Thermodynamic relations between N-containing species

The ability of N to exist in its compounds in at least 10 different oxidation states from —3 to +5 poses certain thermodynamic and mechanistic problems that invite systematic treatment. Thus, in several compounds N exists in more than one oxidation state, e.g. [N- H4]+[N 02]-, [N- H4]+[N 03], [N- 2H5]+[N 03]-, [N- H4]+[N-53]-, etc. Furthermore, we have seen (p. 423) that, under appropriate conditions, NH3 can be oxidized by O2 to yield N2, NO or NO2, whereas oxidation by OCl yields N2H4 (p. 427). Likewise, using appropriate reagents, N2H4 can be oxidized either to N2 or to HN3 (in which the average oxidation number of N is — ). The thermodynamic relations between these various hydrido and 0x0 species containing N can be elegantly codified by means of their 

2e ------ 2NH3OH+) can readily be calculated by appropriate combinations (in this case, 

Chemical reactions can often formally be expressed as the sum of two or more half-reactions in which electrons are transferred from one chemical species to another. Conventionally these are now almost always represented as equilibria in which the forward reaction is a reduction (addition of elecu-ons)  

The electrochemical reduction potential (E volts) of such an equilibrium is given by 

Remembering that AG = —nEF, it follows that the standard free energy change for the half reaction is AG = —nE°F. e.g.  

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