ionisation energies of carbon.

the dissociation energy of

\ce{H2}

and enthalpy of sublimation of carbon (graphite).

the first four ionisation energies of carbon and electron affinity of hydrogen.

Answer and worked solution

Official answer: C

$\Delta_fH$ is from C(graphite) and $\ce{H2}$ (g). To reach gaseous atoms you add the sublimation enthalpy of carbon and twice the dissociation energy of $\ce{H2}$ :
$4\,\varepsilon_{\ce{C-H}}=\Delta_{sub}H(\ce{C})+2D(\ce{H2})-\Delta_fH(\ce{CH4}).$

Topics:Bond enthalpy Hess's law and enthalpies of reaction

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JEE Main 2014 · 12 Apr · Shift 1, Chemistry question 40