The key difference between enthalpy of atomisation and bond dissociation is that enthalpy of atomisation describes the energy required to separate a molecule into its atoms whereas the enthalpy of bond dissociation describes the dissociation of chemical bonds in a molecule.
Sometimes, enthalpy of atomisation and enthalpy of bond dissociation are the same for some simple compounds if there are simple bonds. This is because, in simple compounds, dissociation of bonds forms the atoms from which the molecule is made of.
CONTENTS
1. Overview and Key Difference
2. What is Enthalpy of Atomisation
3. What is Enthalpy of Bond Dissociation
4. Side by Side Comparison – Enthalpy of Atomisation vs Bond Dissociation in Tabular Form
5. Summary
What is Enthalpy of Atomisation?
Enthalpy of atomisation is the change in enthalpy that occurs when separating a chemical substance completely into its atoms. This chemical substance can be either a chemical element or a chemical compound. We can denote this enthalpy change as ΔHat. During the process of atomisation, all types of chemical bonds are broken down, and none are formed. Therefore, the enthalpy of atomisation is always a positive value. The standard enthalpy value for this enthalpy change is “standard enthalpy of atomisation”. The standard conditions considered in this context are the 268.15 K temperature and 1 bar pressure.
For example, enthalpy of atomisation for water molecule refers to the energy required to separate the two hydrogen atoms and the oxygen atom in the water molecule. In other words, the enthalpy of atomisation for water is the sum of the bond dissociation energies of two O-H bonds. Similarly, the enthalpy of atomisation for an elemental solid is the enthalpy of sublimation for that substance because sublimation involves the conversion of the solid into a monoatomic gas upon evaporation.
What is Enthalpy of Bond Dissociation?
Enthalpy of bond dissociation describes the enthalpy change that occurs during the dissociation of a chemical bond. In other words, it is the measure of the strength of a chemical bond. Therefore, we can say that standard enthalpy change that occurs when chemical bond A-B breaks down by hemolysis and fragments A and B is the enthalpy of bond dissociation. If the molecule we are considering is a diatomic molecule, then the bond dissociation enthalpy is equal to the enthalpy of atomisation. Usually, the A and B fragments given by this bond dissociation are radical species. We can denote the enthalpy of bond dissociation as DH0.
There are different methods we can use to measure the bond dissociation such as spectrometric determination of energy levels, generation of radicals by pyrolysis or photolysis, measurements of chemical kinetics and equilibrium, various calorimetric and electrochemical methods, etc.
What is the Difference Between Enthalpy of Atomisation and Bond Dissociation?
The key difference between enthalpy of atomisation and bond dissociation is that enthalpy of atomisation describes the energy required to separate a molecule into its atoms whereas the enthalpy of bond dissociation describes the dissociation of chemical bonds in a molecule. The bond dissociation enthalpy and enthalpy of atomisation are always positive values. Sometimes, the enthalpy of atomisation and enthalpy of bond dissociation are the same for some simple compounds if there are simple bonds. However, these terms differ from each other most of the times.
Below infographic tables more differences between enthalpy of atomisation and bond dissociation.
Summary – Enthalpy of Atomisation vs Bond Dissociation
Enthalpy of atomisation and bond dissociation can be used interchangeably for some simple compounds, but not always they are equivalent. The key difference between enthalpy of atomisation and bond dissociation is that enthalpy of atomisation describes the energy required to separate a molecule into its atoms whereas the enthalpy of bond dissociation describes the dissociation of chemical bonds in a molecule.
Reference:
1. “Enthalpy of atomisation.” Wikipedia, Available here.
Image Courtesy:
1. “Heterolysis (Chemistry)” By Jürgen Martens – Jürgen Martens (CC BY-SA 3.0) via Commons Wikimedia