The key difference between bond energy and bond dissociation energy is that the bond energy is an average value whereas the bond dissociation energy is a particular value for a particular bond.
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 group 18 of the periodic table); therefore, they are not stable. Hence, these atoms tend to react with each other, to become stable. It can occur by forming ionic bonds, covalent bonds or metallic bonds depending on the electronegativity of atoms. When two atoms have similar or very low electronegativity difference, react together, they form a covalent bond by sharing electrons. Bond energy and bond dissociation energy are two concepts regarding covalent chemical bonds.
CONTENTS
1. Overview and Key Difference
2. What is Bond Energy
3. What is Bond Dissociation Energy
4. Side by Side Comparison – Bond Energy vs Bond Dissociation Energy in Tabular Form
5. Summary
What is Bond Energy?
When bonds form, some amount of energy releases. In contrast, bond breaking requires some amount of energy. For a certain chemical bond, this energy is constant. And we name it as the bond energy. Thus, bond energy is the amount of heat required to break one mole of molecules into its corresponding atoms.
Moreover, we can observe the energy of a chemical bond in various forms as chemical energy, mechanical energy or electrical energy. However, ultimately, all these energies convert into heat. Therefore, we can measure the bond energy in kilojoule or kilocalorie.
Figure 01: Bond Energy
Further, the bond energy is an indicator of the bond strength. For instance, 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). Furthermore, when the bond energy is low bond distance is higher. As mentioned in the introduction electronegativity plays a part in bond formation. Hence, the electronegativity of the atoms also contributes to the bond energy.
What is Bond Dissociation Energy?
Bond dissociation energy is also a measurement of the bond strength. We can define it as the enthalpy change taking place when a bond undergoes cleavage by homolysis. Bond dissociation energy is specific to a single 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.
Figure 02: Some Bond Dissociation Energies for Coordination Complexes
Hence, in the methane molecule, bond dissociation energies for C-H bonds are 439 kJ/mol, 460 kJ/mol, 423 kJ/mol and 339 kJ/mol. It is because the first bond breakage forms a radical species via homolysis, thereby the second bond breakage occurs from a radical species, which requires more energy that the first one. Likewise, step by step the bond dissociation energies change.
What is the Difference Between Bond Energy and Bond Dissociation Energy?
Bond energy is the average value of the gas-phase bond dissociation energies (usually at a temperature of 298 K) for all bonds of the same type within the same chemical species. However, bond energy and bond dissociation energy are not the same. Bond dissociation energy is the standard enthalpy change when a covalent bond is cleaved by homolysis to give fragments; which are usually radical species. Therefore, the key difference between bond energy and bond dissociation energy is that bond energy is an average value whereas bond dissociation energy is a particular value for a particular bond.
For example, in methane molecule, bond dissociation energies for C-H bonds are 439 kJ/mol, 460 kJ/mol, 423 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. Further, 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 are the same.
Below infographic on the difference between bond energy and bond dissociation energy provides more details on the differences.
Summary – Bond Energy vs 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. Hence, the key difference between bond energy and bond dissociation energy is that the bond energy is an average value whereas the bond dissociation energy is a particular value for a particular bond.
Reference:
1. “Bond-Dissociation Energy.” Wikipedia, Wikimedia Foundation, 5 Jan. 2019. Available here
2. Libretexts. “Bond Energies.” Chemistry LibreTexts, National Science Foundation, 26 Nov. 2018. Available here
Image Courtesy:
1.”Bond Energy Diagrams”By Fbarreyro – Own work, (Public Domain) via Commons Wikimedia
2.”Bond energies”By Chem540f09grp8 – Own work, (Public Domain) via Commons Wikimedia