The standard enthalpy of formation (ΔfHo298) for methane, CH4, is -74.9 kJmol⁻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?

To calculate the average energy given out in the formation of a C-H bond in methane (CH4), we need to consider the energy changes involved in forming the molecule from its constituent elements in their standard states. The standard enthalpy of formation of methane is given as -74.9 kJ/mol. This represents the enthalpy change for the reaction:

C(s, graphite) + 2H2(g) → CH4(g) ΔfHo298 = -74.9 kJ/mol

The enthalpy change for this reaction involves:

1. Sublimation of carbon: The transformation of solid carbon (graphite) to gaseous carbon atoms requires energy (enthalpy of sublimation).
2. Dissociation of hydrogen molecules: Breaking the H-H bonds in two molecules of hydrogen gas (H2) requires energy (dissociation energy of H2).
3. Formation of C-H bonds: The formation of four C-H bonds releases energy.

Therefore, to calculate the average energy of a C-H bond, we need to know the enthalpy of sublimation of carbon and the dissociation energy of H2. These values will allow us to account for the energy input needed to convert the reactants to gaseous atoms before the C-H bonds are formed. The energy released in the formation of the C-H bonds can then be calculated using Hess's Law.