Chemical reactions very slow

When the chemical reaction step occurs very rapidly (virtually instantaneously upon collision of the reactants), one speaks of a diffusion-controlled reaction and in this case, the reaction rate constant is typically on the order of 1010 M-1 s-1. When the chemical reaction is slow as compared to the collisional process, the reaction is often called an activation-controlled reaction because a high activation energy is needed to yield the products. The rate constant is thus on the order on 1 M-1 s 1. In the general case, the reaction rate constant is a combination of the two processes and is described by the following expression ... 

Such descriptions can be appropriate for instantaneous and very fast reactions. In contrast, if the chemical reaction is slow, the reaction rate dominates the whole process, and therefore, the reaction kinetics expression has to be integrated into the mass and energy balances. This concept has been used in a number of RD studies (e.g., [58, 59]). 

The suppression of the separator is possible in the cell because this reaction, which could evolve chemically, is very slow and thus does not preclude the two electrode reactions. This cell is used in the operational definition of pH. A variant of the preceding cell is the following ... 

When the chemical reaction step occurs very rapidly (virtually instantaneously upon collision of the reactants), one speaks of a diffusion-controlled reaction and in this case, the reaction rate constant is typically on the order of 10 s When the chemical reaction is slow... 

If the rate of chemical reaction is very slow compared to the rate of group diffusion (kj. kj, k ), then Eq. (5.8) reduces to... 

Positive-Tone Photoresists based on Dissolution Inhibition by Diazonaphthoquinones. The intrinsic limitations of bis-azide—cycHzed mbber resist systems led the semiconductor industry to shift to a class of imaging materials based on diazonaphthoquinone (DNQ) photosensitizers. Both the chemistry and the imaging mechanism of these resists (Fig. 10) differ in fundamental ways from those described thus far (23). The DNQ acts as a dissolution inhibitor for the matrix resin, a low molecular weight condensation product of formaldehyde and cresol isomers known as novolac (24). The phenoHc stmcture renders the novolac polymer weakly acidic, and readily soluble in aqueous alkaline solutions. In admixture with an appropriate DNQ the polymer s dissolution rate is sharply decreased. Photolysis causes the DNQ to undergo a multistep reaction sequence, ultimately forming a base-soluble carboxyHc acid which does not inhibit film dissolution. Immersion of a pattemwise-exposed film of the resist in an aqueous solution of hydroxide ion leads to rapid dissolution of the exposed areas and only very slow dissolution of unexposed regions. In contrast with crosslinking resists, the film solubiHty is controUed by chemical and polarity differences rather than molecular size. 

Few mechanisms of liquid/liquid reactions have been established, although some related work such as on droplet sizes and power input has been done. Small contents of surface-ac tive and other impurities in reactants of commercial quality can distort a reac tor s predicted performance. Diffusivities in liquids are comparatively low, a factor of 10 less than in gases, so it is probable in most industrial examples that they are diffusion controllech One consequence is that L/L reactions may not be as temperature sensitive as ordinary chemical reactions, although the effec t of temperature rise on viscosity and droplet size can result in substantial rate increases. L/L reac tions will exhibit behavior of homogeneous reactions only when they are very slow, nonionic reactions being the most likely ones. On the whole, in the present state of the art, the design of L/L reactors must depend on scale-up from laboratoiy or pilot plant work. 

Chemical Reactivity - Reactivity with Water Very slow reaction, considered non-hazardous. Hydrochloric acid is formed Reactivity with Common Materials Corrodes metals because of acid formed Stability During Transport Stable Neutralizing Agents for Acids and Caustics Not pertinent Polymerization Not pertinent Inhibitor of Polymerization Not pertinent. 

For a chemical reaction such as combustion to proceed, mixing of the reactants on a molecular scale is necessary. However, molecular diffusion is a very slow process. Dilution of a 10-m diameter sphere of pure hydrocarbons, for instance, down to a flammable composition in its center by molecular diffusion alone takes more than a year. On the other hand, only a few seconds are required for a similar dilution by molecular diffusion of a 1-cm sphere. Thus, dilution by molecular diffusion is most effective on small-scale fluctuations in the composition. These fluctuations are continuously generated by turbulent convective motion. 

Although Mg is generally considered non-toxic, the inhalation of fumes of freshly sublimed Mg oxide may cause metal fume fever. There is no evidence that Mg produces, true systemic poisoning. Particles of metallic Mg or Mg alloy which perforate the skin of gain entry thru cuts and scratched rilay produce a severe local lesion characterized by the evolution of gas and acute inflammatory reaction, frequently with necrosis. The condition Has been called a chemical gas gangrene . Gaseous blebs may develop within 24 hrs of the injury. The lesion is very slow to heal (Ref 23)... 

