Bond dissociation enthalpy can be defined as the standard change in enthalpy when a bond is cleaved via homolytic fission. The products formed from the homolysis of the bond are generally radicals.
What is Bond Dissociation Enthalpy?
The strength of the chemical bond between any two species can be estimated with the help of bond dissociation enthalpy. Although it is generally measured as the enthalpy change at standard conditions (298K), the bond dissociation energy of a chemical bond is often defined as the enthalpy change of the homolytic fission of the bond at absolute zero (0K).
Some important features of the concept of bond dissociation enthalpy include:
- It is the amount of energy which needs to be supplied in order to break a chemical bond between two species.
- It is a means of calculating the strength of a chemical bond.
- In diatomic molecules specifically, it is equal to the value of bond energy
- The bond between silicon and fluorine is said to have the strongest bond dissociation enthalpy.
- Covalent bonds between atoms or molecules are said to have weak bond dissociation energies.
An illustration describing the bond dissociation enthalpy of a Cl2 is provided below.
As discussed earlier, in the case of diatomic molecules, the bond dissociation enthalpy is equal to the bond energy. This is because bond energy is the average value of all the bond dissociation enthalpies of all bonds of the same type in a molecule.
To elaborate, consider a molecule of methane from which all hydrogen atoms must be removed. To remove the first hydrogen, the energy requirement is 105 kcal/mol whereas the second hydrogen can only be removed when 110 kcal/mol of energy is present. The bond dissociation enthalpy of the third and fourth hydrogens are 101 kcal/mol and 81 kcal/mol respectively. Averaging out these values, the total bond energy is found to be 99 kcal/mol, which is not equal to any of the bond dissociation energies of the C-H bonds.
The Weakest and the Strongest Chemical Bonds
With the help of the concept of bond dissociation enthalpy, the weakest and the strongest chemical bonds can be found. The strongest chemical bond is found to be the bond between silicon and fluorine, as discussed earlier.
The bond dissociation energy required to break the first silicon-fluorine bond in a silicon tetrafluoride molecule is estimated at 166 kcal/mol. This can be explained by the huge difference in electronegativities of the silicon and fluorine atoms, and the fact that fluorine is the most electronegative element in the entire periodic table.
Moving on to neutral compounds, it is observed that the bond with the highest strength is the carbon-oxygen bond in carbon monoxide with a bond dissociation energy of 257 kcal/mol. The carbon-carbon bond in ethyne also has a relatively high bond dissociation energy, valued at approximately 160 kcal/mol.
Thus, the concept of bond dissociation enthalpy is found to be highly useful in the field of thermochemistry.