Lecture 12: Nitrogen and Sulfur Oxides

You will work in groups on the structure and bonding of some of these element oxides in class. Before coming to class:
  1. Carefully read the information on this page. If it is not clear or if you want additional information, you may also want to review material from your textbook in sections 4.5, 14.8, and 15.8.

  2. Watch the 15 minute video lecture Nitrogen and Sulfur Oxides on the MediaSite page.

  3. Take a short quiz on WebBoard.

Periodic Properties of Nitrogen and Sulfur

Both nitrogen is to the right of oxygen in the periodic table and sulfur is below it. We know, then, that both elements are less electronegative than oxygen. All N-O and S-O bonds are polar bonds with more electron density on the oxygen atom.
    Nitrogen has 5 valence electrons and is in a row with a maximum valence number of 8. It typically forms 3 bonds and has a lone pair (:NH3) or makes 4 bonds with a positive charge (NH4+).

    Sulfur has 6 valence electrons and, because of the 3d orbitals, can have more than 8 electrons in its valence shell. It makes some compounds similar to oxygen (H-S-H) and others with up to 6 bonds (SiF6, for example).


The elements:
  • N2
    Molecular nitrogen makes up 78 % of the Earth's atmosphere. It is "fixed" or transformed into ammonia, NH3, by organisms such as the bacteria in soy bean root nodules and the ammonia is tranformed to organonitrogen compounds by organisms.

    Molecular nitrogen is a very stable molecule. We discussed its bonding and some properties in Lecture 4.

  • Sx
    While oxygen has 2 allotropes (O2 and O3), sulfur has many. We say that sulfur as a strong tendency towards catenation, or formation of bonds with itself. There are many examples of sulfur rings and chains. Some are shown below.



    Elemental sulfur is present on the Earth's surface (usually associated with volcanic activity) but most sulfur in the Earth's crust is in sulfide minerals.

Nitrogen compounds in the atmosphere

Nitrogen is one of the few elements that readily forms strong multiple bonds. There is a very strong triple bond between nitrogen atoms in molecular nitrogen. The N=N bond energy of azides is less than half the energy of a triple bond and N-N bonds are quite weak. Single N-O bonds are considerably weaker than N-H bonds. A summary of N-X bond energies (in kJ/mol) is below.



Most of the nitrogen compounds in the atmosphere, apart from molecular nitrogen, are nitrogen oxides. The oxidation state of nitrogen varies from I to V in the oxides.
    (a) N2O, or nitrous oxide, and some NO, or nitric oxide, are produced by bacteria in the soil and in the oceans. Nitrous oxide is relatively unreactive in the troposphere and is the main source of nitric acid and other nitrates in the stratosphere. This is important in the chemistry related to ozone destruction in the stratospheric ozone layer. Nitrous oxide is relatively non-toxic and is used as an anaesthetic and as a propellant for whipped cream in cans.

    (b) Nitric oxide (NO) results from the combination of O2 and N2 in lightning strikes. It is a radical and so is very reactive in the atmosphere. Its unpaired electron can be removed through oxidation to NO+ or the molecule can be reduced to NO-. In biology, NO is important as a signaling agent. Reaction of NO with oxygen produces nitrogen dioxide.

    (c) Nitrogen dioxide (NO2) is a brown gas, responsible for the color of photochemical smog. It is a radical and so is reactive in the atmosphere. Nitrogen dioxide is in equilibrium with its colorless dimer, N2O4.

    (d) Dinitrogen tetraoxide (N2O4) has a very weak N-N bond and is in equilibrium with NO2. It is a diamagnetic substance. It reacts with water to form nitrous and nitric acids and it oxidizes many metals.

    (e) Other neutral nitrogen oxides include N2O5 and N2O3.

    (f) Nitrogen trioxide (NO3) is a radical with an unpaired electron on one of the oxygen atoms. Its reactivity is similar to that of the hydroxy radical (HO).

    (g) With the exception of the unreactive nitrous oxide (NNO), the neutral nitrogen oxides are commonly grouped as NOx. These are toxic to humans and other organisms and are important in the production of ground-level ozone and photochemical smog (more about that later).




The structures of NO2, N2O4, N2O5, and NO3 are below. You should now be able to predict the structure of nitrogen oxides based on Lewis structures and VSEPR.









Sulfur compounds in the atmosphere

There are a number of sulfur-oxygen compounds but the most important, gas-phase oxides of sulfur are SO2 and SO3.



Sulfur dioxide is the initial product from the combustion of sulfur-containing molecules. It reacts further with oxygen in air to form sulfur trioxide. Both molecules are diamagnetic.

Structure and bonding, a review

We talked about the electron count, Lewis structure, and oxidation state of NO3 in Lecture 2. Let's look at N2O5 as another example:



Oxy-acids

The oxides of elements in the upper right part of the periodic table are called acidic oxides. These molecules add water to form acids.

Nitric acid

This is a strong mineral acid. It dissolves readily in water and it is completely dissociated in that solvent.

Nitric acid is an oxidizing agent. For example, it reacts with metals to produce NO or NO2:

3 Cu(s) + 8 HNO3 3 Cu(NO3)2(aq) + 4 H2O + 2 NO(g)
Sulfuric acid is a diprotic acid. In aqueous solution, H2SO4 is completely dissociated to a solvated proton and the conjugate bisulfate, [HSO4]-.



Bisulfate still contains an acidic proton but it is only partially dissociated in water.



Other (less important) acids include hyponitrous acid (H2N2O2) nitrous acid (HNO2), dithionous acid (H2S2O4), sulfurous acid (H2SO3), disulfurous acid (H2S2O5), and dithionic acid (H2S2O6).