Bond Dissociation Enthalpy

Dhristi JEE 2022-24

Bond Dissociation Enthalpy Definition

The Bond Dissociation Enthalpy refers to the amount of energy that is required during an endothermic process to break a chemical bond and produce two separated atoms, each with one electron of the first mutual pair. Bond dissociation enthalpy can be characterized as the standard change in enthalpy when a bond is broken using homolytic separation. The products obtained from the homolysis of a chemical bond are generally radicals.

The change in enthalpy is temperature-dependent, and the bond-dissociation enthalpy is characterized to be the enthalpy change of the homolysis at 0 K (supreme zero), even though the enthalpy change at 298 K (standard conditions) is a common parameter. As a common model, the bond-dissociation enthalpy of one of the C−H bonds in ethane (C2H6) is characterized as the standard enthalpy change of the cycle.

Features of the Concept of Enthalpy of Dissociation

The following are some of the chief characteristics of Bond Dissociation enthalpy.

  • It is a method for figuring the quality of a compound bond.

  • It is the measure of energy that should be provided to break a chemical bond. 

  • In diatomic molecules explicitly, it is equivalent to the estimation of bond enthalpy.

  • Covalent bonds between particles or atoms are said to have feeble bond dissociation enthalpies.

Difference between Bond Enthalpy and Bond Dissociation Enthalpy

The following emphasises the significant difference between bond enthalpy and bond dissociation enthalpy.

Apart from the diatomic molecules, the bond-dissociation enthalpy contrasts with the bond enthalpy. While the bond-dissociation enthalpy is the energy of a single chemical bond, the bond enthalpy is the average of all the bond-dissociation enthalpy of the bonds of the similar type for a given molecule. For a homoleptic compound EXn, the E–X bond enthalpy is (1/n) increased by the enthalpy change of the response EXn → E + nX. Bond Enthalpy can thus, also be known as Average Bond Dissociation Enthalpy and Bond Dissociation Enthalpy can be called Standard Enthalpy of Dissociation.

Average bond enthalpies given in tables are the normal estimations of the bond enthalpy of an assortment of species categories containing "typical" instances of the bond being referred to. For instance, separation of an HO−H bond of a water molecule(H2O) requires 118.8 kcal/mol (497.1 kJ/mol). The separation of the rest of the hydroxyl requires 101.8 kcal/mol (425.9 kJ/mol). The enthalpy of the O−H covalent bonds in water is supposed to be 110.3 kcal/mol (461.5 kJ/mol), basically the average of these values.

Similarly, for eliminating progressive hydrogen atoms from methane the bond-dissociation enthalpies are 105 kcal/mol (439 kJ/mol) for D(CH3−H), 110 kcal/mol (460 kJ/mol) for D(CH2−H), 101 kcal/mol (423 kJ/mol) for D(CH−H) lastly 81 kcal/mol (339 kJ/mol) for D(C−H). The bond enthalpy is, in this way, 99 kcal/mol, or 414 kJ/mol (the normal of the bond-dissociation enthalpies). None of the individual bond-dissociation enthalpies approaches the bond energy of 99 kcal/mol.

The Weakest and the Strongest Chemical Bonds

With the assistance of the idea of bond dissociation enthalpy, the weakest and the strongest chemical bonds can be found. 

As indicated by BDE information, the strongest single bonds are Si−F bonds. The BDE for H3Si−F is 152 kcal/mol, practically half more grounded than the H3C−F bond (110 kcal/mol). The BDE for F3Si−F is much bigger, at 166 kcal/mol. One result of this information is that numerous responses create silicon fluorides, for example, glass carving, deprotection in the natural blend, and volcanic eruptions. The quality of the bond is credited to the considerable electronegativity contrast between silicon and fluorine, which prompts a substantial contribution from both ionic and covalent bonding to the strength of the bond.

For a neutral compound, including various bonds, the strongest bond is found in carbon monoxide at 257 kcal/mol. The protonated types of CO, HCN, and N2 are said to have significantly stronger bonds; however, another examination contends that the utilization of BDE as a measure of bond energy in these cases is deluding.

