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The standard enthalpy of formation (ΔfH°) at 298 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

1. the first four ionization energies of carbon and electron gain enthalpy of hydrogen
2. the dissociation energy of hydrogen molecule, H2
3. the dissociation energy of H2 and enthalpy of sublimation of carbon
4. latent heat of vapourization of methane

Correct answer: the dissociation energy of H2 and enthalpy of sublimation of carbon

Solution

To calculate the average energy for C−H bond formation, we need the dissociation energy of H2 to understand the energy required to form hydrogen atoms, and the enthalpy of sublimation of carbon to convert solid carbon into gaseous carbon atoms, which are both essential for forming methane.

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