Bond Energy

Bond energy is defined as the amount of energy required to break all bonds of a particular type in one mole of the given substance. It is expressed in units of kilojoules per mole (kJmol-1).

The formation of a bond between two atoms releases energy into the surroundings (exothermic process), and the same amount of energy is required to break that very bond into its constituent atoms (endothermic process).

The strength of a bond is determined by its bond energy. A stronger bond requires more energy to be broken. Bond energy is determined experimentally by measuring the heat involved in a reaction.

With the energy change occurring at 298K, bond energy can also be called bond enthalpy. The enthalpy change required to split a molecule into its constituent atoms is known as the enthalpy of atomization.

Moreover, bond energy is the energy required to break one mole i.e. 6.02 x 1023 (Avogadro’s number) of bonds. Generally, multiple bonds are stronger than single bonds.

However, a double bond is exactly not twice as strong nor is a triple bond thrice as strong as a single bond. It implies that a σ bond (head-on overlap) is stronger than a 𝜋 bond (sidewise overlap). Moreover, a polar covalent bond is stronger than a non-polar covalent bond.

Factors Affecting Bond Energy

Bond energy is a measure of the strength of a bond. Higher bond energy corresponds to a stronger bond. Bond strength depends upon the following factors:

  1. Electronegativity difference
  2. Size of the bonded atoms
  3. Bond length
  4. Bond order

The first factor that influences bond energy is the electronegativity difference between the two bonded atoms. Greater the difference in electronegativity between the two bonded atoms, stronger is the bond and higher the bond energy.

Bond Energy and Electronegativity difference

From the table, it can be seen that HF has a higher bond energy due to a greater difference in electronegativity between H and F, which is 1.9.

Bond Energies

Another factor affecting the strength of the bond, and hence the bond energy, is the size of the bonded atoms. The smaller size of bonded atoms corresponds to a stronger bond and higher bond energy. This can be explained by comparing the bond energies of H2 and Cl2.

H-H has a bond energy of 436 kJmol-1 whereas Cl-Cl has a bond energy of 242 kJmol-1. This is because of the larger size of the chlorine atom as compared to the hydrogen atom.

This very factor can also be explained in terms of bond length, with the H-H bond length being smaller than the Cl-Cl bond length. Bond length is the distance between the nuclei of two bonded atoms. In fact, a bond is formed at the distance where there the potential energy is minimum (bond energy) considering the electrostatic forces of attraction and repulsion. 

Graph of bond length

Bond order is the fourth factor influencing bond strength. Multiple bonds will have a greater bond energy than a single bond.

Bond Energy and Bond Order

Bond Energy and Ionic Character

Generally, polar bonds are stronger than non-polar ones and so require more energy to be broken. The ionic character increases the strength of the bond. It can be shown by comparing the theoretical and experimental values for the bond energy of HCl.

H2 + Cl2 → 2HCl

Bond energy of H2 = 436 kJmol-1

Bond energy of Cl2 = 242 kJmol-1

Predicted bond energy of HCl (assuming non-polar) = (436 + 242) kJ / 2 mol

Theoretically, the bond energy of HCl should be 339 kJmol-1. Experimentally, this value is determined to be 431 kJmol-1. This shows that the actual bond energy inclusive of the ionic character is essentially greater than the predicted bond energy assuming non-polar bonds.

This implies that an ionic bond is more stable. The additional bond energy originates from the electronegativity difference between the two bonded atoms. Hence, greater the difference in electronegativity between the two bonded atoms, greater will be the ionic character and higher the bond energy.

Electronegativity is the tendency of a bonded atom to pull the bonded electrons towards itself. So the electrons are not equally shared between the hydrogen atom and the chlorine atom. The chlorine atom, being more electronegative, will have the bonded electrons closer to it than the hydrogen atom. This causes polarity in the molecule that serves as an additional binding force.

Concepts Berg

What determines bond energy?

Bond energy is primarily dependent on the strength of the bond. The factors that influence the bond strength are electronegativity, bond order, and bond length (or size of the atoms).

Which has higher bond energy?

In terms of bond order, triple bonds have more bond energy than double bonds, which in turn have greater bond energy than single bonds generally.

Covalent bonds with a higher ionic character (greater electronegativity difference between atoms) are stronger and possess more bond energy.

Smaller bond length also corresponds to greater bond energy.

What is the bond energy of C = O?

In general, the average bond energy of C = O is 745 kJmol-1. However, in the case of carbon dioxide, CO2, the average bond energy is 799 kJmol-1.

What is bond dissociation energy?

Bond dissociation energy is the energy absorbed when a bond is broken to form two separate atoms or molecules, each in possession of one electron from the bond. It is an endothermic process.

What is the bond energy of glucose?

In a glucose molecule, there are 7 C – H bonds (7 x 414 kJmol-1), 5 C – O bonds (5 x 358 kJmol-1), one C = O bond (803 kJmol-1), 5 O – H bonds (5 x 463 kJmol-1), and 5 C – C bonds (5 x 348 kJmol-1). The sum of all these is 9,546 kJmol-1, which is the bond energy of glucose.

What is bond stabilization energy?

In simple terms, bond stabilization energy is the difference between the amount of energy of a delocalized structure and that considering localized bonds.

How is electronegativity related to bond energy?

A greater difference in electronegativity between the two bonded atoms creates polarity in the bond. This ionic character provides an additional binding force and increases the strength of the bond. In turn, a higher bond energy results.

References

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