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Consider the following data: ΔfH°(methane, g) = −X kJ mol−1 Enthalpy of sublimation of graphite = Y kJ mol−1 Dissociation enthalpy of H2 = Z kJ mol−1 The bond enthalpy of C − H bond is given by:
- (X + Y + 2Z)/4
- (X + Y + 4Z)/2
- X + Y + Z
- (−X + Y + Z)/4
Correct answer: (X + Y + 2Z)/4
Solution
The bond enthalpy of the C−H bond is derived from the enthalpy changes involved in forming methane from its elements, which includes the enthalpy of sublimation of graphite and the dissociation of hydrogen molecules. The correct formula accounts for the contributions of these enthalpies appropriately, dividing by 4 to reflect the four C−H bonds in methane.
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