Pre-equilibrium approximation

A second-order reaction in equilibrium followed by a first-order one of the type 

A system of the above type is characterised by a rapid pre-equilibrium step. A typical 

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The flux expression in Equation (4.16) displays the canonical Michaelis-Menten hyperbolic dependence on substrate concentration [S], We have shown that this dependence can be obtained from either rapid pre-equilibration or the assumption that [S] [E]. The rapid pre-equilibrium approximation was the basis of Michaelis and Menten s original 1913 work on the subject [140], In 1925 Briggs and Haldane [24] introduced the quasi-steady approximation, which follows from [S] 2> [E], (In his text on enzyme kinetics [35], Cornish-Bowden provides a brief historical account of the development of this famous equation, including outlines of the contributions of Henri [80, 81], Van Slyke and Cullen [203], and others, as well as those of Michaelis and Menten, and Briggs and Haldane.)... 

Distinguish between a pre-equilibrium approximation and a steady-state approximation. 

E7.19 first-order in HjOj and in Br", second-order overall E7.21 (ii) Both the pre-equilibrium approximation and the steady-state approximation predict that the reaction is first-order in A, first-order in B, and second-order overall... 

The pre-equilibrium approximation (PEA also called the partial equilibrium approximation or fast-equilibrium approximation) is applicable when the species participating in a pair of fast-equilibrium reactions are consumed by slow reactions. After the onset of an equilibrium, the rates of the, forward and backward reactions become equal to each other, and therefore the ratios of the concentrations of the participating species can be calculated from the stoichiometry of the reaction steps and the equilibrium constant. According to the pre-equilibrium approximation, if the rates of the equilibrium reactions are much higher than the rates of the other reactions consuming the species participating in the equilibrium reactions, then the concentrations of these species are determined, with good approximation, by the equilibrium reactions only. 

Here the rate of production of the final product (usually an essential biomolecule) can be derived using the pre-equilibrium approximation to be... 

However, there are other features of the kinetic system of differential equations that may simplify the situation. The application of kinetic simplification principles (see Sect. 2.3) may result in the situation where it is not that the individual parameters have an influence on the solution, but only some combinations of these parameters. A simple example occurs when species B is a QSS-species within the A B C reaction system, and its concentration depends only on ratio kilk2-Also, when the production rate of species C is calculated using the pre-equilibrium approximation (see Sect. 2.3.2) within reaction system A B C, it depends only on equilibrium constant K = kjk2 and does not depend on the individual values of ki and 2-... 

Three types of fundamental simplification can be used the method of isolation of one of the reactants, the pre-equilibrium approximation and the steady-state approximation. In addition, in many mechanisms it is possible to identify the one step that exercises a pronounced effect on the velocity of the overall reaction. This step is normally termed as the rate-determining step of the reaction, and when it exists, it allows us to obtain very simple relationships for the kinetics of the overall reaction. 

The zeroth-order rates of nitration depend on a process, the heterolysis of nitric acid, which, whatever its details, must generate ions from neutral molecules. Such a process will be accelerated by an increase in the polarity of the medium such as would be produced by an increase in the concentration of nitric acid. In the case of nitration in carbon tetrachloride, where the concentration of nitric acid used was very much smaller than in the other solvents (table 3.1), the zeroth-order rate of nitration depended on the concentrationof nitric acid approximately to the fifth power. It is argued therefore that five molecules of nitric acid are associated with a pre-equilibrium step or are present in the transition state. Since nitric acid is evidently not much associated in carbon tetrachloride a scheme for nitronium ion formation might be as follows ... 

You should verify for yourself that the three expressions in the first line do combine to give the final expression.) Because step 2 is slow relative to the fast pre-equilibrium, we can make the approximation fc,[01[0 ] [02 [0, or equivalently by canceling the 0, 2fO J [Pg.673]

The rate law of an elementary reaction is written from the equation for the reaction. A rate law is often derived from a proposed mechanism by imposing the steady-state approximation or assuming that there is a pre-equilibrium. To be plausible, a mechanism must be consistent with the experimental rate law. 

The pre-equilibrium and the steady-state approximations are two different approaches to deriving a rate law from a proposed mechanism, (a) For the following mechanism, determine the rate law by the steady-state approximation, (c) Under what conditions do the two methods give the same answer (d) What will the rate law become at high concentrations of Br ... 

