Enthalpy of Atomisation

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What is the Enthalpy of Atomisation?

Each spontaneous process tends to lead to newer products formation. Out of all the known processes to us, some processes tend to absorb energy, whereas some processes just simply result in the evolution of energy. Therefore, we tend to experience some kind of an enthalpy or heat change on completing these processes. This enthalpy change is because of the enthalpy of atomisation, solution, etc. The enthalpy of atomization refers to the amount of change in the heat when the bonds of a compound tend to break and elements of that compound get reduced to singular atoms. 


The heat of atomisation is a positive value always and can never be a negative number. The enthalpy of atomisation is denoted by a symbol ΔHa. In this article, we will learn about what is the enthalpy of atomisation, the different enthalpy changes, and how to calculate the enthalpy change of atomisation. Let us first discuss what are the different changes in the heat of atomization.


Enthalpy of Atomization

Given below are some of the common enthalpy changes.

Enthalpy of atomization, denoted by ΔₐH⁰ , refers to the change in enthalpy if one mole of bonds is entirely broken down for obtaining atoms in the gaseous state. Consider, for example, the atomisation of the methane molecule.


CH₄(g) → C(g) + 4H(g)

Here, ΔₐH⁰ is 1665.0 kJ/mol


In the case of the diatomic molecules, the heat of atomisation is equal to that of the enthalpy of the bond dissociation. Consider, for example, the atomization of the dihydrogen molecule.


In diatomic molecules, the heat of atomization equals the enthalpy of bond dissociation. For example, atomization of dihydrogen molecule.


H₂(g) → 2H(g)

Here, ΔₐH⁰ is 435 kJ/mol


1. Enthalpy of the Solution:

The enthalpy of a solution is denoted by Δ\[_{sol}\]H⁰. It refers to the change in the heats when one mol substance is entirely dissolved in a solvent. Consider, for example, the enthalpy of the dissolution of an ionic compound in water.


2. Heat Change During the Phase Transition:

If a substance tends to change the phase or a phase transition, which means that one phase of that substance changes into a different phase, then some amount of energy is either released or absorbed. Consider a simple example, when ice tends to melt from a solid-state to water, energy is required for it to melt. The common heat change during any kind of phase transition includes the following:


3. Enthalpy Change During Phase Transition:

When the phase of a substance changes from one form to another, some energy is released or absorbed. When ice melts into water, it needs energy for melting. 


4. Standard Enthalpy of Vaporization:

The standard enthalpy of vaporization, denoted as Δ\[_{vap}\]H⁰, refers to the amount of enthalpy which is required for vaporising one mol of a given liquid at a constant temperature and under a standard pressure, which is 1 bar. 


5. Standard Enthalpy of Sublimation:

The standard enthalpy of sublimation, denoted as Δ\[_{sub}\]H⁰ refers to the change in the heat when one mole of any given solid substance tends to sublime at a constant temperature and under a standard pressure of 1bar.


How To Calculate Enthalpy Change of Atomisation?

Let us now learn about the calculation of the enthalpy change of atomization.


When pressure is held constant, the heat change is equal to the change in the system’s internal energy. Hence, the enthalpy of atomisation is equal to the sum of the total enthalpies of vaporization and fusion. 


Consider, for example, the enthalpy of atomisation for the chlorine gas, which is an atomic molecule, denoted as Cl₂, refers to the total bond energy of Cl₂. The only thing needed to atomize the gas is breaking the bonds between all the molecules of chlorine.


If you consider the soum metal (Na) at standard conditions, the atomisation needs breaking the atoms that are joined by the metallic bonds. The enthalpy of atomisation here refers to the total sum of the enthalpy of vaporization and the enthalpy of fusion of sodium. For any given elemental solid, the heat of atomisation is the same as that of the enthalpy of sublimation.

FAQ (Frequently Asked Questions)

1. Define Enthalpy of Atomisation.

Ans: Enthalpy of atomization is defined as the amount of heat change that happens when the bonds of a compound are broken which results in the component atoms being reduced to the individual atoms. The heat of atomisation is denoted by the symbol ΔatH. The enthalpy change of atomisation for the gaseous H₂O is the sum of both the OH-H and the O-H enthalpies of bond dissociation.


The heat of atomisation of any given elemental solid is the same as that of the heat of atomization of sublimation for a solid which tends to become a monatomic gas on evaporation.


When a diatomic element gets converted to any gaseous atoms, it’s only half a mole of molecules which are required. This is because the standard enthalpy change is purely dependent on the production of one mole of the gaseous atoms. When the atoms in a particular molecule are different isotopes of that same element, the calculation tends to become non-trivial.

2. What Do You Mean by the Standard Enthalpy of Atomisation?

Ans: The standard enthalpy of atomization for atoms, denoted by ΔH⁰atom, is just a process to convert the number of moles of the standard state of the element which is needed to make one mol of the element’s gaseous state. For example, 


ΔH⁰atom of Cl₂(g) → Cl(g) is given by,


½Cl₂(g) → Cl(g)  


Which determines half of the enthalpy to break the Cl-Cl bond of 1 mol Cl₂(g) at standard conditions.


For the transition metal Cu(s), the ΔHatom depends on the reaction.