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.

** Because bond length increases down a column of the periodic table, bond dissociation energies are a quantitative measure of the general phenomenon—shorter bonds are stronger bonds.

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

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 forms 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