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The bond length is the average (because atoms vibrate) distance between two atoms (taken from the nuclei) in a stable molecule.
The heats of reactions in which bonds are broken are measures of strengths of chemical bonds. Bond energies are determined by measuring the heats of reactions in the gaseous state, where the atoms formed are distinctly separate and free from attraction or repulsion by other atoms, molecules, or ions.
Bond Dissociation Energy
is the enthalpy per mole required to break exactly one bond of the same type per molecule. When it is only possible to rupture several bonds in a single reaction, bond energies that are averages must be used.
For example; four C-H bonds are broken in the gas-phase dissociation of methane:
CH4(g) ® C(g) + 4H(g) D Ho = 1663 kJ
One fourth of the enthalpy of the preceding reaction is the average bond energy of the C-H bond, 415.8 kJ/mol.
Bond energy is the average enthalpy per mole for breaking one bond of the same type per molecule. Bond breaking is an endothermic process, and bond energies are positive enthalpies. In general, the larger the bond energy, the stronger the bond.
The standard enthalpy of a reaction is equal to the sum of the energy required to break all of the bonds in the reactants and the energy released by the formation of all of the bonds in the products.
D Ho =
The first term is the sum of the bond energies for all of the bonds in the reactants. Bond formation is an exothermic process and is the reverse of bond breaking, meaning that the heat of forming a bond has the same value as the bond energy, but is negative rather than positive.
The second term is, therefore, equivalent to - (sum of bond energies for all of the products). These relationships allow the above equation to be rewritten in the following;
D Ho =
However, the strength of a chemical bond is not independent of its surroundings, that is, of the molecule in which it exists. Bonds between different atoms in different compounds, or even in the same compound, may have different strengths, and the energy to break them may vary with the reaction sequence.
NH3(g) ® NH2(g) + H(g) D Ho = 435 kJ
NH2(g) ® NH(g) + H(g) D Ho = 377 kJ
NH(g) ® N(g) + H(g) D Ho = 356 kJ