Bond Energy

Introduction

The amount of energy produced when one mole of bonds is created in isolated gaseous atoms to form a gaseous compound is known as bond formation energy or bond energy. The amount of energy necessary to break the link between two gaseous compounds and generate isolated gaseous atoms is known as bond dissociation energy. These two values are usually the same for a diatomic molecule, hence the phrase "bond energy" is employed. The phrase ‘average bond energy’ is used to describe the bond energy of a polyatomic molecule.

Bond Energy Corresponding to a Chemical Bond

The average bond energy involved with breaking the individual bonds of a molecule is measured by measuring the heat necessary to split one mole of molecules into their constituent atoms. The bond between the two atoms is said to be 'stronger' when the bond energy is higher, and the distance between them (bond length) is lower.

The HO-H bond in a water molecule, for example, needs 494 kJ/mol to break and produce the hydroxide ion (OH–). An extra 425 kJ/mol is required to break the O-H bond in the hydroxide ion.

As a result, the average of the two values, or 459 kJ/mol, is given as the bond energy of covalent O-H bonds in water. The energy values necessary to break successive O-H bonds in the water molecule are known as 'bond dissociation energies,' and they differ from the bond energy. The bond energy is the sum of a molecule's bond dissociation energies.

The nature of the other bonds in the molecule influences the exact parameters of a certain form of bond; for example, the energy and length of the C–H bond change depending on which other atoms are connected to the carbon atom. Similarly, the length of the C-H bond can vary by as much as 4-5 % between molecules.

As a result, the values reported in bond energy and bond length tables are often averages of a range of compounds containing a specific atom pair.

Bond	Bond length (angstrom)	Bond energy (kJ/mol)
C-C	1.54	348
C=C	1.34	614
C≡C	1.20	839

Calculating Bond Energy

• For diatomic molecule (HCl)

HCl is formed as result of association between hydrogen and chlorine gas as shown below:

$$\mathrm{H-H + Cl-Cl\:\rightarrow\:2 HCl}$$

The bond energy corresponding to each bond has been mentioned below in the table:

Bond	Bond energy (kJ/mol)
H-H	437
Cl-Cl	244
H-Cl	433

Energy Change = (437 + 244) – 2 × 433 kJ/mol = (681 – 866) kJ/mol = - 185 kJ/mol

• For polyatomic molecules $\mathrm{(H_2O)}$

Let us calculate the bond energy for O-H bond in a water molecule. The reaction can be represented as

$$\mathrm{H_2O + BE\:\rightarrow\:H + OH}$$

The bond energy of every O-H bond in water molecules could be considered as the average bond energies of every individual O-H bond. It could be calculated in the following manner:

$$\mathrm{H_2O + BE_1\:\rightarrow\:H + OH}$$

$$\mathrm{OH + BE_2\:\rightarrow\:H + O}$$

$$\mathrm{Hence,\:BE (O−H)=\frac{BE_1+ BE_2}{2}}$$

Where, $\mathrm{BE_1}$ indicates the energy required to break one O-H bond in $\mathrm{H_2O }$ and $\mathrm{BE_2}$ indicates the energy required to break one O-H bond in OH.

Factors Affecting Bond Energy

• As the atom's size increases, the bond length increases and the bond energy decreases, lowering bond strength.

• The bond energy of a bond between two identical atoms increases as the bond multiplicity increases.

• As the number of lone pairs of electrons on bonded atoms increases, the repulsion between them increases, and the bond energy decreases.

• As the bond energy increases, the s orbital contribution on the hybrid orbital increases. As a result, bond energy drops in the sequence listed below: $\mathrm{sp\:\gt\: sp^2\:\gt\:sp^3}$

• The higher the electronegativity difference, the higher the bond polarity and hence the bond strength, or bond energy. Thus, halides follow the order: $\mathrm{H-F\:\gt\:H-Cl\:\gt\:H-Br\:\gt\:H-I.}$

Conclusion

Bond Energy is a measurement of the bond strength required to disassemble one mole of a compound into its component atoms. The bond enthalpy, or average bond enthalpy, is another name for it. The stability of a chemical bond is directly proportional to its bond energy.

FAQs

Q1. Define Bond Energy.

Ans: The amount of energy produced when one mole of bonds are created in isolated gaseous atoms to form a gaseous compound is known as bond formation energy or bond energy.

Q2. What is the difference between bond energy and bond dissociation energy?

Ans: Bond Dissociation Energy would indicate the amount of energy required to break down a particular bond in hemolysis. While Bond Energy refers to the average amount of energy necessary to disassemble all the bonds which exist between same two types of atom in a compound. For a diatomic molecule, the bond energy is equal to bond dissociation energy.

Q3. What would be the expression of Bond Energy for a C-H bond in $\mathrm{CH_4}$?

Ans: The expression for Bond Energy in $\mathrm{CH_4}$ would be given as:

$$\mathrm{BE(C-H)=\frac{BE_1+BE_2+BE_3+BE_4}{4}}$$

Where BE1 indicates the bond energy required to break one C-H bond in $\mathrm{CH_4,\:BE_2}$ indicates the bond energy required to break one C-H bond in $\mathrm{CH_3,\: BE_3}$ indicates the bond energy required to break one C-H bond in $\mathrm{CH_2,\: BE_4}$ indicates the bond energy required to break one C-H bond in CH.

Q4. What are the factors that could affect Bond Energy?

Ans: The factors effecting bond energy are:

(i) Atomic Radius

(ii) Electronegativity

(iii) Number of Lone Pair on bonded atoms

Q5. What would be the effect of polarity on bond energy?

Ans: A more polar bond would have greater separation of charge (dipole moment) between the two atoms due to greater difference in electronegativity. Thus, the bond will have greater ionic tendencies compared to the covalent character. As the polarity increases, the bond energy would increase.