Bond Order and Bond Energy?

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:

Gasstd. heat of formation
X(g)389 kJ/mol
H(g)217 kJ/mol
X2H4(g)137 kJ/mol

We also know that the X-H bond energy is 342 kJ/mol.
Use this information to estimate the X-X (single-)bond energy in the X2H4 molecule.

Please show your work! =)

Thanks a lot in advance.

Answer:
X2 --> 2 X 778 kJ Bond dissociation energy for X triple bond X
H2 --> 2 H 434 kJ Bond dissociation energy for H - H

X2 (g) + 2 H2 (g) --> X2H4 (g) 137 kJ

So 137 kJ = [X triple bond X + 2 (H - H)] - [ 4(X - H) + X - X]

137 = [778 + 2 (434)] - [4(342) + X - X]
137 = 1646 - 1368 - (X - X)
137 = 278 - (X - X)
137 - 278 = -(X - X)
-141 = -(X - X)
So, 141 kJ/mol is bond dissociation energy.

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Bond Order and Bond Energy?

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