Bond Energy

The N-N triple bond in molecular nitrogen is one of the strongest chemical bonds and, because of this, the N2 molecule is essentially inert. It doesn't react under normal circumstances with other molecules in the troposphere. However, there is one event in that can provide the energy needed to break the bonds of the N2 molecule: lightning.

The electric discharge in lightning heats the surrounding air to 30,000 degrees C. This heat energy is more than enough to provide the 941 kJ/mol needed to break the N-N bond. The amount of energy required to break a bond is called the bond energy. It is usually reported in kJ/mol. Bond energies for a few other molecules are shown are right.

Although bond energies are given here, it is usually easier to measure the bond enthalpies. Remember that enthalpy change H is the heat change at constant pressure. Energy is the sum of the heat and the work but the work term for most chemical processes is small.

Breaking the N-N bond forms two highly reactive nitrogen atoms. Nitrogen atoms can combine with oxygen atoms that are also formed in the electric discharge to make nitric oxide, NO.

Formation of Nitric Oxide

The overall enthalpy change can be measured. It is the sum of the reaction that breaks the N-N bond in N2, the reaction that breaks the O-O bond in O2, and the reaction that makes the N-O bond in NO. The enthalpy change for the formation of any molecules from the elements in their standard states in called the enthalpy of formation.

The reaction coordinate diagram for the reaction is below. The final product, NO, is at a higher energy than the reactants, 1/2 N2 and 1/2 O2. The reaction is called endothermic because net heat energy is added to the reaction system.

Let's look at the electron dot structure of the product. Compare it to that of N2 and O2.

The nitrogen in NO uses 2 of its valence electrons to make 2 bonds (sigma and pi) to the oxygen atom. Oxygen has no more half-filled orbitals so it can't make a triple bond with the nitrogen. The nitrogen atom is left with one of its orbitals half-filled. Any molecule that has an unpaired electron on an atom is called a radical. Radicals are much more reactive than other molecules.

Formation of Ozone

After a lightning storm you may detect a sharp odor in the air. This is due to the presence of ozone, O3. This forms when the oxygen atoms react with oxygen molecule.

In order to make an electron dot structure of ozone, it is necessary to move an electron from one oxygen atom to another. If oxygen atom #2 donates an electron to oxygen atom #3, it becomes positively charged and #3 becomes negatively charged. Oxygen atom #2 can use two of its electrons to form a double bond (1 sigma bond, 1 pi bond) to oxygen atom #1. It uses another electron to form a sigma bond to oxygen atom #3. Now all three oxygen atoms have a total electron count of 8.

There is another possibility. Oxygen atom #2 could donate an electron to #1. It would form a sigma bond with oxygen atom #1 and form a double bond with oxygen atom #3. The top electron dot structure and the bottom are equivalent. Any time it is possible to make two or more of these equivalent structures (resonance structures), the true structure of the molecule is the average of them. In ozone, there is a sigma bond between each oxygen atom and 1/2 of a pi bond.

Professor Patricia Shapley, University of Illinois, 2011