What is the Difference Between Enthalpy of Atomisation and Bond Dissociation

The main difference between enthalpy of atomisation and bond dissociation is that enthalpy of atomisation is the enthalpy change when one mole of a substance in its standard state is formed from its individual gaseous atoms, whereas enthalpy of bond dissociation is the energy change associated with breaking a specific chemical bond in a molecule to form two separate radicals.

The enthalpy of atomization and bond dissociation are crucial concepts in physical chemistry, providing insights into the energetics of chemical reactions.

Key Areas Covered

1. What is Enthalpy of Atomisation 
      – Definition, Features,  Calculation
2. What is Enthalpy of Bond Dissociation
      – Definition, Features, Calculation
3. Similarities Between Enthalpy of Atomisation and Bond Dissociation
      – Outline of Common Features
4. Difference Between Enthalpy of Atomisation and Bond Dissociation
      – Comparison of Key Differences
5. FAQ: Enthalpy of Atomisation and Bond Dissociation
      – Frequently Asked Questions

Key Terms

Enthalpy of Atomisation,  Enthalpy of Bond Dissociation

What is Enthalpy of Atomisation

The enthalpy of atomization (ΔH_at​) is mathematically represented as follows:

Molecule(g)→n×Atoms(g)

Here, “Molecule(g)” represents one mole of a substance in its standard state, and n×Atoms(g) represents the corresponding number of gaseous atoms formed from breaking all the bonds within the molecule. Moreover, the enthalpy change (ΔH_at​) is associated with this process.

The process of atomization involves breaking all the bonds within a molecule to convert it into its constituent atoms. This requires energy input because chemical bonds are generally stable, and breaking them necessitates overcoming the attractive forces that hold the atoms together. The enthalpy of atomization is, therefore, typically positive, indicating that it is an endothermic process.

Figure 1: Enthalpy of Atomisation

The positive sign implies that energy must be supplied to break the bonds and separate the atoms. This input energy is later recovered when the atoms come together to form a new molecule, a process known as bond formation.

The enthalpy of atomization is also a crucial factor in understanding the chemical reactivity of substances. It also provides information about the energy required to break bonds, which is fundamental to predicting how readily a substance will undergo chemical reactions. Higher enthalpies of atomization generally indicate more stable molecules. Stable molecules are less likely to undergo spontaneous reactions, as they require more energy input to break their bonds. Furthermore, enthalpies of atomization are useful in various thermochemical calculations, including the determination of standard enthalpies of formation (ΔH_f^∘​). In addition, standard enthalpies of formation are essential for calculating reaction enthalpies and studying reaction mechanisms.

What is Enthalpy of Bond Dissociation

The enthalpy of bond dissociation is mathematically represented as:

∙A−B→A∙+B∙

Here, A−B represents the molecule with the bond being broken, and A∙ and ∙B∙ represent the resulting radicals. Moreover, the enthalpy change (ΔHdiss​) associated with this process is indicative of the strength of the specific bond being broken.

Breaking a chemical bond requires overcoming the attractive forces that hold the atoms together. The enthalpy of bond dissociation is a measure of the energy for disrupting this bond. It is generally positive, indicating that the process is endothermic and energy must be supplied to break the bond.

Figure 2: Homolysis

The positive value of ΔHdiss​ signifies that breaking the bond requires an input of energy. Furthermore, this energy is needed to overcome the forces of attraction between the atoms and to provide kinetic energy to the resulting radicals. Conversely, when bonds are formed, energy is released, and the process is exothermic.

Moreover, the nature of the bond that breaks significantly influences the bond dissociation energy. Different types of bonds (e.g., single, double, or triple bonds) have distinct bond dissociation energies. The environment in which the measurement is conducted can impact the bond dissociation energy. For example, gas-phase measurements may differ from values obtained in solution or in a condensed phase. The atoms surrounding the bond of interest can affect the bond dissociation energy. Additionally, electron-withdrawing or donating groups can influence the stability of the bond.

Similarities Between Enthalpy of Atomization and Bond Dissociation

• Both enthalpy of atomization and enthalpy of bond dissociation involve energy changes associated with breaking and forming chemical bonds.
• Higher values for both parameters generally indicate stronger and more stable chemical species.

Difference Between Enthalpy of Atomisation and Bond Dissociation

Definition

Enthalpy of atomisation refers to the enthalpy change when one mole of a substance in its standard state is formed from its individual gaseous atoms whereas enthalpy of bond dissociation measures the energy required to break a specific chemical bond in a molecule, resulting in the formation of two separate radicals.

Represented By

Enthalpy of atomization is represented as Molecule(g)→n×Atoms(g), where one mole of a molecule is broken into n moles of individual gaseous atoms. Meanwhile, enthalpy of bond dissociation is represented as A−B→A∙+B∙, indicating the breaking of a specific bond (A−B) to form two radicals (A∙ and B∙).

Breaking of Bonds

Enthalpy of atomisation involves the breaking of all the bonds in a molecule, resulting in the conversion of the entire molecule into individual atoms, while enthalpy of bond dissociation involves breaking only a specific bond, leaving the rest of the molecule intact.

FAQ: Enthalpy of Atomisation and Bond Dissociation

What is the difference between bond dissociation and bond enthalpy?

Bond dissociation energy refers to the energy required to break a specific bond in a molecule, while bond enthalpy is the average energy required to break a particular type of bond in a variety of molecules.

What is an example of a bond dissociation enthalpy?

An example of bond dissociation enthalpy is the energy required to break the H–H bond in molecular hydrogen (H₂), which is approximately 436 kJ/mol.

What is the relationship between bond dissociation enthalpy and enthalpy of formation?

The relationship between bond dissociation enthalpy and enthalpy of formation lies in the fact that the enthalpy of formation is related to the bond dissociation enthalpies of the bonds broken and formed during a reaction, as per Hess’s law.

Conclusion

In brief, enthalpy of atomisation is the enthalpy change when one mole of a substance in its standard state is formed from its individual gaseous atoms, whereas enthalpy of bond dissociation is the energy change associated with breaking a specific chemical bond in a molecule to form two separate radicals. Thus, this is the main difference between enthalpy of atomisation and bond dissociation 

Reference:

1. “Bond Dissociation Enthalpy.” Byju’s.
2. “Atomisation Enthalpy Solution.” Byju’s.

Image Courtesy:

1. “Enthalpy of atomization of block d elements” By Albris – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Homolysis (Chemistry)” By Jürgen Martens – Jürgen Martens (Public Domain) via Commons Wikimedia