Difference Between Bond Energy and Bond Dissociation Energy Enthalpy

Bond energy and bond dissociation enthalpy may sound like they mean the same thing but they don’t.

Even though they are both thermodynamic properties of a specific system that comes under scrutiny, their energy uses are different.

Sometimes both the terms are commonly grouped under the umbrella term standard enthalpy change to define them both. They both give an energy value after bond cleavage. 

Bond Energy vs Bond Dissociation Energy Enthalpy 

The main difference between bond energy and bond dissociation energy enthalpy is that bond energy always depicts not a single bond cleavage value but that as a whole for a specific molecule or compound thereby only giving an average value for the energy released. In the case of bond dissociation energy enthalpy, the energy value given is only for a single bond cleavage and not for a whole molecule. 

Bond Energy vs Bond Dissociation Energy Enthalpy

Bond energy is the average value of all the bonds that are broken in a system.

For example, if in a specific thermodynamic environment, ten bonds are awaiting to be broken, then the bond energy value that is calculated after all the bonds have been broken would be the sum of individual energy released divided by ten. 

Bond dissociation energy is a single value that need not be further calculated after the cleavage and release of the required energy.

In a thermodynamic environment, after a bond between two atoms in a molecule or compound is broken, a small amount of energy is released and this single energy unit is called bond dissociation energy enthalpy. 

Comparison Table Between Bond Energy and Bond Dissociation Energy Enthalpy

Parameters of ComparisonBond EnergyBond Dissociation Energy Enthalpy
Represented ByEH
SpecificityAny number of bonds can be broken with no limitOnly a single cleavage value
ValueAverage valueSingle and specific
MeasuresBond strengthIndividual cleavage properties
Used forFormation of atomsFormation of free radicals

What is Bond Energy? 

The breaking of the bond between two atoms that are usually in the gaseous phase of state releases a certain amount of energy. 

This when repeated for an entire compound has many values of energy that is released. 

An average of all the energy released is called the bond energy of a specific thermodynamic system. 

It could also be defined as the energy to break down all the bonds that are present in a system to create a free atomic system. 

The optimum temperature for carrying out such a cleavage to release the bond energy is usually 298K.

But this temperature can vary depending upon the pressure around a system or even depending on the molecular nature of the compound. 

A graph that could indicate the bond energy is between the potential energy of a system and the distance between the atoms in question. 

A possible outcome of such a graph is the distances at which the energy required is either too much or too little. 

This distance shown also gives the bond length between two atoms in a theoretical form. 

The greater the bond energy, the higher is the bond strength and the lesser is the distance between them. 

In chemistry, for ease, bond energy is represented with the English alphabet E. 

The bond energy is used to form all the atoms that might come in useful in a later reaction that might even be a part of a cascade reaction. 

There is a single carbon molecule and four hydrogen molecules. The reaction involved is breaking CH4 into its individual five atoms. 

The eventual and average value of energy released is the bond energy and is 414KJ/mol.

What is Bond Dissociation Energy Enthalpy? 

Bond Dissociation Energy is commonly referred to as standard enthalpy. 

The energy released is due to the breaking of the covalent bond. 

Covalent bonds are those bonds present between nonmetallic atoms that are formed due to the sharing of electrons between atoms. 

Bond dissociation energy is released only after the atoms are cleaved by homolysis. 

Homolysis refers to the breakdown of molecules into equal structured particles which are usually free radicals. 

Another definition of bond dissociation energy is the changes in the enthalpy that occurs due to cleavage of a molecule by the process of homolysis. 

The bond dissociation energy is usually associated with individual cleavages. 

No sum of all cleavages happening in a thermodynamic system gives the bond dissociation energy. 

It is usually represented by the English letter H to make the word easier to use. 

The major function of bond dissociation energy is to create the energy needed for the formation of free radicals that come into use in other reactions. 

For example in the complete dissociation of a methane(CH4) molecule, there are four different bond enthalpies. 

The four enthalpies are needed to break the four existing bonds in the system to create four free radicals. 

This shows that the energy values are all different for the breaking of bonds in a single molecule

Main Differences Between Bond Energy and Bond Dissociation Energy Enthalpy

  1. While bond dissociation energy always gives the energy value of a single bond cleavage whereas, in the case of bond energy, the energy given or released is the average of all the cleavage values of a system. 
  2. Bond energy is represented by the alphabet E while bond dissociation energy enthalpy is represented by the letter H. 
  3. During the cleaving of methane, there are four bond dissociation energy values that are 439, 469,423, and 339 KJ/mol but it has a singular bond energy value of 414KJ/mol.
  4. The energy value would vary in bond energy but it wouldn’t in the case of bond dissociation enthalpy as it is calculated for individual cleavages. 
  5. There is a specificity in the case of bond dissociation enthalpy as it can have only one bond while there is no specificity for bond energy as it is an average. 

Conclusion

Bond energy and bond dissociation energy are two terms that could easily be confused. 

At times both the values might coincide and be the same. 

These cases are only for diatomic molecules like nitrogen or oxygen molecules. 

But more often than not, both values are always different. 

The unit of expression is the same for both bond energy and bond dissociation energy. 

The unit is KJ/mol.

Bond energy is the measure of bond strength.

The strength of the bond between two atoms and also the general distance between the atoms could be deciphered from the bond energy. 

An example of the perfect bond energy illustration is methane(CH4). 

References

  1. https://pubs.acs.org/doi/pdf/10.1021/ed072p423
  2. https://pubs.acs.org/doi/pdf/10.1021/ar00068a004
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