❒ When a bond between two atoms is formed, there is a release of energy. The same amount of energy is absorbed when the bond is broken.
❒The bond energy is defined as: the average amount of energy required to break all bonds of a particular type in one mole of the substance.
❒Thus the bond energy of H – H bond is the energy required to break all the bonds in one mole of the gas.
❒ The bond energy is expressed in kcal mol–1 or kJ mol–1.
❒ For example, the bond energy of H – H bond is 433 kJ mol–1or 103.58 kcal mol–1.
❒ The bond energies of some common bonds are listed below:
Bond Energy is a Measure of Strength of the Bond
❒ In other words, bond energy is the force with which the atoms are bonded together. It depends upon :
(1) Size of the atom
(3) Bond length
❒ A knowledge of bond enthalpy is useful for calculating heats of reaction for gaseous reactions for which no thermal data is available and which involve substances having covalent bonds.
❒ Suppose we desire to determine the bond energy of C–H bond in methane. For this purpose we need to know the enthalpy change for the reaction:
C(g) + 4H(g) → CH4(g)
❒ This is obtained by combining the heat of formation of methane from C(s) + H2(g) with the heat of sublimation of carbon i.e., C(s) → C(g) and the heat of dissociation of hydrogen into atoms i.e., H2(g) → 2H(g), which have been determined by spectroscopic methods. The value so obtained is 398 kcal mol–1 (or 1663.64 kJ mol–1). This represents the bond energy of four C–H bonds. Since all the bonds in methane are identical, the bond energy of C–H bond is 398/4 = 99.5 kcal mol–1.
❒ In a similar manner the bond energies of other types of bonds have been calculated. When a bond is broken, the bond energy is positive because heat is absorbed. It is written with a minus sign when a bond is formed and heat is evolved.
❒The calculation of heat of reaction with the help of bond energies is illustrated in the following examples.
Problem (1): Given that energies for H–H, O=O and O–H bonds are 104, 118 and 111 kcal mol–1 respectively, calculate the heat of the reaction:
H2(g) + ½ O2(g) → H2O(g)
In this reaction, two O–H bonds are formed and one H–H bond is broken. Therefore we can write for ΔH
The heat of the given reaction is – 59.0 kcal mol–1
Problem (2): Calculate the bond energy of HCl, given that H–H bond energy is 433 kJ mol–1, Cl – Cl bond energy is 242 kJ mol–1and ΔHf for HCl is – 91 kJ mol–1.
Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolour edition.