Bond Energy and Dissociation Energy
Bond energy is the average value of dissociation energies of the given bond in the series of different dissociating species. But bond dissociation energy is the required energy for breaking an atom from a common bond of the molecule. Hence the bond dissociation energy depends on the nature of the whole molecule. Therefore, the bond energy and bonds dissociation energy diatomic molecules are identical but polyatomic molecules are different. For example, the dissociation energy for breaking the oxygen hydrogen bond in the water molecule will be 120 kcal. But when the second hydrogen atom separated from residual OH-group the dissociation energy is the different value equal to 101 kcal.
Similarly, the dissociation energy of the C-H bond in methane and benzene are different because the residual portion of these two molecules is different. From the above discussion, the definition of dissociation energy is the energy or heat required to break a particular bond in polyatomic molecules in the gas phase to neutral fragments or free radicals.
Average Bond Energy of Molecules
In hydrocarbon methane, electronic configuration of carbon atom in the excited state, 2px1 2py1 2pz1. On mixing these four atomic orbitals give four equivalent sp3-hybrid orbitals. Therefore, each sp3-hybrid orbitals overlap with the 1s-atomic orbitals of four hydrogen atoms by the formation of four equivalent sigma bonds. But the energy required to break the first bond is not the same as the second bond. Hence we can take the average of all the different dissociation values and this average value termed as bond energy.
Therefore, the dissociation energy for breaking C-H bond in methane = 102 kcal but bond energy or the average energies for separate 4 successive atoms of hydrogen = 397/4 = 99 kcal.
|Chemical Bonds||Bond Energies|
|CH3 – H||426.8||102|
|MeCH2 – H||405.8||97|
|Me2CH – H||393.3||94|
|Me3C – H||374.5||89.5|
|C – C||347.3||83|
|C = C||606.7||145|
|N – N||163.2||39|
|N = N||418.4||100|
Pauling Scale of Bond Energy
Greater the polarity of a bond greater will be bond dissociation energy. This extra energy appearing in A – B molecules arise by electrostatic attraction between A and B dipole. Hence this extra energy possessed by molecules by polarization is called ionic resonance energy. These values used by pulling for determination of electronegativity or electron affinity difference of elements in the periodic table. Hence the empirical formula for the calculation of bond energy of A – B molecule is, XA – XB = 0.208 √Δ + α. Where XA and XB = electronegativity of A and B and α = arbitrary constant which nearly zero for elements other than carbon, nitrogen, oxygen, and fluorine, and. helium.
Bonds Energies and Resonance
The elementary factor that responsible for energies of given bonds is resonance and steric effects. The property of the molecules in the ground state describes by resonance energy. Most of the resonance energies obtained from the different types of hydrogenation of bonds. Resonance can occur only when all the atoms involved lie in the same plane. But any change in structure which prevents the planarity diminishes or inhibit resonance. Therefore, the following points useful for learning chemistry to describe the links between resonance hybrid and different bond energies.
- The more stable structure is the larger contribution in resonance. Generally the structure with the largest number of bonds more stable. Because the sum of these energies in the molecules gives the stability of the molecule.
- If different resonating structures have the same number of bonds, but some are charged species. In this case, the charged structure will less stable than the uncharged structure due to the bond moment.
Calculation of Bond Energies
The standard state of the element is the state in which the elements exist at 25°C temperature and one atmosphere pressure. The quantity of heat required to produce one mole of atoms in the gaseous atomic state in the standard state is called the heat of atomization of the element and calculated from the spectrum measurement. In learning chemistry, the thermodynamics heat of formation and atomization or ionization data use for calculation of the bond energy of the molecule.
For example, the heat of formation of methane = -17.9 kcal is an exothermic reaction. The heat of atomization solid carbon element to the gaseous carbon atom and hydrogen gas to gaseous hydrogen atom = 170.9 kcal and 4 × 52.1 kcal.
C (s) → C (g) ΔH = 170.9 kcal
2H2 (g) → 4H (g) ΔH = 4 × 52.1 kcal
CH4 → C (s) + 4H (g) ΔH = 17.9 kcal
From the above data required energy for breaking four C-H bonds = 170.9 + 4 × 51.1 + 17.9 = 397.2 kcal. Therefore, the C-H bond energy = 397.2/4 ≈ 99 kcal. Such calculation shows that C-H bond energy in hydrocarbon like methane, ethane, ethylene, etc are the same but this is not strictly correct. This energy varies due to environmental linking and the stereochemistry or structural dimensions of molecules in chemical science. But the variation values is however often small.
Heat of Formation and Bond Energies
Automation and bond energy data use for calculation of heat of formation of the molecule. Let us take one example, for the formation of methyl alcohol from carbon, oxygen, and hydrogen.
C (s) + 2H2 (g) + ½O2 (g) → CH3OH (l)
Atomization of 1-mole carbon = 170.9
For 2-mole hydrogen = 4 × 52.1
For half-mole oxygen = 59.6
Formation of 3 C-H bonds = – 3 × 99
For 1 C-O bond = – 84
For 1 O-H bond = -110.5
Liquefaction of 1 mole CH3OH = -8.4
From the above bond energies and atomization energies data in chemical science, the heat of formation of liquid alcohol = -61 kcal. If there exist one or more resonating structure, the resonance energy also taken for this calculation.