Hope this still helps!
For part (a), we need to know deltaHof (at 298K) for each of the products and reactants. Remember that the heat of formation of "any" element in its thermodynamic standard state is defined as zero.
deltaHof HCl(g) = -92.307 kJ/mole
deltaHof O2(g) = 0 kJ/mole
deltaHof Cl2(g) = 0 kJ/mole
deltaHof H2O(g) = -241.818 kJ/mole
deltaHof rxn = summation(deltaHof products) - summation(deltaHof reactants)
deltaHof rxn = [(2)(deltaHof H2O(g)) + 2(deltaHof Cl2(g) )] - [4(deltaHof HCl(g)) + 1(deltaHof O2(g))]
deltaHof rxn = [2(-241.818) + 2(0)] - [4(-92.307) + 1(0)]
= -483.636 + 369.228
deltaHof rxn = -114.408 kJ/mole
For part (b), we need to know the bond dissociation energies (BDE) at 298 K for each of the bonds in the equation.
BDE O-H = 463 kJ/mole
BDE Cl-Cl = 243 kJ/mole
BDE O=O = 485 kJ/mole
BDE H-Cl = 432 kJ/mole
Energy needed to break bonds = 4(energy to break H-Cl bond) + 1(energy to break O=O bond)
Energy to break bonds = 4(432) + 1(485) (positive since energy required)
Energy to break bonds = 1728 + 485 = 2213 kJ/mole
Do same for bond formation.
Energy to form bonds = 4(energy to form O-H bond) + 2(energy to form Cl-Cl bond)
Energy to form bonds = 4(-463) + 2(-243) (negative since energy released)
Energy to form bonds = -1852 + -486 = -2338 kJ/mole
deltaHof rxn = energy to break bonds + energy released by bond formation
deltaHof rxn = 2213 + (-2338) kJ/mole
deltaHof rxn = -125 kJ/mole
For question (c):
Most useful bond energies are determined by averaging the values determined from entire classes of compounds, not just individual compounds. For example, the C-H and C-C bond energies are obtained by looking at hydrocarbons in general, not just from methane or ethane. As a result of the averaging, differences will be seen between both methods.