Bond Dissociation Energy
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
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