On the other hand, there is no unmistakable limit between an exceptionally powerless covalent bond and an intermolecular association. The acid-base transition between metal fragments and noble gases are among the weakest of bonds with a substantial covalent character, with (CO)5W: Ar having a W–Ar bond dissociation enthalpy of under 3.0 kcal/mol. 20 Held together completely by the van der Waals power, helium dimer, He₂, has the least estimated bond separation energy of just 0.021 kcal/mol.

More about Bond Enthalpy

Enthalpy of bond formation (also referred to as bond-dissociation enthalpy, average bond energy, or bond strength) depends on how much energy is stored in a bond between two atoms in a molecule. This is the energy that must be added to a bond in order to undergo homolytic or symmetrical cleavage. When the bond is broken, it is homolytic or symmetrical, meaning each atom that originally participated in the bond gets one electron and becomes a radical, rather than an ion.

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In order to break chemical bonds, additional energy must be applied. Chemical bonds form due to their thermodynamic favorability. Because of this, bond enthalpy values are always positive and they usually contain units. An increased bond enthalpy indicates a stronger bond and a greater amount of energy is required to break it. By bringing the bond enthalpy value down, we can determine the amount of energy that will be released when forming or breaking a new bond.

Bond enthalpy values are of great value, and they are readily accessible in reference tables as averages for common bond types. In reality, the actual amount of energy involved in forming and breaking bonds depends on neighboring atoms within a chemical molecule. Nevertheless, the average values provided in these tables can be used as an approximation.

FAQs (Frequently Asked Questions)

1. What are the Four Types of Chemical Bonds?

Ionic Bonds, Covalent Bonds, Hydrogen Bonds, and van der Waals interactions are the four fundamental Bonds required for existence. These types of bonds are generally formed through either the loss, gain or sharing of electrons amongst two molecules or atoms. These bonds fluctuate in their qualities and strengths. In Chemistry, we consider Ionic Bonds and Covalent bonds as having similar qualities. Inorganic chemistry, every reaction happens through the existence of water. This implies that Ionic bonds will in general separate in water. In this way, we can think about these bonds in the chronology (strongest to weakest): Covalent, Ionic, Hydrogen, and van der Waals.

2. What are Homolytic and Heterolytic Bonds?

It tends to be seen that the heterolytic bond dissociation enthalpy is impressively higher than the homolytic separation for a homolytic bond. The Heterolysis of a neutral particle yields a positive and a negative particle. Be that as it may, the partition of these charges which are inverse requires a lot of energy. In the gas stage bond, dissociation happens by a simpler course, in particular homolysis. Further, an ionizing dissolvable heterolysis is the favored sort of breakage.

3. How do you calculate Bond Dissociation Enthalpy?

The bond dissociation enthalpy will allow you to calculate the strength of any chemical bond between two species. It is generally measured as the change in enthalpy at standard conditions (298K), but it is also sometimes described as the change in enthalpy of the homolytic fission of a chemical bond at absolute zero (0K).

The bond dissociation enthalpy concept has some important components, including:

  • A bond between two species requires a large amount of energy to break.

  • A chemical bond strength can be determined by using this equation.

  • A chemical bond strength is equal to bond energy in diatomic molecules specifically.

  • The strongest bond dissociation enthalpy is said to exist between silicon and fluorine.

  • Molecular or atomic bonds with weak bond dissociation energies are said to exist between atoms or molecules.

4. Which chemical bonds are weak and which are strong?

By using bond dissociation enthalpy, you can determine which bonds are the weakest and strongest. As we discussed earlier, silicon and fluorine have the strongest chemical bond.

Silicon tetrafluoride molecules are estimated to break their first silicon-fluorine bond with 166 kcal/mol dissociation energy. Because fluorine is the most electronegative element on the entire periodic table, and silicon has a huge difference in electronegativities.

As we move to neutral compounds, we find that the strongest bond is between carbon and oxygen in carbon monoxide, which has a bond dissociation energy of 257 kcal/mol. Additionally, the carbon-carbon bond in ethyne has a relatively high bond dissociation energy, estimated to be around 160 kcal/mol.

So it is found that the concept of bond dissociation enthalpy is extremely useful in the field of thermochemistry.

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).