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Bond Dissociation Energy

A chemical bond can be defined as the interaction force between two bonded atoms in a molecule. The valence electrons of bonded atoms are involved in bonding. We know that all atoms tend to get the octet configuration for stability. Therefore they can either involve in ionic bonding or covalent bonding to get this octet configuration in their valence shell.

For an ionic bond that is also known as electrovalent bond, one of the atoms loses electrons and form cation which carries a positive charge whereas other atom gains electrons to form anion which carries a negative charge. The positively charged atom or cation and anion are attracted towards each other by electrostatic force of attraction that is known as ionic bond. On the contrary, covalent bond involves the equal sharing of electrons between two atoms and forms a chemical bond between them.

The formation of chemical bond requires some energy which is called as bond formation energy. In other words, the same amount of energy is required to cleave the bond. The energy of the bond also determines the strength and stability of it. A strong chemical bond forms more stable molecule whereas a weak interaction between atoms forms an unstable molecule. Hence we have to determine the strength of chemical bonds so that we can predict the stability of the molecule. The bond dissociation energy helps to determine the strength of chemical bonds. 

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Bond Dissociation Energy Definition

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Bond dissociation energy is also abbreviated as BDE in short form. This energy is said to be the measure of the strength of a chemical bond. In scientific terms, bond dissociation energy is classified as the change in the enthalpy of a bond during its homolysis (a term used when a bond in a neutral molecule is dissociated and free radicals are formed), having the compounds involved in the chemical reaction (homolysis) at zero Kelvin. Thus, this is the reason why Bond dissociated energy is also termed as Bond Dissociation Enthalpy.

BDE is also said to be the change in the enthalpy during the homolytic cleavage of chemical bonds under some specific conditions. We can get bond dissociation energy by two ways in chemical bond.

  1. By Homolytic Dissociation of a chemical bond
  2. By Heterolytic Dissociation of a chemical bond

In homolytic dissociation, the chemical bond breaks symmetrically while in Heterolytic dissociation a chemical bond breaks asymmetrically.

Calculating Bond Dissociation Energy

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Let us see the example of the C-H bond dissociation energy in an organic compound like Ethane having the molecular formula is C2H6. The corresponding bond dissociation energy equation and bond dissociation energy calculation are as follows.

CH3CH2-H $\rightarrow$ CH3CH2 + H

Here, the Hydrogen bond gets dissociated from Ethane and the energy used to dissociate this Hydrogen atom from the molecule of Ethane is termed as Bond dissociation energy.

The value of bond dissociation energy for this reaction is

$\Delta$H = 102 Kcal/mol or 424.0 KJ/mol

and in short it is denoted as Do.

It has been seen that Bond energy and Bond dissociation energy have similar values, but they differ in case of some diatomic molecules. This is because, in diatomic molecules, the average of the total energy becomes the bond energy while BDE remains the same. This is how we can calculate bond dissociation energy and form bond dissociation energy table or chart.

Bond dissociation energy should not be confused with Bond energy, as both are different terms. The former one is the measure of the strength of the bond while the later one is the total energy contained in a chemical bond.


Bond Dissociation Energy Table

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The principal measure of the ease of homolytic cleavage is the bond-dissociation energy. The bond dissociation energies indicate the energy required to break a specific bond in a particular molecule, whereas the average bond energies are calculated from a set of experimental data assuming that all C-H bonds have the same energy.

Both types of values are useful bond dissociation energies provide an accurate assessment of the energy required to break a particular bond homolytically; average bond energies can be used to estimate changes in energy for the transformations from one stable species to another, especially in cases where π bonds are broken and made.

Bond Dissociation Energy

Bond dissociation energies vary with the degree of substitution of the atoms involved in the bond being broken.Thus the energy required to break a C-H bond decreases progressively in the series shown above.

Bond Dissociation Energy Equation

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Evaluation of bond dissociation energy is a typical application of thermochemical information. The information on bond dissociation energy may indicate which bond is more likely to break in a molecule and can be used to estimate the temperature range. The reaction of bond dissociation can be written as follows.

A-B $\rightarrow$ A. + B.

For the above equation the differences between enthalpy and energy can be written as
$\Delta$H - $\Delta$E = $\sum$(pV)fragments - (pV)molecule = ($\Delta$n)RT
There is variation in bond energy and bond enthalpy, true bond energy should be taken at 0K and not at 298.3K. However these differences are very small compared to the value of the bond dissociation energy and in general the bond dissociation energy is taken as equal to the negative value for the enthalpy of formation of the bond. The enthalpy of bond dissociation is given by the expression.
$\Delta$Hof(A-B) = $\Delta$Hof(Ao) + $\Delta$Hof(Bo) - $\Delta$Hof(AB)

Bond Dissociation Energy Problems

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Bond Dissociation Energy Problems

Solved Examples

Question 1: N2 is inert at room temperature explain.
Solution:
 
  • N2 is inert at room temperature because triple bond in N2 molecule is very strong and has a very high dissociation energy of 946kJ/mole.
  • The strong bonding in N2 molecule is not disturbed by any collision that takes place at room temperature.

 

Question 2: Bond dissociation energy of N2 > N2+ but that of O2 < O2+ . Explain.
Solution:
 
  • Bond order of N2 is 3.0 while that of N2+ is 2.5 hence bond dissociation energy if N2 > N2+.
  • In O2 molecule of the 16 electrons 10 are present in bonding and 6 in anti-bonding orbitals the bond order is 2.0.
  • One of the anti-bonding electron is lost to give O2+ ion.
  • The bond order changes to 2.5 and hence the bond dissociation energy of O2+ > O2.

 

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