Bond dissociation energy

Bond dissociation energy is defined as the value of required energy to dissociate 1 mol of a specific type of bond in the gas phase. For example, 436 kJ/mol of energy is required to dissociate one H-H bond in hydrogen molecules.

\({\text{H}_2 \text{(g)} + 436\ \text{kJ/mol} \rightarrow \text{H (g)} + \text{H(g)}}\)

Thus, the bond dissociation energy of H-H bond in hydrogen molecules is 436 kJ/mol.

Bond Energy

Bond energy is average value of bond dissociation energy for all bonds of the same type within the same chemical species.

For example, consider the stepwise of methane \(\text{CH}_4\) into atoms.

\(\text{CH}_4 \text{(g)} + 435\, \text{kJ/mol} \rightarrow \text{CH}_3 \text{(g)}+\text{H(g)}\)

\(\text{CH}_3 \text{(g)} + 453\, \text{kJ/mol} \rightarrow \text{CH}_2 \text{(g)} +\text{H(g)}\)

\(\text{CH}_2 \text{(g)} + 425\, \text{kJ/mol} \rightarrow \text{CH(g)}+\text{H(g)}\)

\(\text{CH(g)} + 339\, \text{kJ/mol} \rightarrow \text{C(g)}+\text{H(g)} \)

The bond energy of C-H in methane is \(\frac{435+453+425+339}{4}=413 \:\text{kJ/mol}\)

The following table is average bond energy of various bond. It is not necessary to remember the given values as they will provide this in the IJSO.

Bond energy represents strength of the bond; a higher value of bond energy means a strong bond. For instance, bond energy of N=N (418 kJ/mol) bond is greater than N-N (160 kJ/mol) , which means N=N bond is stronger than the N-N bond. Generally, multiple bonds are stronger than single bonds.

Bond energy and Enthalpy

In chemical reactions, some bonds are broken and new bonds are formed. In order to dissociate bonds, energy should be added to the system. When bonds are formed, energy is released from the system. So, enthalpy change of reaction is (added energy to break bonds)+(released energy when new bonds are formed).

Method of calculate \(\Delta H\) using bond energy:

\(\Delta H = \Sigma (n \times D(break))- \Sigma(n \times D(form))\)

(D: bond energy per mole, n: number of each bond type (mol))

Example:

\(2\text{H}_2 \text{(g)} +\text{O}_2 \text{(g)} \rightarrow 2\text{H}_2\text{O (g)}\)

In this reaction, two H-H bonds and one O=O bond are broken down, and four H-O bonds are formed. The \(\Delta H\) is:

\(\Delta H =2D_{H-H} +D_{O=O} -4D_{O-H}\)

Written by Thenura Dilruk