Process (3) may be slow, hence under industrial conditions it may be broken off before equilibrium is established. This means that chemical reaction (3) is not completed. As a final product, a not very homogeneous nitrocotton is obtained. The higher the nitrating temperature, the more rapid the denitration process, the sooner the reaction balance is established, and hence the more uniform the product ... 

The situation is very different when the redox reaction is slow or coupled with a chemical reaction, hideed, it is these nonideal processes that are usually of greatest chemical interest and for which the diagnostic power of cyclic voltammetry is most useful. Such information is usually obtained by comparing the experimental... 

The trifluoromethyl-tin bond is, however, much less stable chemically (24). Reaction of, e.g., (CF3)2SnBr with an excess of the relatively covalent, methylating agent (CHaljCd results in the very slow substitution for one of the Sn-CFg bonds, but the reaction of CFaSnBr3 with an excess of the more powerful, more ionic reagent methyllithium results in the displacement of all of the ligands, and the formation of (CH3)4Sn as shown in Equations 16 and 17. 

It has been well known for a long time that some reactions taking place at a very slow rate may be markedly accelerated by the simultaneous occurrence of another reaction of measurable velocity. On the suggestion of Kessler this phenomenon is called chemical induction and it is said that the reaction of measurable velocity induces the other slow reaction. 

According to the authors mentioned above, induced chain reactions (Livingston ) or induced catalysis (Bray and Ramsey ) take place when the very slow reaction between the acceptor and actor is catalyzed by the inductor. However, since the chemical characters of the acceptor and the inductor are the same, the actor reacts with the inductor, too, thus a part of it will be excluded from the catalysis. The principal characteristics of reactions of this type according to Luther and Schilow are that the value of Fj largely exceeds 2 and that the plot of Fj rm 5([Ac]/[I])o rises exponentially. 

The spatio-temporal variations of the concentration field in turbulent mixing processes are associated wdth very different conditions for chemical reactions in different parts of a reactor. This scenario usually has a detrimental effect on the selectivity of reactions when the reaction time-scale is small compared with the mixing time-scale. Under the same conditions (slow mixing), the process times are increased considerably. Due to mass transfer inhibitions, the true kinetics of a reaction does not show up instead, the mixing determines the time-scale of a process. This effect is known as mixing masking of reactions [126]. 

The situation is different when the chemical reaction is not very fast. In this case the equilibrium between substances Red and A in the solution layers near the electrode will be disturbed, and the rate at which reactant Red is replenished on account of reaction (13.37) decreases. When the chemical reaction is very slow, the limiting CD will approach the value... 

The last reaction cited above as shown is very effectively catalyzed by bacterial action but is very slow chemically by recycling the spent ferrous liquors and regenerating ferric iron bacterially, the amount of iron which must be derived from pyrite oxidation is limited to that needed to make up losses from the system, principally in the uranium product stream. This is important if the slow step in the overall process is the oxidation of pyrite. The situation is different in the case of bacterial leaching of copper sulfides where all the sulfide must be attacked to obtain copper with a high efficiency. A fourth reaction which may occur is the hydrolysis of ferric sulfate in solution, thus regenerating more sulfuric acid the ferrous-ferric oxidation consumes acid. 

In extreme cases where all electron transfer steps are reversible and the water trapping reactions are very slow, the charge is distributed over the guanines according to the thermodynamic stabilization. From these experiments one cannot deduce the influence of the sequence on the hole transfer rate. Therefore, using a chemical assay of this type leads to results that have to be discussed with great care. 

Some chemical reactions proceed very slowly, others with explosive speed, and still others somewhere in between. The dissolving of underground limestone deposits by water containing carbon dioxide to form caverns is an example of a slow reaction it can take centuries. The explosion of TNT is an example of a very rapid reaction. 

For situations where the reaction is very slow relative to diffusion, the effectiveness factor for the poisoned catalyst will be unity, and the apparent activation energy of the reaction will be the true activation energy for the intrinsic chemical reaction. As the temperature increases, however, the reaction rate increases much faster than the diffusion rate and one may enter a regime where hT( 1 — a) is larger than 2, so the apparent activation energy will drop to that given by equation 12.3.85 (approximately half the value for the intrinsic reaction). As the temperature increases further, the Thiele modulus [hT( 1 — a)] continues to increase with a concomitant decrease in the effectiveness with which the catalyst surface area is used and in the depth to which the reactants are capable of... 

The polymerization of styrene by Zr (benzyl) 4 has the characteristics of producing high molecular weight polymers by a very slow polymerization process. This can be reasonably explained by a slow initiation process followed by a fast propagation reaction. These two processes are probably chemically similar but differ significantly in rate for structural reasons. 

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