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Bond Dissociation Enthalpy: A Detailed Summary

Introduction

Last updated date: 07th Feb 2023
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Most of the chemical conversion is built upon the formation and dissociation of bonds. Chemical bonds are created when electrostatic forces between atoms combine. Chemical bonds can only be broken with the application of energy. The bond dissociation energy is the amount of energy needed to dissolve a chemical bond. Understanding how to calculate reaction enthalpies from bond dissociation enthalpy does not automatically translate into an understanding of bond breaking energies or even of energies and interactions. This article gives a clear view on the bond dissociation enthalpy with examples of halogens.


What is Bond Dissociation Enthalpy?

Bond Dissociation Enthalpy is the term utilised to describe the quantity of energy necessary in an endothermic reaction to dissociate a chemical bond and generate 2 distinct atoms, each having one of the original mutual pair's electrons. Whenever a bond is dissolved through homolytic dissociation, bond dissociation enthalpy is the typical shift in enthalpy.


The concept of bond-dissociation enthalpy, which is linked to the phrase bond-dissociation energy and is frequently utilised interchangeably, is equivalent to that of bond-dissociation energy. While some researchers utilise the phrase "bond-dissociation enthalpy" to apply to the enthalpy shift at 298 K (DH0298), others employ the word "bond-dissociation energy" (D0) to relate to the enthalpy shift at 0 K.


Features of Bond Dissociation Enthalpy

The following are a few crucial aspects of the bond dissociation enthalpy idea:

  • It refers to the quantity of energy required to supply a chemical connection between two molecules.

  • It is a way to determine how strong a chemical bonding is.

  • It is equivalent to the bond energy level in particular for diatomic compounds.

  • According to some, the bond involving silicon and fluorine has the highest bond dissociation enthalpy.

  • The bond dissociation energies of covalent bonds involving atoms or molecules are referred to as weak.


Example of Bond Dissociation Enthalpy

The reaction's enthalpy, that is by design the bond dissociation enthalpy of the molecule AB, DH0298 (AB), equals to the energy needed to dissociate a homolytic bond at 298 K. The following gives an instance of the bond dissociation enthalpy of a diatomic molecule.


The first rule of thermodynamics states that thermal energy equal to the bond dissociation enthalpy is produced if the radicals A and B join once more to create the molecule AB. These concepts make it easy to calculate the energy of the system of a variety of straightforward but significant processes including the transfer of a single bond. Through deducting, the energy needed to dissociate the previous bond from the energy acquired from the newly created bond is accomplished.


Bond Dissociation Enthalpy of Halogens

As previously stated, in the particular instance of diatomic molecules like halogens, the bond dissociation enthalpy of halogens is equivalent to the bond energy. This happens since bond energy is the median of every one of the bond dissociation enthalpies of halogen bonds of the identical form in a molecule.


As the size of the atom rises, the bond dissociation enthalpy of the halogen group falls down the hierarchy. Nevertheless, because of the inter-electronic repulsion occurring in the small atom of fluorine, fluorine's bond dissociation enthalpy is lower than that of chlorine and bromine. Hence, the order of bond dissociation enthalpy of halogen decreases as


Cl2 > Br2 > F2 > I2


This implies the decreasing bond dissociation enthalpy of halogens.


Halogen acids strength

The compounds formed while halogens react with hydrogen gas are recognised as halogen hydracids or halogen acids holding the formula HX. The 4 hydracids are referred to as: HF - Hydrofluoric acid, HCl - Hydrochloric acid, HBr - Hydrobromic acid, and HI - Hydroiodic acid.


Halogen acid's bond dissociation energy and level of ionisation in the aqueous phase both influence the acid strength. Overall, when bond dissociation energy rises, the degree of dissociation correspondingly reduces. This implies increasing bond dissociation enthalpy, by decreasing acidic strength.


Since the order of bond dissociation enthalpy of halogen acids are HI< HBr<HCl<HF. Thus, the above hydracids increasing acidic order is represented as, 

HF< HCl< HBr< HI


Examples of Strong and Weak Chemical Bonds

The strongest single bonds, as per bond dissociation enthalpy research, are SiF bonds. The bond dissociation enthalpy for H3SiF is over 50% stronger than the bond dissociation enthalpy for CH3F. Much more bond dissociation enthalpy is present for SiF3F. These findings have the effect of generating silicon fluorides in a variety of processes, including glassware etching, deportation in chemical synthesising, and volcano releases. Due to the significant variation in electronegativity involving silicon and fluorine, respectively, ionic and covalent bonding significantly contributes to the bond's overall ability. This is why the bond is so strong. Diacetylene is one of the strongest carbon single bonds, connecting two sp-hybridised carbon atoms.


In contrast, it is unclear where an extremely weak covalent bond ends and an intermolecular interaction begins. The weakest bonds with significant covalent characteristics are those formed by Lewis acid-base combinations involving components of transition metal and noble gases.


Conclusion

Therefore, it can be concluded that the bond-dissociation enthalpy (DH°) is the measure of the capacity of a diatomic molecule's chemical bond. The bond-dissociation enthalpy varies from the bond energy with the exception of diatomic molecules.


Key Features

  • Bond-dissociation enthalpy, average bond energy, and bond strength are other names for the enthalpy of bond formation.

  • Extra energy is required to break down chemical bonds, owing to the thermodynamic preference of chemical bonds. As a result, bond enthalpy estimates are often positive and comprise units. 

  • A stronger bond signifies a higher bond enthalpy, and a more force is needed to dissolve it.

FAQs on Bond Dissociation Enthalpy: A Detailed Summary

1. Which one forms the strongest haloacids HF or HCl?

Since the HF bond is shorter and stronger than the HCl bond, HF acid produces less H+ ions in aqueous conditions than HCl acid.  HCl is therefore a stronger acid than HF. 

2. Which one of the halogens possesses the lowest bond dissociation energy?

Because of interelectronic repulsions present in the tiny fluorine atom, fluorine's bond dissociation energy is lower compared to that of chlorine and bromine.

3. Which element has the highest single bond, bond dissociation enthalpy?

For a single bond involving 2 atoms of any element, the H2 bond dissociation enthalpy is the highest.

5. In what ways do bond energy and bond dissociation energy differ?

When diatomic molecules are presented with a bond, the bond-dissociation energy differs from the bond energy. Unlike the bond-dissociation energy that corresponds to a single chemical bond, the bond energy is equal to the average of the bond dissociation energies of the same kind of bonds within a given molecule.


The bond energy is the average of the dissociation energies visible between the same types of atoms in the gas phase (normally at 298 K).  These energies are not the same as bond dissociation energies. It is the enthalpy change resulting from the dissociation of one covalent bond into two or more radical species when the bond is cleaved by homolysis (broken up into equal pieces). Bond energies and bond dissociation energies have the following major difference: bond energies are average values, while bond dissociation energies are specific values for particular bonds. Bond dissociation energy (enthalpy) (H) is the amount of energy needed to break down a specific bond in homolysis. A bond undergoes cleavage by homolysis. This is defined as the change in enthalpy that occurs. A single bond is defined as having separate dissociation energy.


Known as bond enthalpy, or bond dissociation energy, it describes the standard enthalpy change that occurs when a bond is cleaved (separated or divided) by homolysis (separation into small pieces) by reactants and products of the homolysis reaction at 0 K (absolute zero).

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