Bond Dissociation Energy: Definition, Equation, Problems

– In this subject, we will discuss the Bond Dissociation Energy: Definition, Equation, Problems

Bond Dissociation Energy

 – Bond breaking can be quantified using the bond dissociation energy.

– The bond dissociation energy is the energy needed to homolytically cleave a covalent bond.

– The energy absorbed or released in any reaction, symbolized by ΔH°, is called the enthalpy change or heat of reaction.

– When ΔH° is positive (+), energy is absorbed and the reaction is endothermic.

– When ΔH° is negative (–), energy is released and the reaction is exothermic.

– The superscript (°) means that values are determined under standard conditions (pure compounds in their most stable state at 25 °C and 1 atm pressure).

– A bond dissociation energy is the ΔH° for a specific kind of reaction—the homolysis of a covalent bond to form two radicals.

– Because bond breaking requires energy, bond dissociation energies are always positive numbers, and homolysis is always endothermic.

– Conversely, bond formation always releases energy, so this reaction is always exothermic.

– The H– H bond requires +435 kJ/mol to cleave and releases – 435 kJ/mol when formed.

Bond dissociation energy and bond strength

– The following Table (1) contains a representative list of bond dissociation energies for many common bonds.

– Additional bond dissociation energies for C – C multiple bonds are given in Table (2)

– Comparing bond dissociation energies is equivalent to comparing bond strength.

– The stronger the bond, the higher its bond dissociation energy.

– For example, the H – H bond is stronger than the Cl – Cl bond because its bond dissociation energy is higher [Table: 435 kJ/mol (H₂) versus 242 kJ/mol (Cl₂)].

– The data in Table (1) demonstrate that bond dissociation energies decrease down a column of the periodic table as the valence electrons used in bonding are farther from the nucleus.

– Bond dissociation energies for a group of methyl–halogen bonds exemplify this trend.

Bond dissociation energy and enthalpy change

– Bond dissociation energies are also used to calculate the enthalpy change (ΔH°) in a reaction in which several bonds are broken and formed.

– ΔH° indicates the relative strength of bonds broken and formed in a reaction.

– When ΔH° is positive, more energy is needed to break bonds than is released in forming bonds.

– The bonds broken in the starting material are stronger than the bonds formed in the product.

– When ΔH° is negative, more energy is released in forming bonds than is needed to break bonds.

– The bonds formed in the product are stronger than the bonds broken in the starting material.

To determine the overall ΔH° for a reaction

 (1) Beginning with a balanced equation, add the bond dissociation energies for all bonds broken in the starting materials.

– This (+) value represents the energy needed to break bonds.

(2) Add the bond dissociation energies for all bonds formed in the products.

– This (–) value represents the energy released in forming bonds.

(3) The overall ΔH° is the sum in Step (1) plus the sum in Step (2).

Important Trends

Solved Problems on Bond Dissociation Energy

Problem (1): Use the values in Table (1) to determine ΔH° for the following reaction.

answer:

Because ΔH° is a negative value, this reaction is exothermic and energy is released.

The bonds broken in the starting material are weaker than the bonds formed in the product.

Problem (2): Use the values in Table (1) to calculate ΔH° for each reaction. Classify each reaction as endothermic or exothermic.

answer:

– The oxidation of both isooctane and glucose, the two molecules form CO₂ and H₂O. 

– ΔH° is negative for both oxidations, so both reactions are exothermic.

– Both isooctane and glucose release energy on oxidation because the bonds in the products are stronger than the bonds in the reactants.

Limitations of Bond Dissociation Energies

– Bond dissociation energies have two important limitations.

– They present overall energy changes only.

– They reveal nothing about the reaction mechanism or how fast a reaction proceeds.

– Moreover, bond dissociation energies are determined for reactions in the gas phase, whereas most organic reactions are carried out in a liquid solvent where solvation energy contributes to the overall enthalpy of a reaction.

– As such, bond dissociation energies are imperfect indicators of energy changes in a reaction.

– Despite these limitations, using bond dissociation energies to calculate ΔH° gives a useful approximation of the energy changes that occur when bonds are broken and formed in a reaction.

Reference: Organic chemistry / Janice Gorzynski Smith, University of Hawai’i at Manoa / (Third edition), 2011. USA