Pseudo zero-order reactions

Eor a pseudo-zero-order reaction a plot of [A]( versus time should be linear with a slope of -k, and a y-intercept of [A]o (equation 13.8). A plot of the kinetic data is shown in figure 13.7. Linear regression gives an equation of... 

A third method, or phenomenon, capable of generating a pseudo reaction order is exemplified by a first-order solution reaction of a substance in the presence of its solid phase. Then if the dissolution rate of the solid is greater than the reaction rate of the dissolved solute, the solute concentration is maintained constant by the solubility equilibrium and the first-order reaction becomes a pseudo-zero-order reaction. 

Finally, although rare, we mention the occurrence of zero-order reactions. The special case of a pseudo-zero order reaction arises if a reactant is present in large excess, and the reaction does not noticeably change the concentration of the reactant. The differential and integral rate equations for a zero-order reaction R —> P are... 

The adiabatic induction time can be approximately evaluated from graphs in Fig. 5.4-68. They are plotted for the condition qR qp, which is nearly equivalent to adiabatic operation if the initial temperature is greater than Tr.i- Eqn. (5.4-214) is the basis of the graph in Fig. 5.4-68. From both graphs in Fig. 5.4-68 the apparent activation energies (E/Rf.) for pseudo-zero order reactions can be determined. 

It is impossible to conceive of a reaction rate as being independent of the concentration of all the species involved in the reaction. The rate might, however, very easily be independent of the concentration of one of the reactants. If this species, say A, is used in deficiency, then a pseudo zero-order reaction results. The rate —d[Jk]/dt will not vary as [A] decreases, and will not depend on the initial concentration of A. 

A.24.9 Zero-order reactions are discussed in section 25.5.1. [Aq] = 0 for acetaldehyde. The rate for tlie conversion of ethanol into acetaldehyde, i.e. the loss of ethanol, is 25 mmol It s so the rate of the creation of acetaldehyde is the opposite -25 mmol s Rearranging Equation 25.9, we get [A] = [Aq] - kt = 0 - (-25 mmol L 4 ) x 13 s = 325 mmolL The reaction could behave as a zero-order reaction if tlie concentration of ethanol is much higher than alcohol dehydrogenase, in which case, the rate of conversion would be independent of ethanol concentration. This situation is often referred to as a pseudo zero-order reaction. 

The rate of ammonium formation in sediments is discussed in theoretical terms by Berner (1974). He has applied his model to two environmentally different sediment cores. Berner assumes the rate of ammonium formation to be a pseudo-zero-order reaction, since the ammonium concentration in cores is a linear function of depth. A pseudo-zero-order... 

The value of Cfe for di-t-octyl disulfide and for di-n-octyl disulfide at a given temperature can be computed from Eqns 11-la and 11-1b. Because these equations were fitted to data for 25% additive in white oil, adjustment for concentration uses the fact that the rate of a pseudo-zero-order reaction is a linear function of concentration. Let us select 498 K (225 C) as the temperature the computed values of Ck are... 

Nitration of aromatics has been studied by many workers (25-28) and complied by many workers (25). In general, all nitration reactions involve two steps formation of nitronium ion (NOj) and reaction between nitronium and the aromatic substance. Normally, the second step is the controlling one and the nitration of many aromatics, such as chlorobenzenes, is too slow to involve any diffusional resistance. On the other hand, depending on the conditions, the nitration of benzene and toluene may well fall into the fast reaction regime. Indeed, in the case of toluene nitration, at high concentrations of toluene, the reaction may be controlled by the formation of the nitronium ion, leading to pseudo zero order reaction. 

Chan and Reineccius [35] also have published some kinetic work on other aroma compounds (isovaleraldehyde, phenylacetaldehyde, 2-acetyl-l-pyrroline, 2-acetyl-furan, and di(H)di(OH)-6-methyl pyranone). All of these volatiles followed pseudo zero order reaction kinetics in their early stages of reaction (Table 5.2). The fact that the concentrations of isovaleraldehyde, phenylacetaldehyde, and the methylpyr-anone reached plateaus late in heating suggests that a first order fit, as proposed by Jusino et al. [32], might be more appropriate. 

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