Oxides and oxoanions of nitrogen

Key point: Reactions of nitrogen oxygen compounds that liberate or consume N₂ are generally very slow at normal temperatures and pH = 7. We can infer the redox properties of the compounds of the elements in Group 15 in acidic aqueous solution from the Frost diagram in Fig. 15.6. The steepness of the slopes of the lines on the far right of the diagram show the thermodynamic tendency for reduction of the 5 oxidation states of the elements. They show, for instance, that Bi₂O₅ is potentially a very strong oxidizing agent, which is consistent with the inert-pair effect and the tendency of Bi(V) to form Bi (III). The next strongest oxidizing agent is NO₃⁻. Both As(V) and Sb(V) are milder oxidizing agents, and P(V), in the form of phosphoric acid, is a very weak oxidant.

The redox properties of nitrogen are important because of its widespread occurrence in the atmosphere, the biosphere, in dustry, and the laboratory. Nitrogen chemistry is quite complex, partly because of the large number of accessible oxidation states but also because reactions that are thermodynamically favour able are often slow or have rates that depend crucially on the identity of the reactants. As the N₂ molecule is kinetically inert, redox reactions that consume N₂ are slow. Moreover, the for mation of N₂ is often slow and may be sidestepped in aqueous solution (Fig. 15.7). As with several other p-block elements, the barriers to reaction of high oxidation state oxoanions, such as NO3 −, are greater than for low oxidation state oxoanions, such as NO₂⁻. We should also remember that low pH enhances the oxidizing power of oxoanions (Section 5.6). Low pH also often accelerates their oxidizing reactions by protonation, and this step is thought to facilitate subsequent NO bond breaking. Table 15.2 summarizes some of the properties of the nitrogen oxides, and Table 15.3 does the same for the nitrogen oxoanions. Both tables will help us to navigate through the details of their properties.

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مواضيع ذات صلة

Condensed phosphates

Oxoanions of phosphorus, arsenic, antimony, and bismuth

Oxides of phosphorus, arsenic, antimony, and bismuth

Nitrogen(V) oxides and oxoanions

Oxohalides

Halides of the heavy elements

Phosphine, arsane, and stibine

Hydrazine and hydroxylamine

Ammonia

Phosphides

Azides

Nitrides