How To Calculate Bond Energy

Bond energy is a measurable attraction between the atoms in a molecule and can be used to predict the outcomes of reactions. A ​chemical bond​ is a ​stable arrangement of electrons​, and the energy that is required to break each bond can be looked up in a reference table and used in bond energy calculations to find the total energy change expected in a reaction.

Bond strength is affected by bond length, which is affected by atomic radius, nuclear charge, electronegativity and whether the bond is a single, double or triple bond. Note that there are exceptions, but it provides an overall trend.

​Atomic radius​, if large, will mean that the outer electrons are far from the pull of the positively charged nucleus. Two small atoms will be physically closer to each other than large ones, so the bond will be stronger.

​Nuclear charge​ is affected by the number of protons in the nucleus. Compare neon Ne (atomic number 10) and sodium ion Na+ (atomic number 11). Both have 10 electrons, but Na+ has 11 protons, and neon has only 10 protons, resulting in a higher nuclear charge for Na+.

On the periodic table, the elements that are closer to the right hand side have more ​electronegativity​ and will therefore form stronger bonds than those closer to the left side. Also, elements that are closer to the top of the periodic table have more electronegativity than those closer to the bottom. As an example, fluorine is much more reactive than iodine and carbon is more reactive than lithium.

​Double bonds​ require substantially more energy before they can be broken. Note the difference in the bond energies listed below for carbon.

Single bond: C—C bond energy is 346 kJ/mol
Double bond: C=C bond energy is 602 kJ/mol
Triple bond: C ≡ C bond energy is 835 kJ/mol

Using the table of bond energies given, what is the energy change when HCl is added to C₂H₄ to produce C₂H₅Cl?

The energy change of the reaction is equal to the sum of the bond energy of the bonds broken minus the sum of the bond energy of the bonds formed.

\(\Delta E_{reaction}=\Sigma BE_{bonds broken}-\Sigma BE_{bonds formed}\)

Draw the molecules: H₂C=CH₂ + H-Cl ⟹ H₃C—CH₂-Cl

You can see that the double bond between the carbons breaks and becomes a single bond. You know that the hydrochloric acid, HCl, will dissociate into ions H+ and Cl-, and these ions will bond with the carbon chain structure.

Bonds Broken (bond energy kJ/mol):

C=C (602)
H—CL (432)
Now add these together:

\(\Sigma BE_{bonds broken}=602+432=1034\)

Bonds Formed (bond energy kJ/mol):

C—C (346)
C—Cl (339)
C—H (413)
Now add these together:

\(\Sigma BE_{bonds formed}=346+339+413=1089\)

\(\Delta E_{reaction}=\Sigma BE_{bonds broken}-\Sigma BE_{bonds formed}=1034-1089=-55\text{ kJ}\)

The final result, ​-55 kJ​, is negative, indicating that the reaction was exothermic (heat released).