Energy changes and bonding

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i've been told that the molar bond enthalpy for F-F is 159 kJ and for O=0 is 498 kJ. The overall enthalpy change for the reaction of 2 moles of fluorine with 1 mole of oxygen is -44 kJ. So how do I go about finding the mean molar bond enthalpy of the O-F bonds in oxygen difluoride?
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Corrie writes .....
 
1. Breaking the bonds in two moles of F₂ will cost 2 x 159 kJ. Breaking up one O₂ molecule costs 498 kJ. Add these two amounts of energy together.
 
2. The difference between the energy needed to break the old bonds and the energy released to make the new bonds here is 44 kJ, according to your data. Thus more energy is released by forming new bonds than is needed to break the old ones.
 
3. Therefore the energy released by forming four O-F bonds is 44 kJ more than the amount of energy in 1. Divide this total amount of energy by 4 to find the mean bond enthalpy for O-F in kJ/mol.
 

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updated: 21 February 2008