bond energy?

Question

the standard enthalpy of formation of H2O2 is -136 kj/mol. Use the info, along with the bond energies below, to estimate the bond energy of an O-O bond.

H-H 432
H-O 467
O=O 495
O-O ???

someone please help at least a starting point would be nice and a full out explanation.

Answer 1

Bond breaking process is endothermic (requires energy).
Bond formation process is exothermic (releases energy).

The standard enthalpy of formation of any compound is the energy released when 1 mole of that compound is formed from the most stable form of its elements at standard state conditions (25C and 1 atm). [ note: for a few unstable compound it requires energy]

Formation equation of H2O2;

H2 + O2 ------> H2O2
H-H + O=O ----> H-O-O-H

According to the equation; one H-H bond and one O=O bond should be broken and 2 O-H bond and one O-O bond should be formed.

Energy required to break bonds:
H-H : 432
O=O : 495
Total : 927 kJ/mol This is energy required (endothermic)

Energy released during formation of the bonds:
2 H-O : 2 x 467 = 934
O-O : Let's say X
Total : (934 + X) kJ/mol This is energy relesed (exothermic)

The sign of the energy in endothermic processes is + and the sign of the energy in exothermic processes is -

927 - (934 + X) = -136
927 - 934 - X = - 136
927 - 934 + 136 = X
X = 129 kJ/mol
O-O = 129 kJ/mol
Breaking this bond requires 129 kJ of energy (+)
Formation of this bond releases 129 kJ of energy (-)

Answer 2

For the reaction H2 + O2 = H2O2, DeltaH = - 136 kJ/mol

You can also imagine going from H2 + O2 by breaking the molecules into atoms and then sticking them together. This will give you the same total energy.* You will have one unknown in this calculation, which is the energy you are after.

* If you have been told how to correct for the difference between energy and enthalpy, remember to do that. But it won't makemuch difference to your answer.