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Bond dissociation energy is defines as the amount of energy which is required to homolytically fracture a chemical bond. A homolytic fracture usually produces radical species. Shorthand notation for this energy is BDE, D0, or DH°. Bond dissociation energy is often used as a measure of the strength of a chemical bond and to compare different bonds. Note the enthalpy change is temperature dependent. Typical units of bond dissociation energy are kJ/mol or kcal/mol. Bond dissociation energy may be measured experimentally using spectrometry, calorimetry, and electrochemical methods.
Key Takeaways: Bond Dissociation Energy
- Bond dissociation energy is the energy required to break a chemical bond.
- It is one means of quantifying the strength of a chemical bond.
- Bond dissociation energy equals bond energy only for diatomic molecules.
- The strongest bond dissociation energy is for the Si-F bond. The weakest energy is for a covalent bond and is comparable to the strength of intermolecular forces.
Bond Dissociation Energy Versus Bond Energy
Bond dissociation energy is only equal to bond energy for diatomic molecules. This is because the bond dissociation energy is the energy of a single chemical bond, while bond energy is the average value for all the bond dissociation energies of all bonds of a certain type within a molecule.
For example, consider removing successive hydrogen atoms from a methane molecule. The first bond dissociation energy is 105 kcal/mol, second is 110 kcal/mol, third is 101 kcal/mol, and final is 81 kcal/mol. So, the bond energy is the average of the bond dissociation energies, or 99 kcal/mol. In fact, the bond energy doesn't equal the bond dissociation energy for any of the C-H bonds in the methane molecule!
The Strongest and Weakest Chemical Bonds
From bond dissociation energy, it's possible to determine which chemical bonds are strongest and which are weakest. The strongest chemical bond is the Si-F bond. The bond dissociation energy for F3Si-F is 166 kcal/mol, while the bond dissociation energy for H3Si-F is 152 kcal/mol. Th reason the Si-F bond is believed to be so strong is because there is a significant electronegativity difference between the two atoms.
The carbon-carbon bond in acetylene also has a high bond dissociation energy of 160 kcal/mol. The strongest bond in a neutral compound is 257 kcal/mol in carbon monoxide.
There is no particular weakest bond dissociation energy because weak covalent bonds actually have energy comparable to that of intermolecular forces. Generally speaking, the weakest chemical bonds are those between noble gases and transition metal fragments. The smallest measured bond dissociation energy is between atoms in the helium dimer, He2. The dimer is held together by the van der Waals force and has a bond dissociation energy of 0.021 kcal/mol.
Bond Dissociation Energy Versus Bond Dissociation Enthalpy
Sometimes the terms "bond dissociation energy" and "bond dissociation enthalpy" are used interchangeably. However, the two are not necessarily the same. The bond dissociation energy is the enthalpy change at 0 K. The bond dissociation enthalpy, sometimes simply called bond enthalpy, is the enthalpy change at 298 K.
Bond dissociation energy is favored for theoretical work, models, and computations. Bond enthalpy is used for thermochemistry. Note that most of the time the values at the two temperatures are not significantly different. So, even though enthalpy does depend on temperatures, ignoring the effect doesn't usually have a big impact on calculations.
Homolytic and Heterolytic Dissociation
The definition of bond dissociation energy is for homolytically broken bonds. This refers to a symmetrical break in a chemical bond. However, bonds can break asymmetrically or heterolytically. In the gas phase, the energy released for a heterolytic break is larger than for homolysis. If a solvent is present, the energy value drops dramatically.
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- IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997).
- Gillespie, Ronald J. (July 1998). "Covalent and Ionic Molecules: Why Are BeF2 and AlF3 High Melting Point Solids whereas BF3 and SiF4 Are Gases?". Journal of Chemical Education. 75 (7): 923. doi:10.1021/ed075p923
- Kalescky, Robert; Kraka, Elfi; Cremer, Dieter (2013). "Identification of the Strongest Bonds in Chemistry". The Journal of Physical Chemistry A. 117 (36): 8981-8995. doi:10.1021/jp406200w
- Luo, Y.R. (2007). Comprehensive handbook of chemical bond energies. Boca Raton: CRC Press. ISBN 978-0-8493-7366-4.