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Universidad Nacional Experimental

“Francisco De Miranda”

Aprendizaje Dialógico Interactivo

Programa Ingeniera !"mica

U#$% Ingl&s Instr"mental II

Actividad del 'es"men

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7ond energies

The bond energy is the amount of work that must be done to pull two atoms completely apart; in other words, itis the same as the depth of the “well” in the potential energy curve shown above. This is almost, but not quite

the same as the bond dissociation energy actually required to break the chemical bond; the difference is the very

small zeropoint energy. This relationship will be clarified below in the section on bond vibrational frequencies. 

!ow bond energies are measured"

#ond energies are usually determined indirectly from thermodynamic data, but there are two main e$perimentalways of measuring them directly%

1. The direct thermochemical method  involves separating the two atoms by an electrical discharge or some othermeans, and then measuring the heat given off when they recombine. Thus the energy of the &'& single bond

can be estimated from the heat of the recombination reaction between methyl radicals, yielding ethane%&!( ) &!( * !(&+&!(lthough this method is simple in principle, it is not easy to carry out e$perimentally. The highly reactive

components must be prepared in high purity and in a stream of moving gas.

2. The spectroscopic method  is based on the principle that absorption of light whose wavelength corresponds tothe bond energy will often lead to the breaking of the bond and dissociation of the molecule. -or some bonds,

this light falls into the green and blue regions of the spectrum, but for most bonds ultraviolet light is required.

The e$periment is carried out by observing the absorption of light by the substance being studied as thewavelength is decreased. hen the wavelength is sufficiently small to break the bond, a characteristic change in

the absorption pattern is observed.

/pectroscopy is quite easily carried out and can yield highly precise results, but this method is only applicable

to a relatively small number of simple molecules. The ma0or problem is that the light must first be absorbed by

the molecule, and relatively few molecules happen to absorb light of a wavelength that corresponds to a bondenergy. 1$periments carried out on diatomic molecules such as 23 and &/ yield unambiguous values of bond

energy, but for more comple$ molecules there are complications. -or e$ample, the heat given off in the &!(

combination reaction written above will also include a small component that represents the differences in the

energies of the &! bonds in methyl and in ethane. These can be corrected for by e$perimental data on reactionssuch as

&!( ) ! * &!4

&!3 ) ! * &!(

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#y assembling a large amount of e$perimental information of this kind, a consistent set of average bondenergies can be obtained 5see table below.6 The energies of double bonds are greater than those of single bonds,

and those of triple bonds are higher still.

7se of bond energies in estimating heats of reaction

2ne can often get a very good idea of how much heat will be absorbed or given off in a reaction by simply

finding the difference in the total bond energies contained in the reactants and products. The strength of an

individual bond such as 2+! depends to some e$tent on its environment in a molecule 5that is, in this e$ample,on what other atom is connected to the o$ygen atom6, but tables of 8average8 energies of the various common

 bond types are widely available and can provide useful estimates of the quantity of heat absorbed or released in

many chemical reaction.

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