Reaction rates have first-order dependence on both metal and iodide concentrations. The rates increase linearly with increased iodide concentrations up to approximately an I/Pd ratio of 6 where they slope off. The reaction rate is also fractionally dependent on CO and hydrogen partial pressures. The oxidative addition of the alkyl iodide to the reduced metal complex is still likely to be the rate determining step (equation 8). Oxidative addition was also indicated as rate determining by studies of the similar reactions, methyl acetate carbonylation (13) and methanol carbonylation (14). The greater ease of oxidative addition for iodides contributes to the preference of their use rather than other halides. Also, a ratio of phosphorous promoter to palladium of 10 1 was found to provide maximal rates. No doubt, a complex equilibrium occurs with formation of the appropriate catalytic complex with possible coordination of phosphine, CO, iodide, and hydrogen. Such a pre-equilibrium would explain fractional rate dependencies. 

The pre-equilibrium and the steady-state approximations are two different approaches to deriving a rate law from a proposed mechanism. 

The solution of the simultaneous differential equations implied by the mechanism can be expressed to give the time-varying concentrations of reactants, products, and intermediates in terms of increasing and decreasing exponential functions (8). Expressions for each component become complicated very rapidly and thus approximations are built in at the level of the differential equations so that these may be treated at various limiting cases. In equations 2222 and 2323, the first reaction may reach equilibrium for [I much more rapidly than I is converted to P. This is described as a case of pre-equilibrium. At equilibrium, Ar [A][S] = kr[I]. Hence,... 

A second common approximation is the steady-state condition. That arises in the example if A is fast compared with in which case [7] remains very small at all times. If [J] is small then d[I /dt is likely to be approximately zero at all times, and this condition is commonly invoked as a mnemonic in deriving the differential rate equations. The necessary condition is actually somewhat weaker (9). For equations 22a and b, the steady-state approximation leads, despite its different origin, to the same simplification in the differential equations as the pre-equilibrium condition, namely, equations 24a and b. 

SN1 reactions, such as hydrolyses and substitution by anions of tertiary halogen-oalkanes, are good examples of reactions with a reversible first step. In these a carbonium ion is produced, which then reacts with water or anions. Chapter 6, Problem 6.5, illustrates the different rate expressions found after applying the steady state treatment, and after assuming a pre-equilibrium where the equilibrium lies very far to the left, i.e. where K is very small, and only a very, very small amount of R+ is present. It also looks at the conditions under which the amount of R+ present in a steady state could approximate to a pre-equilibrium. The discussion did not include the situation where K is not small. 

The solvent isotope effects on individual rate coefficients computed from the experimental results are fe /fep = 2.32, kfJ°/kf2° = 3.68, kf fe jfe5i/fep fe fe i = 0.28. The first two values are of the magnitude expected for rate-determining proton transfer. The third quantity corresponds to a solvent isotope effect on a reaction with pre-equilibrium proton transfer, and the value is of the expected magnitude. It is approximately equal to Ks D /Ks H since fen % /eft. 

With the liquid mobile phase off and the channel rotating at an appropriate speed, the sample mixture is injected into the channel. The channel is rotated in this mode for a relaxation or pre-equilibrium period that allows the particles to be forced towards the accumulation wall at approximately their sedimentation equilibrium position. Particles denser than the mobile phase are forced towards the outer wall. Diffusion opposite to that imposed by the centrifugal force causes the particles to establish a specific mean thickness near the accumulation wall as a function of particle mass. Liquid mobile phase is then restarted with a parabolic velocity front. Small particles are engaged by the faster moving central streamlines and are eluted first. Large particles near the wall are intercepted by the slower streamlines and are eluted later. Thus particles are eluted from the channel in order of increasing mass. 

Since the rate laws for the two mechanisms (rapid pre-equilibrium or steady-state approximation) are indistinguishable, the mechanism could not clearly be established. 

This is essentially the classic case of the steady-state approximation and pre-equilibrium conditions for a two-step reaction with a reversible step. See Espenson, J. H. Chemical Kinetics and Reaction Mechanisms, 2nd ed. McGraw-Hill New York, 1995 pp 77-90. 

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