Non-equilibrium natural systems

Thus far we have studied thermodynamics and kinetics imder the assumption that the systems of interest are in equilibrium. However, some natural systems have reaction rates so slow that they exist for long periods under non-equilibrium conditions. The formation of nitric oxide serves as an interesting example. 

The net reaction for NO formation is N2 -I- O2 — 2NO, although the actual mechanism by which NO forms does not include the direct reaction of N2 and O2 to any significant extent. This direct reaction would involve the breaking of two strong bonds and the formation of two new bonds, an unlikely event. Rather, the oxidation of nitrogen begins with a simple reaction  

This reaction occurs to only a small extent, but the oxygen atoms thus formed may form NO through the following catalytic cycle. 

The reverse reactions of 1, 2, and 3 are also important in establishing the equilibrium between N2,02, and NO2. 

At low temperatures the rates of these reactions are very slow either because the rate constants are very small or because the concentrations of O and N are very small. For these reasons, equilibrium is not maintained at the low temperatures typical of the atmosphere. However, as the temperature rises, the rate constants for the critical steps increase rapidly because they each have large activation energies -Ea = 494 kj/mol for reaction 1 and 316 kj/mol for reaction 2. The larger rate constants contribute to a faster rate of NO production, and equilibrium is maintained at higher temperatures. The time scale for equilibrium for the overall reaction N2 -I- O2 2NO is less than a second for T 2000 K. 

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