Bond energy is defined as average enthalpy change for the breaking of an A-B bond in a molecule in the gas phase.
When we describe the formation of a covalent bond,we introduced the concept of bond dissociation energy,the energy required to break a particular bond in a molecule.The bond dissociation energy is a measure of strength of perticular bond is essentially enthalpy change for a gas phase reaction in which a bond breaks.
There are three trends consistent in bond strenght:
1. Bond strength increases as more electrons are shared between the atoms : Shared electrons are the 'glue' of chemical bonding,so sharing more electrons strengthens the bond.
2.Bond strength increases as the electronegativity difference between bonded atom increases : Polar bonds gains stability from the electrical attraction between the negative and positive fractional cahrges around the bonded atoms. Bonds between oxygen and other second row elements explains this trend
O-O BE is 145 kj/mol , O-N BE is 200 kj/mol , O-C BE is 360 kj/mol
3.Bond strength decreases as bonds become longer : As atoms become larger,the electron density of a bond is spread over a wider region.This decreases the net attraction between the electrons and nuclei.Following example illiustrates the effect: H-F 92 pm BE is 565 kj/mol, H-Cl is 127 pm BE is 430 kj/mol , H-Br is 141 pm BE is 360 kj/mol
The table above list the values of some bond energies.Because it takes energy to break a bond,bond energies are always positive numbers.When a bond is formed,the energy is equal to the negative of the bond energy (energy is released).
Bond energy is the measure of the strength of a bond;the larger the bond energy,the stronger the chemical bond.Triple bond is stronger than double bond which is turn is stronger than single bond.
The table given above can be used to estimate the heat of reactions or enthalpy changes ΔH.
In general, the enthalpy of reaction is equal to the sum of the bond energies for the bond energies for bonds minus the sum of the bond energies for the bonds formed.
Lets see an example for estimating the ΔH from the Bond energies:
Find the ΔH of the following reaction:
CH4 + Cl2 ---------> CH3Cl + HCl
Step 1 : Imagine that the reaction takes place in steps involving the breaking and forming of bonds.Start with reactant and think that one C-H and Cl-Cl bond break.So, the enthalpy change is BE(C-H) + BE(Cl-Cl) .
Step 2 : now look at the product bond formation, in this case C-Cl and H-Cl bonds are formed and the enthalpy change equals the negative of the bond energies -BE (C-Cl) - BE(H-Cl).
Step 3 :Substituing bond energy values from the table , you get the enthalpy of reaction.
ΔH = BE(C-H) + BE(Cl-Cl) -BE (C-Cl) - BE(H-Cl)
= (411 + 240 -327 -428 ) kj
= -104 kj
the negative sign means that heat is released by the reaction.because the bond energy concept is only approximate.
In general,the enthalpy of reaction is (approximately) equal to the sum of the bond energies for bonds broken minus the sum of the bond energies for bonds formed.