Question

The standard enthalpy of formation ($\Delta$Hf°) at 398 K for methane, CH4(g) is 74.8 kJ mol–1. The additional information required to determine the average energy for C - H bond formation would be :

• A. The dissociation energy of H2 and enthalpy of sublimation of carbon
• B. Latent heat of vapourisation of methane
• C. The first four ionization energies of carbon and electron gain enthalpy of hydrogen
• D. The dissociation energy of hydrogen molecule, H2

Answer 1

Answer: (A)

The dissociation energy of H2 and enthalpy of sublimation of carbon

Answer 2

C + 2H2 $\rightarrow$CH4; $\Delta$H⁰ = – 74.8 kJ mol–1

In order to calculate average energy for C – H bond formation we should know the followng data.

C(graphite) $\rightarrow$ C(g); $\Delta$H ¦o = enthalpy of sublimation of carbon

H2 (g) $\rightarrow$ 2H(g) ; $\Delta$Ho bond dissociation energy of H2.