Bond Order
 The number of bonding electron pairs shared by two
atoms in a molecule.
 Can be whole numbers as well as fractional numbers.
 Draw H2, NH3, and CH4.

Each have a bond order of 1
 Draw CO2 and C2H4

The C=O bond in CO2 has a bond order of 2, as does the C=C
bond in C2H4.
 Draw N2 and CO

Bond orders of 3
Bond Order
 Draw O3
 It has 2 resonance structures, so how do I calculate the
bond order?


Bond Order = _[Number of shared pairs in all X-Y bonds]_
[Number of X-Y links in the molecule or ion]
Bond Order = _[Number of bonds]_
[Number of structures]
 Each O-O bond has a total of 3 shared pairs/2 structures

Bond Order = 1.5
Bond Order
 Draw C2O4-2 …
 How many resonance structures are there?
 What’s the bond order between Carbon and Oxygen?
 Draw OCN- …
 How many resonance structures are there?
 What’s the bond order between C and O? C and N?
Bond Length
 Is the distance between the nuclei of two bonded
atoms.
 Related to 2 things:
 Size of the atom
 The larger the atom, the longer the bond
 H-F < H-Cl < H-Br < H-I
 Bond Order
 When comparing bonds between the same two atoms, the
larger the bond order, the shorter the bond
 C-O > C= O > C= O
 Single bonds are longer than double bonds, which are longer
than triple bonds
Bond Length
 Compare the bond lengths between carbon monoxide,
carbon dioxide, and carbonate ion. Arrange them in
order of increasing bond length.
 Carbon Monoxide < Carbon Dioxide < Carbonate Ion
Bond order:
3
2
1.3
Bond Enthalpy
 Bond Dissociation Enthalpy
 the enthalpy change for breaking a bond in a molecule
with the reactant and products in the gas phase
 Enthalpy (ΔH)


The process of breaking bonds in a molecule is always
endothermic
 ΔH is positive. Energy must be put in to break a bond.
The process of forming bonds in a molecules is always
exothermic
 ΔH is negative. Energy is released when a bond is formed
Bond Enthalpy
ΔHrxn = [∑(Bond enthalpies bonds broken)] –
[∑(Bond enthalpies bonds formed)]
or
ΔHrxn = [∑ ΔHbonds broken ] – [ ∑ ΔHbonds formed]
Practice Problems
 Compare the nitrogen-nitrogen bond in hydrazine,
H2NNH2, with that in “laughing gas”, N2O. In which
molecule is the nitrogen-nitrogen bond shorter? In
which is the bond stronger?
Practice Problems
 The compound oxygen difluoride is quite reactive,
giving oxygen and HF when treated with water.
OF2(g) + H2O(g)  O2(g) + 2HF(g)
ΔHrxn = -318kJ/mol●rxn
Using bond dissociation enthalpies, calculate the bond
dissociation energy of the O-F bond OF2.
H-O
O=O
H-F
= 463 kJ/mol
= 498 kJ/mol
= 565 kJ/mol