# Bond energy

Last updated

In chemistry, bond energy (BE), also called the mean bond enthalpy [1] or average bond enthalpy [2] is the measure of bond strength in a chemical bond. [3] IUPAC defines bond energy as the average value of the gas-phase bond-dissociation energy (usually at a temperature of 298.15 K) for all bonds of the same type within the same chemical species. [4]

## Contents

The bond dissociation energy (enthalpy) [5] is also referred to as bond disruption energy, bond energy, bond strength, or binding energy (abbreviation: BDE, BE, or D). It is defined as the standard enthalpy change of the following fission: R - X → R + X. The BDE, denoted by Dº(R - X), is usually derived by the thermochemical equation,

${\displaystyle {\begin{array}{lcl}\mathrm {D^{\circ }(R-} X)\ =\Delta H_{f}^{\circ }\mathrm {(R)} +\Delta H_{f}^{\circ }(X)-\Delta H_{f}^{\circ }(\mathrm {R} X)\end{array}}}$

The enthalpy of formation ΔHfº of a large number of atoms, free radicals, ions, clusters and compounds is available from the websites of NIST, NASA, CODATA, and IUPAC. Most authors prefer to use the BDE values at 298.15 K. [6]

For example, the carbonhydrogen bond energy in methane BE(C–H) is the enthalpy change (∆H) of breaking one molecule of methane into a carbon atom and four hydrogen radicals, divided by four. The exact value for a certain pair of bonded elements varies somewhat depending on the specific molecule, so tabulated bond energies are generally averages from a number of selected typical chemical species containing that type of bond. [7]

Bond energy (BE) is the average of all bond-dissociation energies of a single type of bond in a given molecule. [8] The bond-dissociation energies of several different bonds of the same type can vary even within a single molecule. For example, a water molecule is composed of two O–H bonds bonded as H–O–H. The bond energy for H2O is the average of energy required to break each of the two O–H bonds in sequence:

${\displaystyle {\begin{array}{lcl}\mathrm {H-O-H} &\rightarrow &\mathrm {H\cdot +\cdot O-H} &,D_{1}\\\mathrm {\cdot O-H} &\rightarrow &\mathrm {\cdot O\cdot +\cdot H} &,D_{2}\\\mathrm {H-O-H} &\rightarrow &\mathrm {H\cdot +\cdot O\cdot +\cdot H} &,D=(D_{1}+D_{2})/2\\\end{array}}}$

Although the two bonds are the equivalent in the original symmetric molecule, the bond-dissociation energy of an oxygen–hydrogen bond varies slightly depending on whether or not there is another hydrogen atom bonded to the oxygen atom.

When the bond is broken, the bonding electron pair will split equally to the products. This process is called homolytic bond cleavage (homolytic cleavage; homolysis) and results in the formation of radicals. [9]

## Predicting the bond strength by radius

Metallic radius, ionic radius, and covalent radius of each atom in a molecule can be used to estimate the bond strength. For example, the covalent radius of boron is estimated at 83.0  pm, but the bond length of B–B in B2Cl4 is 175 pm, a significantly larger value. This would indicate that the bond between the two boron atoms is a rather weak single bond. In another example, the metallic radius of rhenium is 137.5 pm, with a Re–Re bond length of 224 pm in the compound Re2Cl8. From this data, we can conclude that the bond is a very strong bond or a quadruple bond. This method of determination is most useful for covalently bonded compounds. [10]

## Factors affecting ionic bond energy

The electronegativity of the two atoms bonding together affects ionic bond energy. [11] Greater differences in electronegativity correspond to stronger ionic bonds.

## Related Research Articles

A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs, and the stable balance of attractive and repulsive forces between atoms, when they share electrons, is known as covalent bonding. For many molecules, the sharing of electrons allows each atom to attain the equivalent of a full valence shell, corresponding to a stable electronic configuration. In organic chemistry, covalent bonding is much more common than ionic bonding.

A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking of chemical bonds between atoms, with no change to the nuclei, and can often be described by a chemical equation. Nuclear chemistry is a sub-discipline of chemistry that involves the chemical reactions of unstable and radioactive elements where both electronic and nuclear changes can occur.

Electronegativity, symbolized as χ, is the tendency for an atom of a given chemical element to attract shared electrons when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the distance at which its valence electrons reside from the charged nucleus. The higher the associated electronegativity, the more an atom or a substituent group attracts electrons. Electronegativity serves as a simple way to quantitatively estimate the bond energy, and the sign and magnitude of a bond's chemical polarity, which characterizes a bond along the continuous scale from covalent to ionic bonding. The loosely defined term electropositivity is the opposite of electronegativity: it characterizes an element's tendency to donate valence electrons.

In chemistry, a hydrogen bond is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a lone pair of electrons—the hydrogen bond acceptor (Ac). Such an interacting system is generally denoted Dn−H···Ac, where the solid line denotes a polar covalent bond, and the dotted or dashed line indicates the hydrogen bond. The most frequent donor and acceptor atoms are the second-row elements nitrogen (N), oxygen (O), and fluorine (F).

An intermolecular force (IMF) is the force that mediates interaction between molecules, including the electromagnetic forces of attraction or repulsion which act between atoms and other types of neighbouring particles, e.g. atoms or ions. Intermolecular forces are weak relative to intramolecular forces – the forces which hold a molecule together. For example, the covalent bond, involving sharing electron pairs between atoms, is much stronger than the forces present between neighboring molecules. Both sets of forces are essential parts of force fields frequently used in molecular mechanics.

A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and biochemistry, the distinction from ions is dropped and molecule is often used when referring to polyatomic ions.

