Table of Contents
What is Bond Energy?
Bond Energy, also known as average bond enthalpy or simply bond enthalpy, is a quantity that offers insight into the strength of a chemical bond. The IUPAC definition of the term ‘bond energy’ can be written as: “the average value obtained from the bond dissociation enthalpies (in the gaseous phase) of all the chemical bonds of a specific type in a given chemical compound. Therefore, the bond energy of a chemical bond in a given compound can be visualized as the average amount of energy required to break one such chemical bond.
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Example
It is important to note that the bond energy of a chemical bond in a compound is the average value of all the individual bond dissociation enthalpies of the chemical bonds. For example, the bond energy of the carbon-hydrogen bond in a methane (CH4) molecule is equal to the average of the bond dissociation energies of each individual carbon-hydrogen bond. It can be calculated as follows.
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- CH4 + BDE1 → CH3 + H
- CH3 + BDE2 → CH2 + H
- CH2 + BDE3 → CH + H
- CH + BDE4 → C + H
- BE(C-H) = (BDE1 + BDE2 + BDE3 + BDE4)/4
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Here, BDE1 denotes the energy required to break one carbon-hydrogen bond in the CH4 molecule, BDE2 denotes the energy required to break one carbon-hydrogen bond in the CH3 molecule, BDE3 denotes the energy required to break one carbon-hydrogen bond in the CH2 molecule, and BDE4 denotes the energy required to break the only carbon-hydrogen bond in the CH molecule. Finally, the term BE(C-H) denotes the bond energy of the carbon-hydrogen bond in the methane molecule.
Therefore. the bond energy of the carbon-hydrogen bond in the methane molecule can be visualised as the change in enthalpy (usually denoted by ΔH) associated with the breaking of one CH4 molecule into one carbon atom and four hydrogen atoms, totally divided by four (since there are a total of four carbon-hydrogen bonds in the methane molecule).
Bond Energy and Stability
The bond energy of a chemical bond in a given compound can be visualized as the average amount of energy required to break one such chemical bond. Therefore, the bond energy of a chemical bond is directly proportional to the stability of that bond. This implies that the greater the bond energy of a given chemical bond between two atoms, the greater the stability of that chemical bond.
Comparison Between Bond Energy and Bond Dissociation Energy
The bond dissociation energy of a chemical bond (sometimes abbreviated to BDE) can be defined as the change in enthalpy associated with the breakage of the chemical bond via homolytic cleavage. For example, the bond dissociation energy of a molecule A-B is the amount of energy that is required to facilitate the homolytic cleavage of the bond between A and B, resulting in the formation of two free radicals. It is important to note that the bond dissociation energy of a chemical bond is dependent on the absolute temperature of the environment. Therefore, bond dissociation energy is usually calculated under standard conditions (where the temperature is roughly equal to 298 K). On the other hand, the bond energy of a chemical bond in a compound is the average value of all the bond dissociation enthalpies of that bond in the molecule.
Example: Bond Energy and Bond Dissociation Energy of the Hydrogen-Oxygen Bond in Water
The bond dissociation energy of the hydrogen-oxygen bond in a water molecule is given by:
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- H2O + BDE → OH + H
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Thus, the bond dissociation energy of the hydrogen-oxygen bond in a water molecule is the amount of energy required to split it into an H free radical and an OH free radical.
On the other hand, the bond energy of the hydrogen-oxygen bond in the water molecule is given by:
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- H2O + BDE1 → OH + H
- OH + BDE2 → H + O
- BE(O-H) = (BDE1 + BDE2)/2
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Thus, the bond energy of the hydrogen-oxygen bond in the water molecule is the amount of energy required to split all the hydrogen-oxygen bonds in the molecule, totally divided by two.
Frequently Asked Questions on Bond Energy
Will the bond energy of a chemical bond always have a positive value?
Since breaking a bond requires energy, bond energies are usually positive numbers. When a bond is formed, the energy equals the bond energy negative (energy is released). Bond energy is the measure of a bond’s strength; the bigger the bond energy, the stronger the chemical bond. If the bond energy of a chemical bond is negative, the bond cannot exist because it will be too unstable. Therefore, it is almost impossible for a chemical bond to have a negative value for its bond energy.
What is the primary difference between bond dissociation energy and bond energy?
The key difference between bond dissociation energy and bond energy is that bond energy is the average amount of energy required to break down all the bonds in a compound between the same two types of atoms while bond dissociation energy is the amount of energy needed to break down a particular bond via homolytic cleavage. The bond energy of a chemical bond in a compound is the average value of all the bond dissociation energies of that bond in the compound. For example, the bond energy of the magnesium-chlorine bond in the MgCl2 molecule is the total amount of energy required to split the molecule into magnesium and chlorine atoms divided by two.
What is the importance of bond energy?
Bond energy provides insight into the strength of a chemical bond and, therefore, the stability of a chemical compound. Energy is often needed to break a bond, known as bond energy. Although the term can sound basic, bond energy plays a very important role in explaining a molecule’s structure and characteristics.
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