Bond energy (definition, Illustration, solved Problems)

Bond energy (definition, Illustration, solved Problems)

Bond energy 

– 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–1 or 103.58 kcal mol–1.

– The bond energies of some common bonds are listed below:

Bond energy (definition, Illustration, solved Problems)

Bond Energy is a measure of the 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

(2) Electronegativity

(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.

Solved Problem

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–1 and ΔHf for HCl is – 91 kJ mol–1.


Bond energy (definition, Illustration, solved Problems)

Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolour edition.

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