bond dissociation energy

(noun)

The energy required to separate two atoms joined by a particular bond. Expressed in terms of a mole of such bonded atoms. Indicates the strength of the bond.

Related Terms

Examples of bond dissociation energy in the following topics:

  • Bond Enthalpy

    • Bond enthalpy, also known as bond dissociation energy, is defined as the standard enthalpy change when a bond is cleaved by homolysis, with reactants and products of the homolysis reaction at 0 K (absolute zero).
    • For instance, the bond enthalpy, or bond-dissociation energy, for one of the C-H bonds in ethane (C2H6) is defined by the process:
    • Each bond in a molecule has its own bond dissociation energy, so a molecule with four bonds will require more energy to break the bonds than a molecule with one bond.
    • As each successive bond is broken, the bond dissociation energy required for the other bonds changes slightly.
    • Bond dissociation energies for different element pairings are listed.

  • Halogenation

    • In the case of carbon-hydrogen bonds, there are significant differences, and the specific dissociation energies (energy required to break a bond homolytically) for various kinds of C-H bonds have been measured.
    • The hydrogens bonded to the aromatic ring (referred to as phenyl hydrogens above) have relatively high bond dissociation energies and are not substituted.
    • The covalent bond homolyses that define the bond dissociation energies listed above may are described by the general equation:
    • The argument that the stability of alkyl radicals is simply derived from R-H bond dissociation energies is flawed.
    • We assume that the bond dissociation energies (BDEs) of elemental hydrogen and chlorine truly reflect the covalent bonding of the corresponding diatomic molecules.

  • Bond Energy

    • Bond energy is the energy required to break a covalent bond homolytically (into neutral fragments).
    • Bond energies are commonly given in units of kcal/mol or kJ/mol, and are generally called bond dissociation energies when given for specific bonds, or average bond energies when summarized for a given type of bond over many kinds of compounds.
    • Tables of bond energies may be found in most text books and handbooks.
    • The following table is a collection of average bond energies for a variety of common bonds.
    • Such average values are often referred to as standard bond energies, and are given here in units of kcal/mole.

  • Bond Energy

    • Bond energy is the measure of bond strength.
    • Bond energy is a measure of a chemical bond's strength, meaning that it tells us how likely a pair of atoms is to remain bonded in the presence of energy perturbations.
    • These energy values (493 and 424 kJ/mol) required to break successive O-H bonds in the water molecule are called 'bond dissociation energies,' and they are different from the bond energy.
    • The bond energy is the average of the bond dissociation energies in a molecule.
    • Identify the relationship between bond energy and strength of chemical bonds

  • Methods of Generating Free Radicals

    • The homolytic cleavage of covalent bonds produces radicals, and since this is an endothermic process, it requires the introduction of energy from the surroundings.
    • Heat serves this purpose by collisional interconversion of kinetic energy into vibrational energy, and the temperature required for bond homolysis will be proportional to the bond dissociation energy.
    • As expected, weaker covalent bonds dissociate into radicals more readily than stronger covalent bonds.
    • The following table lists standard bond energies (D) for the C–C, C–O and C–H bonds commonly found in organic compounds, together with bond energies for some weaker bonds that have been found useful for generating radicals.
    • Examples include the halogens Cl2, Br2 & I2 (bond dissociation energies are 58, 46 & 36 kcal/mole respectively), alkyl hypochlorites, nitrite esters and ketones.

  • Carbon Oxides and Carbonates

    • The bond dissociation energy of CO is 1072 kJ/mol and represents the strongest chemical bond known.
    • The two C=O bonds are equivalent and short (116.3 pm), consistent with double bonding.
    • CO2 is an end product of the metabolism of organisms via the cellular respiration process, in which energy is obtained from the breaking down of sugars, fats, and amino acids.
    • The Lewis structure of the carbonate ion has two single bonds to negative oxygen atoms and one short double bond to a neutral oxygen.
    • That's because it reacts with water to produce H2CO3, a small amount of which will dissociate into H+ and a bicarbonate ion.

  • ATP: Adenosine Triphosphate

    • When the chemical bonds within ATP are broken, energy is released and can be harnessed for cellular work.
    • The more bonds in a molecule, the more potential energy it contains.
    • The two bonds between the phosphates are equal high-energy bonds (phosphoanhydride bonds) that, when broken, release sufficient energy to power a variety of cellular reactions and processes.
    • Unless quickly used to perform work, ATP spontaneously dissociates into ADP + Pi, and the free energy released during this process is lost as heat.
    • To harness the energy within the bonds of ATP, cells use a strategy called energy coupling.

  • Isotopes of Hydrogen

    • The H–H bond is one of the strongest bonds in nature, with a bond dissociation enthalpy of 435.88 kJ/mol at 298 K.
    • As a consequence, H2 dissociates to only a minor extent until higher temperatures are reached.
    • At 3000K, the degree of dissociation is only 7.85%.
    • Chemically, deuterium behaves similarly to ordinary hydrogen (protium), but there are differences in bond energy and length for compounds of heavy hydrogen isotopes, which are larger than the isotopic differences in any other element.
    • It is radioactive, decaying into helium-3 through beta-decay accompanied by a release of 18.6 keV of energy.

  • Bond Order

    • Bond order is the number of chemical bonds between a pair of atoms.
    • Bond order indicates the stability of a bond.
    • Bond order is also an index of bond strength, and it is used extensively in valence bond theory.
    • In the second diagram, one of the bonding electrons in H2 is "promoted" by adding energy and placing it in the antibonding level.
    • By adding energy to an electon and pushing it to the antibonding orbital, this H2 molecule's bond order is zero, effectively showing a broken bond.

  • Nitrogen Fixation Mechanism

    • The enzymatic reduction of N2 to ammonia therefore requires an input of chemical energy, released from ATP hydrolysis, to overcome the activation energy barrier.
    • The association of nitrogenase component I and II and later dissociation occurs several times to allow the fixation of one N2 molecule (see step B and D).
    • Nitrogenase ultimately bonds each atom of nitrogen to three hydrogen atoms to form ammonia (NH3).
    • A) Components I and II are dissociated; II is ready for reduction.
    • D) The protein complex dissociates, and nitrogenase reduces dinitrogen to ammonia and dihydrogen.