Nitrate vs Nitrite
Both nitrate and nitrite are oxy anions of Nitrogen. Although they both seem to look alike, there are differences between them. These differences can be with respect to their structure, bonding, geometrical shape, oxidation state of Nitrogen, chemical reactivity, uses of them and etc. These differences are mainly discussed here.
NO3- ion is the conjugate base of HNO3 (nitric V acid) which is a strong acid. It is a planer molecule with sp3 hybridization in Nitrogen atom. All three oxygen atoms are equivalent. There are 24 electrons in the valence shells. NO3- ion has the molecular weight of 62.004 g mol-1.
NO2- ion is the conjugate base of HNO2 (nitric III acid) which is a weak acid (pKa=3.5). It is also a planer molecule with sp2 hybridization. In the valence shells, there are only 18 electrons. NO2- ion has the molecular weight of 46.006 g mol-1.
When the bonding in both the ions are considered, in NO3- ion the pi bonding involves four 2pz atomic orbitals (one orbitla from N and three orbitals from 3 oxygen atoms). These four atomic orbitals form 4, four-centred pi molecular orbitals. The molecular orbital with the lowest energy is the bonding orbital. The orbital with the largest energy is the anti-bonding orbital. Other two orbitals are degenerate (equal in energy) which are non-bonding orbitals. In the case of NO2- ion, the pi bonding involves only three 2pz atomic orbitals. Here, the orbital with the lowest energy is the bonding orbital, the middle one is the non-bonding orbital, and the other one is the anti-bonding orbital. In both the ions, the bonding orbitals involve in making the σ-bond (sigma-bond) and non-bonding orbitals involve in making π-bonds (pi-bonds). Therefore, in NO3- ion each bond has a bond order of 11/3. 1 from σ-bond and 1/3 from the π-bonds. In NO2- ion, the bond order is 11/2. 1 from σ-bond and 1/2 from the π-bonds.
Although, both the ions are from the same electronic geometry, as the bond order is different, their geometrical shapes are also different. The NO3- ion has a shape of planer triangular and the NO2- ion has an angular shape (or V shape). The bond angles are also different. ONO bond angle of NO3- ion and NO2- ion are 1200 and 1150 respectively. The oxidation state of Nitrogen in both NO3- ion and NO2- ion are +5 and +3 respectively. Because of these reasons (especially the difference in bonding), the chemical reactivity (such as basicity, oxidizing/reducing ability, products of thermal decomposition of the compounds consist of these ions) is also different.
In practice, NO2- ion is a weaker base while NO3- ion is a very weaker base. When the oxidation state of Nitrogen atom in these ions is taken in to consideration, NO2- ion can act as a reducing agent as well as an oxidizing agent while NO3- ion can only act as an oxidizing agent.
The examples for products obtained from thermal decomposition of NO3- and NO2- ions containing compounds are given below to show the difference in reactivity.
Even, when they form complexes with metal ions they behave in a dissimilar manner. That is, NO2- ion acts as a monodentate ligand while NO3- ion acts as a bidentate ligand.
These differences in chemical reactivity help us to differentiate the uses of them. For example, nitrites are generally used in the processing of meat (sometimes nitrates are also used), and nitrates are used for producing of explosives. Nitrates are naturally occurring and are cancerous. But Nitrites are not cancerous.