Dependence of reaction rates on

In order to elucidate the reaction mechanism, the influence of the acid concentration on the oxidation rates should be examined. An increase in the acid concentration was found to accelerate the 

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The dependence of reaction rates on pH and on the relative and absolute concentrations of reacting species, coupled with the possibility of autocatalysis and induction periods, has led to the discovery of some spectacular kinetic effects such as H. Landolt s chemical clock (1885) an acidified solution of Na2S03 is reacted with an excess of iodic acid solution in the presence of starch indicator — the induction period before the appearance of the deep-blue starch-iodine colour can be increased systematically from seconds to minutes by appropriate dilution of the solutions before mixing. With an excess of sulfite, free iodine may appear and then disappear as a single pulse due to the following sequence of reactions ... 

The dependence of reaction rate on concentration is readily explained. Ordinarily, reactions occur as the result of collisions between reactant molecules. The higher the concentration of molecules, the greater the number of collisions in unit time and hence the faster the reaction. As reactants are consumed, their concentrations drop, collisions occur less frequently, and reaction rate decreases. This explains the common observation that reaction rate drops off with time, eventually going to zero when the limiting reactant is consumed. 

In the search for a better approach, investigators realized that the ignition of a combustible material requires the initiation of exothermic chemical reactions such that the rate of heat generation exceeds the rate of energy loss from the ignition reaction zone. Once this condition is achieved, the reaction rates will continue to accelerate because of the exponential dependence of reaction rate on temperature. The basic problem is then one of critical reaction rates which are determined by local reactant concentrations and local temperatures. This approach is essentially an outgrowth of the bulk thermal-explosion theory reported by Fra nk-Kamenetskii (F2). 

The solvent dependence of the reaction rate is also consistent with this mechanistic scheme. Comparison of the rate constants for isomerizations of PCMT in chloroform and in nitrobenzene shows a small (ca. 40%) rate enhancement in the latter solvent. Simple electrostatic theory predicts that nucleophilic substitutions in which neutral reactants are converted to ionic products should be accelerated in polar solvents (23), so that a rate increase in nitrobenzene is to be expected. In fact, this effect is often very small (24). For example, Parker and co-workers (25) report that the S 2 reaction of methyl bromide and dimethyl sulfide is accelerated by only 50% on changing the solvent from 88% (w/w) methanol-water to N,N-dimethylacetamide (DMAc) at low ionic strength this is a far greater change in solvent properties than that investigated in the present work. Thus a small, positive dependence of reaction rate on solvent polarity is implicit in the sulfonium ion mechanism. 

A number of authors (29-32) have studied the dependence of reaction rate on pressure in the reaction mixture. Almost all of them [see, e.g., references (30-32) ] have obtained the first order with respect to hydrogen and deuterium. Pines and Ravoire (29) noted the order close to unity (0.7). 

Reaction of Ru(CO)g with H2 has been observed by high-pressure IR spectroscopy to produce H2Ru(CO)4 (18). The involvement of H2 in an equilibrium process such as step 2 could be the root of the observed non-integral dependence of reaction rate on H2 pressure. 

Verify Eq. (2.62) for the power-law dependence of reaction rate on temperature. 

Studies of the reaction kinetics at low concentrations of dibutylmagnesium (<0.35 M) reveal a first-order dependence of reaction rate on both zirconocene and dibutylmagnesium concentration, which is consistent with ratedetermining transmetallation. At higher concentration of dibutylmagnesium (>0.35 M), the reaction is first order in... 

Some reactions proceed explosively. The explosion are of two types (i) thermal explosion and (ii) explosion depends on chain reaction. The basic reason for a thermal explosion is the exponential dependence of reaction rate on the temperature. In an exothermic reaction, if the evolved energy cannot escape, the temperature of the reaction system increases and this accelerates the rate of reaction. The increase in reaction rate produces heat at an even greater rate. As the heat cannot escape, hence the reaction is even faster. This process continues and an explosion occurs. 

This is fundamentally different from the dependence of reaction rate on temperature that you learned about in Unit 3. The reaction between Hg( ), 02(g), and HgO(s) does not just change its rate with a change in temperature, it changes its direction. 

This functional form of k(T) predicts a very strong dependence of reaction rates on temperature, and this fact is central in describing the complexities of chemical reactions, as we win see throughout this book. 

Reaction weight is a reaction rate calculated at unit concentrations of intermediates, i.e. it is either the reaction constant or the reaction constant multiplied by power product corresponding to the "slow" components (either reagents or products). Thus, the dependencies of reaction rate on temperature and concentrations are "hidden" in reaction weights. 

