Question #8a5a1

1 Answer
Aug 14, 2015

Yes, there is a very important distinction to be made between bond energy and bond-dissociation energy.


Bond energy and bond-dissociation energy both revolve around the energy (or enthalpy) needed to break chemical bonds, only they do so from different perspectives.

Moreover, bond energy actually builds upon the concept of bond-dissociation energy.

For a chemical compound, the bond-dissociation energy refers to the energy needed to break a particular bond in the compound by homolysis. This value is given per mole of substance.

Now, when a bond between two atoms is being broken by homolysis, each of the two atoms gets to keep the electrons they contributed to the bond.

Simply put, in terms of the bonding electrons, each atom gets what it put in. The products of a homolytic cleavage are free radicals.

Bond energy, on the other hand, will tell you the average value of the bond-dissociation energies for all the bonds that exist between the same two atoms in a chemical compound.

Let's take water, for example. Water has two hydrogen atoms bonded to one oxygen atom.

The bond-dissociation energy for water will tell you the energy you need to give it (per mole) in order to break one #"O-H"# bond by homolysis.

#"H"_2"O" + color(blue)("energy") -> "HO" * + *"H"#

The bond energy for water will be the average energy needed to break both #"O-H"# bonds.

These two energies will be a little different because the first bond that's being broken is actually #"H-OH"#. The presence of a hydrogen atom bonded to the oxygen will influence the energy needed to break the other oxygen - hydrogen bond.

The bond energies you see listed in tables actually represent the average value needed for a wide range of compounds, not just for one.

The bond energy for the #"O-H"# bond differs depending on what molecule you're actually dealing with, that is why an average value is calculated to begin with.