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Atoms require more energy to stand on their own than when in a bond. So, they bond together to form compounds. Bonded atoms require less energy than single atoms. Energy is released when a bond is made, and energy is utilised to break a bond. Thus, when a chemical reaction takes place, numerous molecular bonds are broken and new bonds are also formed. This released energy exists primarily as heat.
Bond energy is an average value measuring the bond strength of a chemical bond, while bond dissociation energy breaks a particular bond only.
Bond Energy (E)
Bond energy is an average value of bond strength in a chemical reaction. It is written as (E). It provides the required energy to make or break a bond between the same atoms in a compound i.e. it may be endothermic or exothermic.
When bond energy is high, it means the bond is strong. Higher bond strength requires higher bond energy to break. For example, a carbon-carbon single bond requires 80 kcal/mol bond energy, and a much stronger carbon-carbon double bond needs the energy of 145 kcal/mol.
When energy is consumed, it will be an endothermic reaction. When energy is released, it is an exothermic reaction.
2H2 + O2 →2 H2O +135 kcal is an exothermic reaction where energy is released as heat.
N2 + O2 + 45 kcal →2NO is an endothermic reaction where energy is utilised.
When the bond energy (E) linked with a pair of atoms is high, the stronger the bond is and the lesser the distance between the 2 atoms.
When a reaction takes place, bond energy takes all the bond formation and dissociation into consideration. Suppose a chemical reaction had 2, 3 kcal on the reactant side and 3, 4 kcal on the product side. Then, the bond energy is -2 kcal (average of the total energy absorbed – total energy emitted). Since the value is negative, it is an exothermic reaction.
We can calculate bond energy (E) as:
ΔH = ∑H(bonds broken) – ∑H(bonds formed)
In short, every chemical bond has a specific bond energy with it. It is the amount of energy required to break or form the corresponding bond. The heat released or absorbed in a reaction can be measured using this bond energy.
Bond Dissociation Energy (H)
Bond dissociation energy is the energy required to break a particular bond. It dissociates atoms into free radicals. It is written as (H). Bond dissociation energy is the energy change that occurs when a bond undergoes dissociation. It is specific to a single bond.
As mentioned earlier, energy 1, 2, 3, 4 kcal in a chemical reaction is the bond dissociation energies (used to break a bond). The average of those energies is the bond energy.
Measurements for bond dissociation energy began in the 1930s. The experimental bond dissociation energy value changed from time to time. Many important organic compounds were also re-measured many times. C-H in methane is a typical example of this. It was 98 kcal/mol in the beginning, but now, it has been changed to 105 kcal/mol. The equilibrium constant is very sensitive to any errors associated with the bond dissociation energy.
The methods of experimental measuring of bond dissociation energy include spectrometry, pyrolysis kinetics, chemical equilibrium and kinetics, photolysis and mass spectrometry.
Previous Years’ Questions
One H-H bond and one I-I bond are combined to form 2 H-I bonds. (H-H =436 KJ/mol, I-I = 151 KJ/mol, H-I = 297 KJ/mol). What is the enthalpy change and what kind of a reaction is this?
H2+I2→2HI
ΔH = ∑H(bonds broken) – ∑H(bonds formed)
∑H(bonds broken)=436 + 151 = 587 KJ/mol
∑H(bonds formed)=2 x 297
E = 587 – (2 x 297)
= -7 KJ/mol
It is an exothermic reaction.
A2 + B2 2C (bond energy of A = 430 KJ/mol, bond energy of B = 240 KJ/mol, bond energy of C = 425 KJ/mol). Calculate the energy change.
ΔH = ∑H(bonds broken) – ∑H(bonds formed)
= (430+ 240) – (2 x 425)
= 670 – 850
= -180 KJ/mol
-180 KJ/mol is the calculated energy change. Therefore, it is an exothermic reaction.
2C A2 + B2 (bond energy of C = 360 KJ/mol, bond energy of A = 430 KJ/mol, bond energy of B = 200 KJ/mol). Calculate the energy change
ΔH = ∑H(bonds broken) – ∑H(bonds formed)
= (2 x 360) – (430+200)
= 720 – 630
= +90 KJ/mol
+90 KJ/mol is the calculated energy change. Hence, it is an endothermic reaction.
H2 + ½ O2 H2O (H-H is 432 KJ/mol , 0 = 0 is 496 KJ/mol, H-O is 463 KJ/mol). Calculate the energy change.
ΔH = ∑H(bonds broken) – ∑H(bonds formed)
= (432 + (0.5)(496)) – (2 x 463)
= 680 – 926
= -240 KJ/mol
The calculated energy change is – 240 KJ/mol.
Therefore, it is an endothermic reaction.
Conclusion
Bond energy is an average value of bond strength in a chemical reaction. It is written as (E).
We can calculate bond energy (E) as:
ΔH = ∑H(bonds broken) – ∑H(bonds formed)
Bond energy provides the required energy to make or break a bond. It can either be endothermic or exothermic. While bond dissociation energy is the energy required to break a particular bond. It is written as (H). It is the energy change that occurs when a bond undergoes dissociation.
The methods of experimental measuring of the bond dissociation energy include spectrometry, pyrolysis kinetics, chemical equilibrium and kinetics, photolysis and mass spectrometry.
