Rate-controlled reaction

Consider the alkaline pH region of the pH-rate profile in Fig. 8.4 (p. 459), which indicates a rate independent of pH. The rate-controlling reaction in this region is... 

The reaction does not feature a bimolecular step, such as direct Sn2 attack of the hydroxide nucleophile on the cobalt center. Rather, hydroxide ion participates in a prior-equilibrium reaction, and the actual rate-controlling reaction is believed to be the uni-molecular expulsion of the leaving group from a species that contains a coordinated... 

The reaction between cellulose and acrylamide was studied by quantitative, chromatographic separation of the substituted D-glucoses obtained on acid hydrolysis of the reaction product,320 followed by an analysis by Spurlin s method.249 Although, apparently, no check was made on the stability of the ethers to the conditions of hydrolysis, it might be expected that the ethers would isomerize only under basic conditions. The ratios of the relative equilibrium-constants for reaction at 0-2, 0-3, and 0-6 were 9 1 19, and these are attributable to the high, relative stability of the primary ether, together with the low reactivity of 0-3, also observed in rate-controlled reactions. 

Finally, in the Rh catalysed carbonylation of MeOAc to AcjO it is the control of [Acl] rather than [HI] which determines the overall carbonylation rate because Acl is involved in the rate controlling reaction with MeOAc. 

The oxidation rate of methanol in SCW and the subsequent production and destruction of the primary intermediate, formaldehyde, has been investigated using Raman spectroscopy as an in situ analytical method. Effluent samples were also examined using gas chromatography. An elementary reaction mechanism, which reproduces accurately the quantitative features of methanol oxidation and formaldehyde production, is used to identify key rate controlling reactions during the induction period and the transition to the primary oxidation path (Rice et al., 1996). 

A rate-controlled reaction is one whose major product is formed through the transition state with the lowest Aff. A thermodynamic-controlled reaction is one whose major product has the lower (more negative) Af/ of reaction. Reactions may shift from rate to thermodynamic control with increasing temperature, especially when the formation of the rate-controlled product is reversible. 

According to this theory the rate-controlling reactions are associated with the gasification (pyrolysis) of the solid oxidant and the solid binder which in a composite propellant are essentially independent of each other. 

Since the peroxide decomposition may be the rate-controlling reaction step in the preceding, it is of paramount importance to choose the peroxide that has the required decomposition rate at the real or expected melt processing temperatures. Such rates for dialkylperoxides are determined from decomposition kinetic data carried out in dilute decane or dodecane solutions in the form of half-lives, fy2, which is the time required for the decomposition of 50% of the POX (34), as shown in Figure 11.3. 

Interpret these facts to favor the view that photoelectrocatalysis occurs because of a rate-controlling reaction at the metal/solution interface, or, alternatively, that it occurs because the metal added to the surface shifts the Fermi level of the semiconductor. (Bockris)... 

For the case where diffusion of the corrosive ions is the rate controlling reaction, it has been found that P = po (1 + Ac/Aa) where p is the penetration that is proportional to the corrosion rate and p0 is the corrosion rate of the less noble uncoupled metal Ac and Aa are the areas of the more noble and active metal respectively (Uhlig and Revie, pp. 101-103).7 If a galvanic cell is not avoidable, a large anode and a limited size of cathode are recommended. Stagnant conditions and weak electrolytes may lead to pitting in spite of the large area of the exposed active metal. 

The rate at which a particular reaction occurs is important because it could provide real-time prediction capabilities. In addition, it could identify a particular reaction in a given process as the rate-controlling reaction of the process. For example, chemical mobility in soils, during rain events, is controlled by the rate at which a particular species desorbs or solubilizes, Similarly, the rate at which a particular soil chemical biodegrades is controlled by the rate at which the soil chemical becomes available substrate. 

Reductive Precipitation. Reductive precipitation involves the production of reduced species with limited solubility. An example of reductive precipitation in the environment involves the reduction of S04 to H2S and the precipitation of metals as metal-sulfides. In nature, the process of reductive precipitation is mostly microbiologically controlled. Production of H2S is the rate-controlling reaction of metal-sulfide precipitate formation. 

This can be interpreted in terms of a rate-controlling reaction between weakly sorbed O2 and strongly sorbed CO which saturates the surface ... 

It is possible to show that a two-sloped graph such as that shown in Fig. 5.56 can be treated in such a way that the rate-controlling reaction in the above series can be identified as the slow dissociation of CdBrj and the corresponding lifetime of the entity involved evaluated, also. The lifetime determined for the CdBr was 0.3 s in the nitrate... 

Experimentally it has been shown that the threshold pressure at which combustion instability can be induced artificially in composite proplnts by pulsing is a function of the burning rate of the proplnt (in a motor size of 5-inch diameter and 40-inch length) (Ref 45). This relationship is shown in Figs 17 and 18 for both aluminized and non-aluminized composite proplnts. It was also found that potassium perchlorate, lithium perchlorate and AN proplnts were resistant to this induced instability. Since AP composites were the only proplnts, other than double-base, which were driven unstable, the rate controlling reactions and response function are those related to AP decompn and perhaps the diffusion flame between oxidizer and binder... 

The time scale of the rate controlling reactions described by R ci,...,Cn) is much larger than the Lagrangian time scale of the turbulence. 

The given rate expressions for the rate-controlling reactions are... 

R and S configurations, 12ff Racemization, 69, 82, 124 Radicals, 31, 61, 222 Raney nickel, 295 Rate-controlled reaction, 150 Reaction mechanism, 31,41 Reaction rate, 39, 48 Reaction, spontaneity of, 36 Reactions, addition-elimination, 2 ljf Reactions, aromatic, 99ff Reactivity-selectivity principle, 59, 211 Redox, 33... 

For large samples of pure tungsten at temperatures >2200 °C (0.65 7] ), the stress exponent n = 5 and A// = 585kJ-mol . This activation enthalpy is similar to that of self-diffusion in tungsten. It was therefore assumed that under these conditions the deformation of the grains by dislocation climb or glide processes is the rate-controlling reaction [1.63]. 

Ketogenesis in the liver. All reactions occur in mitochondria the rate-controlling reactions (not shown) are release of fatty acids from adipose tissue and uptake of acyl-CoA into mitochondria, in particular, the CPTI reaction (see Figure 18-1). Acetoacetyl-CoA may regulate ketogenesis by inhibiting the transferase and the synthase. 

Because the pH of typical alkaline floods falls within the range of the existence of Si(OH)30-, Bunge and Radke (1982) presumed that solid silica dissolves according to the following rate-controlling reaction ... 

Two fundamentally different regimes can exist 1) those characterized by transport-controlled reaction and 2) those characterized by kinetic rate-controlled reaction (2.)- In the case of transport-controlled reaction, the reaction rate constant is much faster than any of the transport processes involved so that the length scale over which a moving fluid comes to equilibrium is small. In this regime, therefore, the walls of a dissolution channel are essentially discontinuities in permeability while in the kinetic rate-controlled case, where equilibrium between the fluid and the reacting mineral occurs over some distance, the boundaries of a channel are blurred by a more gradual permeability change. 

While the formation of coherent channels is favored in the transport-controlled case because of the ability of the channel to capture flow and restrict permeability change to the walls and tip of the channel, in the case of kinetic rate-controlled reaction, the permeability change is... 

A kinetic deuterium isotope effect would be expected if transfer of hydrogen were the rate-controlling reaction. Initial attempts by Hammond and co-workers (5,6) to observe such an isotope effect in the... 

---