Bond energy (BE) is the average amount of energy required to break all the chemical bonds between the same two types of atoms in a molecule (e.g., carbon and hydrogen, hydrogen and oxygen). It is also called average bond enthalpy or mean bond enthalpy. Typical units are kilocalories per mole (kcal/mol) or kilojoules per mole (kJ/mol). Bond energy is a measure of the bond strength of a chemical bond.
Difference Between Bond Energy and Bond Dissociation Energy
Bond energy and bond dissociation energy are related concepts. Bond energy is the average of all of the bond dissociation energies for a single type of bond in a molecule.
You might assume the bond dissociation energy is a constant value for a type of bond (e.g., C-H, O-H, O=O), but it actually changes after each chemical bond is broken. The composition of the rest of the molecule also affects bond dissociation energy values.
For example, the bond dissociation energy for O-H in water (H2O) is different when the first bond is broken compared to when the second bond is broken. The bond energy is the average of these values.
Bond energy is a single value for a certain bond in a molecule (e.g., O-H), while bond dissociation energy can change depending on whether it’s the first broken bond or not and what’s going on in the rest of the compound.
It’s worth noting tables of bond dissociation values are for homolytic bond cleavage. What this means is that electrons participating in a bond divide equally between products when the bond breaks. In reality, some bonds break heterolytically, where the shared electrons go to one product and not the other. This occurs with transition metals and some ligands.
Relationship Between Bond Energy and Bond Strength
Large bond energy values indicate strong chemical bonds and stable molecules. High bond energy correlates with short distances between two atoms participating in a covalent bond. Small bond energy values indicate relatively weak chemical bonds and less-stable molecules. There is a larger distance between two atoms when the bond energy is small. So, if you know the distance between two atoms in a chemical bond, you can predict the bond energy. Also, short bonds tend to be double or triple bonds, while long bonds are single bonds.
It’s a little bit different in ionic compounds because the ions often arrange in a lattice. Distance isn’t as reliable an indicator of bond strength. Bond energy increases as the difference between the electronegativity values of the two atoms increases. In other words, the strongest ionic bonds form between atoms with large electronegativity differences.
Is Energy Released When Bonds Are Broken or Formed?
Breaking a chemical bond always requires an input of energy. The bond absorbs energy so that the atoms can separate. Bond-breaking is an endothermic process. Its values always have a positive sign.
Forming a chemical bond always releases energy. Bond formation is an exothermic process. Its enthalpy change is negative.
Whether a chemical reaction is exothermic or endothermic depends on the difference between the energy absorbed to break bonds and the energy released to form new ones. If bond breaking absorbs less energy than bond formation releases, then the reaction is exothermic. If bond breaking absorbs more energy than bond formation releases, then the reaction is endothermic.