Bond Energy with Enthalpy of Formation?

Question

An unknown gas, X2(g), which behaves much like N2(g) (NN), is analysed and the following enthalpies of formation are obtained:

Gas |enthalpies of formation
X(g) | 401 kJ/mol
H(g) | 217 kJ/mol
X2H4(g) |201 kJ/mol


We also know that the X-H bond energy is 333 kJ/mol.
Use this information to estimate the X-X (single-)bond energy in the X2H4 molecule.

Answer 1

We know that the value of 201kJ/mol contains energy input and energy output ie 201 = bonds broken - bonds formed.

Bonds broken = (2 x 401) + (4 x 217)

Bonds formed = (X-X) + (4 x 333).

Now you can solve it!