Average Bond Enthalpies

The function of bond enthalpies is to show the relationship between the strength and length of bonds. Bond enthalpies can be shown in an equation of H = E + RT, where E is the total bond energy and R is the universal gas constant. On a graph of this equation, three points are used to designate different types of energies: H (enthalpy), dH (heat or thermal energy), and dQ (thermal or kinetic energy).

Key terms to remember –
Thermal Energy: Thermal energy is related to temperature. Thermal points increase as we move from solid – to liquid–gas phases. 

Kinetic Energy: Kinetic energies are related to motion without any external force.

What are the Average Bond Enthalpies?

Average bond enthalpies are calculated based on the energy deposited by bonds in a molecule made up of atoms bonded to ‘f’ electron donors and ‘g’ electron acceptors. This represents a non-bonding (n-bond) system.

Average bond enthalpies can be used to predict the stability of a molecule and view different types of bonds and how they differ in energy. The knowledge of average bond enthalpies is essential in the science of bonding and is linked to the use of computational chemistry.

Average Bond Enthalpies Importance:

Besides the practical applications of average bond enthalpies, they are also helpful in predicting the stability of a molecule. Also, average bond enthalpies are used to study bonding to compare different types of bonds and how they differ in energy. This knowledge is essential in the science of bonding and computational chemistry.

Comparing Average Bond Enthalpies to Molecular Weight:

Average bond energies are often used alongside molecular weights. However, they are two different types of measurements. Molecular weight shows how many molecules in a given sample the atoms make up. In contrast, average bond enthalpies show how much energy is stored in the bonds which hold molecules together. The two measurements are not entirely equal as they both do differ slightly.

When measuring average bond enthalpy, a sample containing a molecule of known molecular weight can be heated under different conditions to determine its average bond enthalpy values. This is often done in laboratories for a variety of experiments.

Calculating Average Bond Enthalpies using Computational Chemistry:

Average bond enthalpies can be obtained using computational chemistry. The method used to obtain the values involves building a potential energy surface (PES) with known geometry and fixed charges, then solving the Schrödinger equation times to find the imaginary time inverse of the possible energy surface. In this case, “zeta” refers to the temperature we have solved that Schröding equation.

Once the imaginary time zeta is found, we can use a computer program to find values for the norm of zeta, which is then used to find bond energies. The measure of distance between two points on the VAS determines their power. In theory, BDEs are equal everywhere, while this is not always the case in practice.

BDEs in computational chemistry can be pretty accurate as it uses experimental data and solves an equation derived from experimental data and a theoretical model.

Average Bond Enthalpy Uses:

Average bond enthalpies can be used to predict the stability of a molecule and view different types of bonds and how they differ in energy.

• Using Average Bond Enthalpies to Predict Stability of a Molecule: The simplest example is that a molecule with a high heat of formation is more stable than one with a lower heat of formation.

The highest energy will be in the bonds furthest away from the central atom, while the lowest point will be in the bonds nearest to the central atom. This can be seen when comparing average bond enthalpies and enthalpy values; high enthalpy means stability, as you have more energy stored in your bonds and therefore present less risk for spontaneity. This theory can then be applied to predict the strength of hypothetical molecules.

• Viewing Different Types of Bonds and How They Differ in Energy:

An example of this is when comparing diatomic (two-atom) bond energy and polyatomic (many atoms) molecules. The bonding interaction is only between two atoms; however, the particles are all bonded together by bonds in a polyatomic molecule. Therefore, in polyatomic molecules, the energy is lower than that found in diatomic molecules as we have decreased our number of bonds.

• The effect of the addition or removal of single bonds on Average Bond Enthalpies: Bond energies can be modified by adding or removing single bonds from a molecule. This can be seen when looking at the effect single bonds have on the heat of formation and bond energies. 

The more bonds are added to a molecule, the more power we need to break those bonds; therefore, this molecule will be less stable than a molecule with fewer bonds.

Double or triple bonded molecules can have higher bond energies than comparable single-bonded molecules. When a double or triple bond forms, there is an overlap of atomic orbitals. This overlap increases the stability of these bonds. 

From this, we can see that diatomic molecules have lower average bond enthalpies than comparable polyatomic molecules, which means that diatomic molecules are more stable and have riskless spontaneity.

Conclusion:

Average bond enthalpies are an essential part of the science of bonding. With the knowledge of BDEs and their properties, researchers can predict the stability of a molecule and view different types of bonds and how they differ in energy. BDEs in computational chemistry is another way to understand these values; they can be pretty accurate in theory because they use experimental data and solve an equation derived from experimental data and a theoretical model. Average bond enthalpies are used in many practical applications, such as measuring the stability of a molecule, predicting bond energies, and understanding types of bonds, for example, single, double, and triple bonds.