The average energy required to break one mole of similar bonds in a substance is called bond energy.
Units: It is expressed in kJ / mol
It is the energy required to break an Avogadro’s number of bonds. The same amount of energy is released when an Avogadro’s number of bonds is formed.
Experimentally bond energies are determined by calorimetry or spectroscopy.
The enthalpy change in splitting a molecule into its component atoms is called enthalpy of atomization
Ionic Character and Bond Energy
Bond energy is a measure of the strength of a bond. The strength of bonds depends upon
- Electronegativity difference of bonded atoms
- Sizes of atoms
- Bond length
A polar covalent bond is stronger than a non-polar covalent bond.
Consider the greater strength of HCI, due to its polar ( ionic ) character.
The H – H bond energy is 436 kJ/mol
i.e H + H ͢ H₂ ∆H = -436 kJ/mol
Thus 6.0210^23 Bond energy has = 436 kJ/mol
1 bond will have energy = 436/6.0210^23 = 72.42×10^-23
Since each hydrogen atom contributes equally, therefore, the contribution of each hydrogen will be 36.21×10^-23kj
Similarly, Cl – Cl bond energy is 240 kJ / mol and for one bond it is 39.86 x 10^-23kJ.
Thus each Cl atom will contribute 19.93 x 10-23 kJ.
Hence In case of H – Cl, bond energy must be 36.21×10^-23kj +19.93×10^-23kj =56.31×10^-23kj/molecule.
For one mole it will be 338.9 kJ / mol. But actually, it is 432 km/mol.
Bond energies generally decrease with the decrease in electronegativity difference of the bonded atoms.
Example: In H – F, H – CI, HBr, and H 1. the greatest Bond electronegativity difference is in the H – F bond i.e. 2.1, while in H – 1, the electronegativity difference is the least Le.0.4.
Hence, H – F most excellent bond energy ( 568 KJ / mol ) while Hl is the least (299k/mol).
Relative electronegativities can be determined by noting the difference between experimental and calculated bond energies e.g. In the case of HX, this difference is largest for HF and least for Hl. It shows that HF is more logical than Hl.
The order of logic character is
HF > HCI > HBr > HI
Bond energies can be used to determine the heat of gaseous reactions of covalent compounds.
Polar covalent bonds have high bond energy than non-polar covalent bonds.
Shorter bonds have higher bond energies.
e.g. The bond energies of C = C, C = C, and C – C are in the order
C = C > C = C > C – C
Thus a triple bond is stronger than a double bond which is stronger than a single bond. However, the bond energy of a triple bond is not thrice of a single bond. Similarly, the bond energy of a double bond is not double that of a single bond. Thus it shows that the sigma bond is stronger than a pi bond.
Bond energy decreases down the group, due to an increase in bond length for a particular type of bond.
The average distance between the nuclei of two atoms forming a covalent bond is called bond length
In the SI system bond lengths are expressed in pm (1 pm = 10-12 m ).
Physical methods measure the bond lengths. e.g. electron diffraction, X-ray diffraction, or spectral studies.
Due to the vibratory motion of atoms, bond lengths are not constant, Thus measured bond length is the average value.
The covalent bond length of a particular bond is almost independent of the nature of molecules. e.g in most aliphatic hydrocarbons C – C bond length is 154 pm. The same bond length is present in diamonds as well.
Relationship of Covalent Radii with Bond Length
One-half of the distance between nuclei of two similar bonded atoms is called Covalent Radius. Covalent radii of different elements are additive.
The covalent radius of Cl is 99 pm, which is half of the Cl – Cl bond length i.e.198 pm.
Similarly, the Covalent radius of C is 77 pm, which is half of the C – C bond length L.e.154 pm.
Thus C – Cl bond length must be 99 + 77 = 176 pm .
However, in certain cases, this rule does not apply.
Example: By adding covalent radii of Si ( 117 pm ) and F ( 64 pm ), the expected bond length of Si – F in SiF4 is 181 pm but the actual value is 154-159 pm.
This deviation is due to the difference in electronegativity of the bonded atoms.
Generally greater the electronegativity difference, the shorter the bond.
Thus calculated values are always higher than actual values due to electronegativity difference. it is because the ionic character is produced in the bond. The poles attract each other and the bond length becomes shorter.
Relationship of hybridization with Bond Length.
In general, bonds are shortened by increasing the s – the character of the hybrid orbitals It is because s – orbital has the smallest mean radius.
e.g Bond hybridization % s – Character Length(pm)
C – C in ethyne sp 50% 120
C – C in ethene sp2 33.33 % 133
C – C in ethane sp3 25% 154
Relationship Of Multiple Bonding With Bond Length
π – bonding also reduces the inter-nuclear bond distance.
Generally, a triple bond is shorter than a double bond, which, in turn, is shorter than a single bond Bond.
e.g Bond Bond length (pm)
Trends in Periodic Table
Generally, bond length decreases from left to right in a periodic table due to a decrease in atomic size and use in nuclear charge, e.g. bond length of C – C is greater than N – N bond length.
In a Group
Generally, bond length increases down the group due to an increase in atomic size.
e.g. Si-Si bond length is more than C – C bond length.
P – P bond length is more than N – N bond length.
It is because due to the increase in atomic radius, effective nuclear charge decreases on electrons. Thus bond length increases.
Read more about the Discovery of the Electron.