Bond Energy vs Bond Dissociation Energy
As proposed by the American chemist G.N.Lewis, atoms are stable when they contain eight electrons in their valence shell. Most of the atoms have less than eight electrons in their valence shells (except the noble gases in the group 18 of the periodic table); therefore, they are not stable. These atoms tend to react with each other, to become stable. Thus, each atom can achieve a noble gas electronic configuration. This can be done by forming ionic bonds, covalent bonds or metallic bonds. Electronegativity of atoms becomes a deciding factor when bonds are forming. When two atoms having similar or very low electronegativity difference, react together, they form a covalent bond by sharing electrons. When the number of sharing electrons is more than one from each atom, multiple bonds result. Bond making releases energy and bond breaking requires energy. This energy is stored in the bond.
When bonds are in making, some amount of energy is released. And when the bonds are breaking some amount of energy is absorbed. For a certain bond, this energy is constant. And it is known as the bond energy. Thus, bond energy can be defined as the amount of heat required to break one mole of molecules into its corresponding atoms. The energy of a bond can be in various forms as chemical energy, mechanical energy or electrical energy. However, ultimately, all these energies are converted to heat. Therefore, bond energy is measured in kilo joule or kilocalorie. Bond energy is an indicator of the bond strength. Stronger bonds are hard to cleave. Therefore, the bond energies of them are larger. On the other hand, weak bonds have small bond energies, and they are easy to cleave. Bond energy also indicates the bond distance. Higher bond energies mean the bond distance is low (therefore, bond strength is high). Further, when the bond energy is low bond distance is higher. As mentioned in the introduction electronegativity plays a part in bond formation, hence, electronegativity of the atoms also contributes to the bond energy.
Bond Dissociation Energy
Bond dissociation energy is also a measurement of the bond strength. It can be defined as the enthalpy change taking place when a bond is cleaved by homolysis. Bond dissociation energy is specific to a one bond. In this case, the same bond can have different bond dissociation energies depending on the situation. For example, there are four C-H bonds in a methane molecule and all the C-H bonds don’t have the same bond dissociation energy.
What is the difference between Bond Energy and Bond Dissociation Energy?
• Bond dissociation energy is different from the bond energy. Bond energy is the average value of all the bond dissociation energies of a molecule.
• For example, in methane molecule, bond dissociation energies for C-H bonds are 435 kJ/mol, 444 kJ/mol, 444 kJ/mol and 339 kJ/mol. However, the bond energy of the C-H of methane is 414 kJ/mol, which is the average of all four values.
• For a molecule, bond dissociation energy may not necessarily be equal to the bond energy (as for above given methane example). For a diatomic molecule, bond energy and the bond dissociation energy is the same.