Estimate Delta_fH of C2H6(g) from the following data in kJ mol^-1: enthalpy of sublimation of carbon (graphite -> C(g)) =717; bond enthalpies H-H=436, C-H=414, C-C=346. Give the answer in kJ mol^-1 (nearest integer).

JEE Advanced levelNumerical answerDifficulty 3 of 3td151

Estimate $Δ_{f} H$ of $C X_{2} H X_{6} (g)$ from the following data in $kJ mo l^{- 1}$ : enthalpy of sublimation of carbon (graphite $\to C (g)$ ) $= 717$ ; bond enthalpies $H - H = 436$ , $C - H = 414$ , $C - C = 346$ . Give the answer in $kJ mo l^{- 1}$ (nearest integer).

Numerical answer type, answer in kJ/mol, tolerance 0.5.

Formation:

2 C (graphite) + 3 H X_{2} (g) C X_{2} H X_{6} (g)

. Energy to atomise reactants

= 2 (717) + 3 (436) = 2742

. Energy released forming bonds in

C X_{2} H X_{6}

6 (414) + 346 = 2830

Δ_{f} H = 2742 - 2830 = - 88 kJ mo l^{- 1}