# Bond energy

In chemistry, bond energy (E), also called the mean bond enthalpy or average bond enthalpy[1] is the measure of bond strength in a chemical bond.[2] IUPAC defines bond energy as the average value of the gas-phase bond dissociation energies (usually at a temperature of 298 K) for all bonds of the same type within the same chemical species.[3]

The bond dissociation energy (enthalpy)[4] 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, Dº(R−X) = ΔfHº(R) + ΔfHº(X) – ΔfHº(RX). The enthalpy of formation ΔfHº 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. [5]

For example, the carbonhydrogen bond energy in methane H(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 elemnts 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.[6]

Bond energy (E) is the average of all bond-dissociation energies of a single type of bond in a given molecule.[7] 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:

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

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

## Standard Bond Energies

Energy is always required to break a bond. Energy is released when a bond is made. In general, the shorter the bond length, the greater the bond energy.[8]

Single Bonds ΔH°* (kJ/mol) Single Bonds ΔH°*(kJ/mol) Multiple Bonds ΔH°*(kJ/mol)
H–H 436 B–F 613 C=C 602
C–C 345 B–O 536 N=N 418
N–N 160 C–N 305 O=O 494
O–O 140 N–O 200 C=N 615
F–F 160 C–O 350 C=O (CO2) 799
Si–Si 230 O–O 140 C=O (aldehyde) 740
P–P 215 C–S 260 C=O (ketone) 744
S–S 215 C–F 439 C=O (ester) 749
Cl–Cl 243 C–Cl 330 C=O (amide) 749
Br–Br 190 C–Br 275 C=O (halide) 740
I–I 150 C–I 240 C=S (CS2) 577
H–C 415 C–B 356 N=O (HONO) 598
H–N 390 C–Si 318 P=O (POCl3) 460
H–O 464 C–P 264 P=S (PSCl3) 292
H–F 569 N–O 201 S=O (SO2) 535
H–Cl 432 S–S 215 S=O (DMSO) 389
H–Br 370 Si–F 565 P=P 351
H–I 295 Si–Cl 381 P≡P 489
H–B 389 Si–O 452 C≡O 1072
H–S 340 P–Cl 326 C≡C 835
H–Si 395 P–Br 264 N≡N 942
H–P 320 P–O 335 C≡N 887

Average Bond Dissociation Enthalpies in kilo joule per mole[9]. (There can be considerable variability in some of these values.) [10]

## Bond energy–distance correlation

Bond strength (energy)[11] can be directly related to the bond length and bond distance. Therefore, we can use the metallic radius, ionic radius, or covalent radius of each atom in a molecule to determine 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.[12]

## Factors affecting ionic bond energy

There are several contributing factors but usually the most important is the difference in the electronegativity of the two atoms bonding together.[13]

## Super-Strong, Super-Modulus Materials

Melting Temperature–Bond Energy Relation[14] The bond energy is directly related to the melting temperature of solids.It is seen that for different types of bonds, the melting temperature scales with the bond energy. Both ionic (e.g. NaCl, MgO) and covalent bonds (e.g. Si, C) have high bond energies and consequently high melting temperature. The metallic bond has a large variation in bond energies, ranging from 68 kJ/mol for Hg to 850 kJ/mol for W, matching that of ionic bonds. The lowest bond energies and melting temperatures are for materials with secondary bonds such as NH3, and H2O. Polymers, in which the chains are interconnected by secondary bonds, have very low melting/transition temperatures. The low melting/transition temperatures are related to the breaking of the Van der Waals bond between molecules rather than the covalent bond between atoms.

## References

1. ^ Clark, J (2013), BOND ENTHALPY (BOND ENERGY), Chemguide, BOND ENTHALPY (BOND ENERGY)
2. ^ March, Jerry (1985), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (3rd ed.), New York: Wiley, ISBN 0-471-85472-7
3. ^ Treptow, R. (1995). Journal of Chemical Education 1995 72 (6), 497. DOI: 10.1021/ed072p497
4. ^ 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. Retrieved 24 June 2016.CS1 maint: date format (link)
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 1498754287. Retrieved 24 June 2016.CS1 maint: date format (link)
6. ^ 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
7. ^ Madhusha (2017), Difference Between Bond Energy and Bond Dissociation Energy, Pediaa, Difference Between Bond Energy and Bond Dissociation Energy
8. ^ Song, Kim. "Bond Energies". chem.libretexts. Libretexts. Retrieved 30 September 2019.
9. ^ "Common Bond Energies". Wired Chemist. Claude Yoder. Retrieved 4 November 2019.
10. ^ Sanderson, R.T. "Standard Bond Energies". This page has tables of standard bond energies and bond dissociation energies. Wayback Machine. Retrieved 22 October 2019.
11. ^ Crawls, Alexa. "Bond Energy". Internet Archive. Wayback Machine. Retrieved 11 July 2003.
12. ^ Alcock, N. W. (1990). Bonding and Structure: Structural Principles in Inorganic and Organic Chemistry. New York: Ellis Horwood. pp. 40–42. ISBN 9780134652535.
13. ^ Handbook of Chemistry & Physics (65th ed.). CRC Press. ISBN 0-8493-0465-2.
14. ^ Banerjee, Shashwat (2012). "Functional Materials". Elsevier: 467-505. doi:10.1016/C2010-0-65659-8. Retrieved 31 October 2019.