Enthalpy calculation help

I need help with 2.4 chemistry
Hi, I just wanted to ask if anyone is familiar with how to calculate the bond energies of a bond eg : calculate bond energy of the C-H bond
heres the link to the question for refrence https://www.nzqa.govt.nz/nqfdocs/ncea-resource/exams/2021/91164-exm-2021.pdf

Question THREE C ( i)

thanks! happy studying

Welcome to StudyIT.
The key aspect of this question is recognising that the overall enthalpy change in a chemical reaction depends on the difference between the total energy required to break existing bonds (the reactants) and the total energy produced when the new bonds are formed in the products.
Because making bonds releases energy this can be written in the form

Note that in this particular calculation we subtract the total bond energies of the products from the reactants. This is because the bond enthalpy is defined as the energy required to break the bond and so is always a positive value (an endothermic reaction). Conversely when bonds are made energy is always released. As most spontaneous reactions are exothermic this means that the more energy is released from making new bonds than is required to break the original bonds. This is consistent with spontaneous reactions usually being exothermic because this results in lower energy, more stable products.
Usually all of the required bond energies are given (as they are already known) and are used to calculate the overall enthalpy change of the reaction. But in this particular question the enthalpy change is given and it is one of the bond enthalpies that is “unknown”.
So the working looks like this where "x"is the unknown C-H bond enthalpy.

One final point - this type of calculation can only be used for reactions where ALL reactants AND products are in the gas phase as there are extra intermolecular forces of attraction involved with solids and liquids and these can’t be easily quantified.

thank you so much!!!