Consider the hypothetical elements X and Y. Suppose the enthalpy of formation for the compound XY is -336 kJ/mol, the bond energy for X2 is 414 kJ/mol, and the bond energy for Y2 is 159 kJ/mol. Estimate the XY bond energy in units of kJ/mol.

Answer 1

I worked this for you earlier but I wasn't sure if the delta Hf was -336 or not. It is; therefore, the delta Hrxn is -672 for the reaction as written.
X2 + Y2 ==> 2XY
(sum B.E.X2 + B.E.Y2)-(sum B.E. 2XY) = deltaHrxn*2
(414+159)-(2*XY) = -672
Solve for XY and I get 622.5 kJ/mol for X-Y.

Check that this way:
(1/2)X2 + (1/2)Y2 = XY
(sum B.E.X2 + B.E.Y2)-(B.E.XY) = ??
(207+79.5)-(622.5) = ??
(286.5)-(622.5) = -336
Check my thinking.

Answer 2

Consider hypothetical elements X and Y. Suppose the enthalpy of formation of the compound XY is −336 kJ/mol, the bond energy for X2 is 421 kJ/mol, and the bond energy for Y2 is 152 kJ/mol. Estimate the XY bond energy in units of kJ/mol.