Pseudo- and Quasi-steady State Modes

We have seen that the calculation of reaction rates from mechanisms quickly led to complex systems of differential equations that carmot be solved analytically or numerically because the numerical values of the rate coefficients involved are unknown. Fortunately, in a very large number of cases, reactions occnr according to permanent or quasi-permanent modes so it is possible to identify them experimentally. This simplifies the equations, thus enabling us to obtain expressions of speed that can then be compared to the experiment in order to validate the mechanisms. 

Concepts of the psendo-steady and quasi-steady states are apparently veiy close to one another. However the difference is fundamental. Whereas the pseudo-steady states define kinetic modes of reaction, the quasi-steady states are simple mathematical approximations. Moreover the difference is palpable in the sense that the first concept can be directly checked by the experiment independently of the kinetic law, the second is checked only by the conformity between the kinetic law speed, concentrations calculated and the experimental corresponding law. But maity other factors must intervene in addition so that these two laws coincide, in particular a correct choice of the mechanism of the reaction. 

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Remark.- In a homogenotts system with constant volirme, the quasi-steady state mode and the pseudo-steady state merge and this explains why some authors use the same term. 

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