Another factor that affects the rate of a chemical reaction is the concentration of reactants. As noted, most reactions take place in solutions. It is expected that as the concentration of reactants increases more collisions occur. Therefore, increasing the concentrations of one or more reactants generally leads to an increase in reaction rate. The dependence of reaction rate on concentration of a reactant is determined experimentally. A series of experiments is usually conducted in which the concentration of one reactant is changed while the other reactant is held constant. By noting how fast the reaction takes place with different concentrations of a reactant, it is often possible to derive an expression relating reaction rate to concentration. This expression is known as the rate law for the reaction. 

The decomposition of hydrogen peroxide illustrates the dependence of reaction rate on concentration. One way to determine how fast hydrogen decomposes is to measure the amount of oxygen generated. By performing a series of trials using different concentrations of hydrogen peroxide, it is found that the reaction rate increases in... 

Because of the overall first-order dependence of reaction rate on pressure (specifically, on hydrogen partial pressure) in combination with the rather complex selectivity relationships among primary products, it is regarded as quite probable that all of the primary products (and the separate inter-... 

Day 2 ), who apparently is completely in Zone III for his studies on the carbon-oxygen reaction, confirms the small dependence of reaction rate on temperature, as shown in Fig. 8. Between 1227 and 2027°, the activation energy is less than 8 kcal./mole at all flow velocities used. 

Because of the inertness of Co(III) and Cr(III) complexes, their substitution reactions were the first among those of octahedral complexes to be extensively studied. Most evidence supports the fd mechanism for substitution in Co(fll) complexes. First, there is little dependence of reaction rates on the nature of the incoming ligand, if bond making were of significant importance, the opposite would be expected. Data are presented in Table 13.4 for the anation reaction of penta-ammineaquacobaltdll) ... 

The distinctive feature of electrochemical kinetics is the strong dependence of reaction rates on the interfacial potential difference. 

Note that both 9.50 and 9.55 show the characteristic dependence of reaction rate on first power of substrate concentration and square root of the ratio of initiation rate to termination rate. 

As the temperature increases, the distribution of collision energies broadens and shifts to higher energies (Figure 12.15), resulting in a rapid increase in the fraction of collisions that lead to products. At 308 K, for example, the calculated value of / for the reaction with Ea = 75 kj/mol is 2 x 10-13. Thus, a temperature increase of just 3%, from 298 K to 308 K, increases the value of / by a factor of 3. Collision theory therefore accounts nicely for the exponential dependence of reaction rates on reciprocal temperature. As T increases (1 /T decreases), / = e E RT increases exponentially. Collision theory also explains why reaction rates are so much lower than collision rates. (Collision rates also increase with increasing temperature, but only by a small amount—less than 2% on going from 298 K to 308 K.)... 

Heat transfer is an extremely important factor in CVD reactor operation, particularly for LPCVD reactors. These reactors are operated in a regime in which the deposition is primarily controlled by surface reaction processes. Because of the exponential dependence of reaction rates on temperature, even a few degrees of variation in surface temperature can produce unacceptable variations in deposition rates. On the other hand, with atmospheric CVD processes, which are often limited by mass transfer, small susceptor temperature variations have little effect on the growth rate because of the slow variation of the diffusion with temperature. Heat transfer is also a factor in controlling the gas-phase temperature to avoid homogeneous nucleation through premature reactions. At the high temperatures (700-1400 K) of most... 

Fig. 3. A schematic picture of the dependence of reaction rate on partial pressure of CO. A case with a strong adsorption (7.5). (Reprinted with permission from Advances in Chemistry Series. Copyright by the American Chemical Society.)...

In the framework of this model, the next effects of the dependence of reaction rates on the local environment will be considered ... 

Static solvent effect — is widely understood as the dependence of -> reaction rate on solvent -> permittivity. The most systematic studies of this effect were stimulated by the early version of -> Marcus theory and mostly consisted in experimental verification of Mar-... 

Dependence of reaction rates on the concentrations of the reactants (and the products in certain cases) can be very useful in understanding the mechanism of catalysis. Some of the ubiquitous mechanistic steps reveal themselves in empirically derived rate expressions. In other words, such rate expressions, once established by experiments, are characteristic signs of such steps. 

Rb Kinetic expression for dependence of reaction rate on temperature and concentration (determined by applicable rate units)... 

Figure 2.3 Dependence of reaction rate on temperature for a reversible reaction (a) endothermic reaction (b) exothermic reaction.

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