Slowest step

Similarly to the response at hydrodynamic electrodes, linear and cyclic potential sweeps for simple electrode reactions will yield steady-state voltammograms with forward and reverse scans retracing one another, provided the scan rate is slow enough to maintain the steady state [28, 35, 36, 37 and 38]. The limiting current will be detemiined by the slowest step in the overall process, but if the kinetics are fast, then the current will be under diffusion control and hence obey the above equation for a disc. The slope of the wave in the absence of IR drop will, once again, depend on the degree of reversibility of the electrode process. 

Like tert butyloxonium ion tert butyl cation is an intermediate along the reaction pathway It is however a relatively unstable species and its formation by dissociation of the alkyloxonium ion is endothermic Step 2 is the slowest step m the mechanism and has the highest activation energy Figure 4 8 shows a potential energy diagram for this step... 

With the potential energies shown on a common scale we see that the transition state for formation of (CH3)3C is the highest energy point on the diagram A reaction can proceed no faster than its slowest step which is referred to as the rate determining step In the reaction of tert butyl alcohol with hydrogen chloride formation of the... 

It IS important to note that although methyl and primary alcohols react with hydro gen halides by a mechanism that involves fewer steps than the corresponding reactions of secondary and tertiary alcohols fewer steps do not translate to faster reaction rates Remember the order of reactivity of alcohols with hydrogen halides is tertiary > sec ondary > primary > methyl Reaction rate is governed by the activation energy of the slowest step regardless of how many steps there are... 

Second order kinetics is usually interpreted m terms of a bimolecular rate determining step In this case then we look for a mechanism m which both the aryl halide and the nucleophile are involved m the slowest step Such a mechanism is described m the fol lowing section... 

Rate determining step (Section 4 9) Slowest step of a multi... 

Rate-determining step (Section 4.9) Slowest step of a multi-step reaction mechanism. The overall rate of a reaction can be no faster than its slowest step. 

Rate-determining Step (r.d.s.) the slowest step in the mechanism of a reaction which thereby controls the rate of the overall reaction. The r.d.s. has the highest activation energy. 

Rate-limiting step (Section 11.4) The slowest step in a multistep reaction sequence. The rate-limiting step acts as a kind of bottleneck in multistep reactions. 

Find the slowest step and equate the rate of the overall reaction to the rate of that step. 

Rate constant The proportionality constant in the rate equation for a reaction, 288 Rate-determining step The slowest step in a multistep mechanism, 308 Rate expression A mathematical relationship describing the dependence of reaction rate upon the concentra-tion(s) of reactant(s), 288,308-309 Rayleigh, Lord, 190... 

Basu and Searcy [736] have applied the torsion—effusion and torsion— Langmuir techniques, referred to above for calcite decomposition [121], to the comparable reaction of BaC03, which had not been studied previously. The reaction rate at the (001) faces of single crystals was constant up to a product layer thickness of 1 mm. The magnitude of E (225.9 kJ mole-1) was appreciably less than the enthalpy of the reaction (252.1 kJ mole-1). This observation, unique for carbonates, led to the conclusion that the slowest step in BaC03 vacuum decomposition at 1160—1210 K is diffusion of one of the reaction components in a condensed phase or a surface reaction of C02 prior to desorption. 

The most common way to generate sulfonyl radicals for spectroscopic studies has been the photolysis of solutions containing di-t-butyl peroxide, triethylsilane and the corresponding sulfonyl chloride in a variety of solvents (equations 4-6). The slowest step in this sequence is the reaction between t-butoxyl radicals and triethylsilane (ks = 5.3 x 106m 1s-1)26 since that for chlorine abstraction (equation 6) is extremely efficient (cf. Table 4). 

These steps occur in the sequence shown and the slowest step determines the deposition rate. The rules of the boundary layer apply in most CVD depositions in the viscous flow range where pressure is relatively high. In cases where very low pressure is used (i.e., in the mTorr range), the rules are no longer applicable. 

The rate is independent of the substrate concentration and first order with respect to enzyme concentration. In this case reaction (3), in which the complex decomposes to form the product, is the slowest step and is therefore rate limiting. Although this discussion has assumed that we have only an isolated enzyme reacting with a substrate, the same principles are applied to the more complex case when an entire organism, or a series of organisms consumes a substrate. 

Reaction 1 is the slowest step in this series of reactions leading to product formation. It is the rate-limiting step. Since this reaction involves bringing E and S together, it is a second-order reaction overall and first order with respect to the total enzyme concentration and the substrate concentration. 

Consider the NO/N02-catalyzed ozone destruction cycle, reactions 5 and 6 in Section 5.4.3. One could perform a calculation to determine which reaction is the rate-limiting step (i.e., the slowest step that determines the rate of the overall reaction) in this cycle. In this case, a theoretical doubling of ks reduces the ozone concentration by about 2%. On the other hand, doubling kf, reduces the ozone concentration by nearly 50%. (a) Which reaction is the rate-limiting step in N0/N02-catalyzed ozone destruction (b) The concentrations of NO and NO2 are [NO] = 2.9 X 10 /cm and [NO2] = 6.1 x 10 /cm. How do these data support or refute your answer to (a) ... 

A further consideration in porous materials is the shape of the pores. Molecules have to diffuse through the pores to feel the effect of the catalytic groups which exist in the interior and, after reaction, the reaction products must diffuse out. These diffusion processes can often be the slowest step in the reaction sequence, and thus pores which allow rapid diffusion will provide the most active catalysts. It is another feature of the MTSs that they have quite straight, cylindrical pores - ideal for the rapid diffusion of molecules. 

Each elementary reaction in a mechanism proceeds at its own unique rate. Consequently, every mechanism has one step that proceeds more slowly than any of the other steps. The slowest elementary step in a mechanism is called the rate-determining step. The rate-determining step governs the rate of the overall chemical reaction because no net chemical reaction can go faster than its slowest step. The idea of the rate-determining step is central to the study of reaction mechanisms. 

The slowest step in any sequential process is rate-determining. Airplanes take off at the rate that ranways become available, and arriving passengers leave the terminal at the rate their baggage is delivered. 

Remember that the overall rate of a reaction is determined by the rate of the slowest step, hi other words, no reaction can proceed faster than the rate-determining step. Any step that comes after the rate-determining step cannot influence the overall rate of reaction. In both proposed mechanisms, the first step of each mechanism is rate-determining. That is, each proposed mechanism predicts an overall rate of NO2 decomposition that is the same as the rate of the first step in the mechanism. 

The rate law predicted by a mechanism can be derived by setting the overall rate of reaction equal to the rate of the slowest step. As described in Section 15-1. any earlier steps are assumed to have equal forward and reverse rates. 

However, current solutions suffer from speed of the slowest step (e.g. filling with robots) or lack of automation [75]. 

It is apparent from the last example cited in previous section that there is not necessarily a connection between the kinetic order and the overall stoichiometry of the reaction. This may be understood more clearly if it is appreciated that any chemical reaction must go through a series of reaction steps. The addition of these elementary steps must give rise to the overall reaction. The reaction kinetics, however, reflects the slowest step or steps in the sequence. An overall reaction is taken as for an example ... 

At high temperatures (> 170 K), the water desorbs and so the autocatalytic reaction cannot be sustained and is an explanation for why the H2 + 02 reaction slows, the formation of OH species now being solely dependent on the H(a) + O(a) reaction, which is the slowest step in the above scheme. That the water + oxygen reaction was fast and facile was evident from the spectroscopic studies at both nickel and zinc surfaces, when the oxygen surface coverage was low and involving isolated oxygen adatoms. 

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