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What chemical compounds might be present in drinking water?
What chemical compounds might be present in drinking water?

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2.1 The nitrogen cycle

Cycling of nitrogen and its compounds through the environment involves a delicate balance of redox, atmospheric and biological processes (Figure 4) generally involving water as a solvent. Plants require nitrogen, which is absorbed in the form of nitrate or ammonium ions, for the synthesis of nitrogenous organic compounds, such as amino acids, which are incorporated into the tissues of the plant. However, some of the intrinsic nitrogen is lost upon removing a crop from the soil. This must be replaced to maintain the fertility of the soil.

Described image
Figure 4 The chemicals present within the nitrogen cycle.
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Figure 4 The bottom third of the diagram is shaded green to represent the land, and the area above that is shaded blue to represent the sky. At the far left, in the blue area, it says 'N subscript 2'. An arrow points vertically down into the green area, and then turns and points to the right where there is a plant, with its roots in the green area. Beneath the plant it says 'N subscript 2 fixing in roots (N H subscript 3 )'.

An arrow points to the right to the words 'amino acids'.

From here another arrow points to the right where it says 'nitrates nitrites'.

From here another arrow points to the right to 'denitrifying bacteria', and from there an arrow points up into the blue layer, where it says 'N subscript 2'.

From 'nitrates nitrites' an arrow points to the left and then up to the surface of the green layer, where there is some grass growing. An arrow points up from the grass to the mouth of a cow. The cow is defecating. An arrow from the faeces down to the words 'amino acids', previously described.

On the top of the green layer is the word 'fertiliser'. An arrow points from the fertiliser down to the words 'nitrates nitrites'.

At the top of the blue layer is a cloud. Rain is falling from the cloud, and there are two lightning bolts below the cloud. An arrow labelled 'N O subscript 3 superscript minus (aq)' points down to the words 'nitrates nitrites' in the green layer.

Figure 4 The chemicals present within the nitrogen cycle.

Nitrogen has a versatile redox chemistry and displays several oxidation numbers in Figure 4.

  • What are the oxidation numbers of the nitrogen in ammonia (NH3), nitrogen gas, nitrite (NO2−) and nitrate (NO3−)?

  • The oxidisation number of hydrogen is generally +1 and oxygen is generally −2. The oxidation numbers of nitrogen in these compounds aree −3, 0, 3 and +5, respectively, to maintain the charge seen on the compounds. Note nitrogen compounds display oxidation numbers ranging from −3 to +5.

Like other elements in the second row of the Periodic Table, the nitrogen atom can form double bonds both to other nitrogen atoms and to atoms of some other elements of the second row, such as boron, carbon and oxygen.

  • Determine the resonance forms in the Lewis structure of nitrite.

  • There are two resonance forms (Structure 1).

    Structure 1