An unknown gas, X2(g), which behaves much like N2(g) (N-triple-bond-N), is analysed and the following enthalpies of formation are obtained:
Gas std. heat of formation
X(g) 483 kJ/mol
H(g) 217 kJ/mol
X2H4(g) 167 kJ/mol
We also know that the X-H bond energy is 378 kJ/mol.
Use this information to estimate the X-X (single-)bond energy in the X2H4 molecule.