In organic chemistry, a peptide bond is an amide type of covalent chemical bond linking two consecutive alpha-amino acids from C1 of one alpha-amino acid and N2 of another, along a peptide or protein chain.

In coordination chemistry, a coordinate covalent bond, also known as a dative bond, dipolar bond, or coordinate bond is a kind of two-center, two-electron covalent bond in which the two electrons derive from the same atom. The bonding of metal ions to ligands involves this kind of interaction. This type of interaction is central to Lewis acid–base theory.

In chemistry, polarity is a separation of electric charge leading to a molecule or its chemical groups having an electric dipole moment, with a negatively charged end and a positively charged end.

The bond-dissociation energy is one measure of the strength of a chemical bond A−B. It can be defined as the standard enthalpy change when A−B is cleaved by homolysis to give fragments A and B, which are usually radical species. The enthalpy change is temperature-dependent, and the bond-dissociation energy is often defined to be the enthalpy change of the homolysis at 0 K, although the enthalpy change at 298 K is also a frequently encountered parameter.

In chemistry, the valence or valency of an element is the measure of its combining capacity with other atoms when it forms chemical compounds or molecules.

In chemistry, homolysis or homolytic fission is the dissociation of a molecular bond by a process where each of the fragments retains one of the originally bonded electrons. During homolytic fission of a neutral molecule with an even number of electrons, two free radicals will be generated. That is, the two electrons involved in the original bond are distributed between the two fragment species. Bond cleavage is also possible by a process called heterolysis.

In chemistry, heterolysis or heterolytic fission is the process of cleaving/breaking a covalent bond where one previously bonded species takes both original bonding electrons from the other species. During heterolytic bond cleavage of a neutral molecule, a cation and an anion will be generated. Most commonly the more electronegative atom keeps the pair of electrons becoming anionic while the more electropositive atom becomes cationic.

In chemistry, a non-covalent interaction differs from a covalent bond in that it does not involve the sharing of electrons, but rather involves more dispersed variations of electromagnetic interactions between molecules or within a molecule. The chemical energy released in the formation of non-covalent interactions is typically on the order of 1–5 kcal/mol. Non-covalent interactions can be classified into different categories, such as electrostatic, π-effects, van der Waals forces, and hydrophobic effects.

Diphenylmethane is an organic compound with the formula (C6H5)2CH2 (often abbreviated CH
2
Ph
2
). The compound consists of methane wherein two hydrogen atoms are replaced by two phenyl groups. It is a white solid.

The covalent radius of fluorine is a measure of the size of a fluorine atom; it is approximated at about 60 picometres.

Molecular binding is an attractive interaction between two molecules that results in a stable association in which the molecules are in close proximity to each other. It is formed when atoms or molecules bind together by sharing of electrons. It often, but not always, involves some chemical bonding.

This glossary of chemistry terms is a list of terms and definitions relevant to chemistry, including chemical laws, diagrams and formulae, laboratory tools, glassware, and equipment. Chemistry is a physical science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions; it features an extensive vocabulary and a significant amount of jargon.

In chemistry, bond cleavage, or bond fission, is the splitting of chemical bonds. This can be generally referred to as dissociation when a molecule is cleaved into two or more fragments.

In theoretical chemistry, the charge-shift bond is a proposed new class of chemical bonds that sits alongside the three familiar families of covalent, ionic, and metallic bonds where electrons are shared or transferred respectively. The charge shift bond derives its stability from the resonance of ionic forms rather than the covalent sharing of electrons which are often depicted as having electron density between the bonded atoms. A feature of the charge shift bond is that the predicted electron density between the bonded atoms is low. It has long been known from experiment that the accumulation of electric charge between the bonded atoms is not necessarily a feature of covalent bonds.

## References

1. Clark, J (2013), BOND ENTHALPY (BOND ENERGY), Chemguide, BOND ENTHALPY (BOND ENERGY)
2. Christian, Jerry D. (1973-03-01). "Strength of chemical bonds". Journal of Chemical Education. 50 (3): 176. doi:10.1021/ed050p176. hdl:. ISSN   0021-9584.
3. March, Jerry (1985), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (3rd ed.), New York: Wiley, ISBN   0-471-85472-7
4. Treptow, Richard S. (1995). "Bond Energies and Enthalpies: An Often Neglected Difference". Journal of Chemical Education. 72 (6): 497. doi:10.1021/ed072p497.
5. Haynes, William (2016–2017). CRC Handbook of Chemistry and Physics, 97th Edition (CRC Handbook of Chemistry & Physics) 97th Edition (97th ed.). CRC Press; 97 edition. ISBN   978-1498754286.
6. Luo, Yu-Ran and Jin-Pei Cheng "Bond Dissociation Energies". In Lide, David R. (ed) 2017, CRC Handbook of Chemistry and Physics, 97th edition (2016–2017). Boca Raton: Taylor & Francis Group. 9-73.
7. IUPAC , Compendium of Chemical Terminology , 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006) " Bond energy (mean bond energy) ". doi : 10.1351/goldbook.B00701
8. Madhusha (2017), Difference Between Bond Energy and Bond Dissociation Energy, Pediaa, Difference Between Bond Energy and Bond Dissociation Energy
9. "Illustrated Glossary of Organic Chemistry - Homolytic cleavage (homolysis)". www.chem.ucla.edu. Retrieved 2019-11-27.
10. Alcock, N. W. (1990). Bonding and Structure: Structural Principles in Inorganic and Organic Chemistry. New York: Ellis Horwood. pp. 40–42. ISBN   9780134652535.
11. Handbook of Chemistry & Physics (65th ed.). CRC Press. 1984-06-27. ISBN   0-8493-0465-2.