Van’t Hoff intermediates

This rate expression for the Arrhenius intermediate is characteristic of surficial and enzymatic catalysis with single substrates. On the other hand, the steady-state treatment of the van t Hoff intermediate for the condition of fc2[A] > /c [B] yields a rate expression of the following form ... 

If we now consider the formation of a van t Hoff intermediate (the steady-state caseX the rate expression for the single catalyst-substrate complex is... 

When a catalyst affects the formation of a van t Hoff intermediate but not the follow reaction, it increases the rate of the reaction by... 

With decrease in pressure, the chance that an activated molecule will lose its energy decreases more rapidly than the chance of the energy becoming so distributed as to allow a reaction to occur. Consequently, at low pressures the rates of unimolecular reactions cannot remain independent of the pressure. The activated molecule tends to become a Van t Hoff intermediate, and the rate to depend on the collision frequency. This corollary of Lindemann s theory has been... 

Figure 3.8. Potential energy diagrams for Arrhenius (left) and van t Hoff intermediates (right).

For the case of case of van t Hoff intermediates the concentration of the intermediate SH+ is considered to be negligible in comparison with BH+, therefore a steady state treatment of mechanism (5.6) requires that the rate of steps 1 and 2 are equal... 

If fc2[W] fc i[Y] in mechanism (13.II-13.Ill), then [X] is small and the steady-state approximation can be applied to this mechanism. Laidler called X under these conditions a van t Hoff intermediate. The energy profiles corresponding to Arrhenius and van t Hoff intermediates are illustrated in Figure 13.2. 

Figure 13.2 Potential-energy profiles for catalysed reactions, (a) The rate-determining step is the second step, occurring after the formation of an Arrhenius intermediate, (b) The rate-determining step is the first step, which leads to a van t Hoff intermediate.

The mechanism of each of the four possible combinations may involve an Arrhenius or a van t Hoff intermediate. This leads to eight possible mechanisms, schematically presented in Table 13.2. Each of these mechanisms can be developed using either the preequilibrium or the steady-state approximation to arrive at the corresponding rate law. The lessons that can be learnt from the treatment of these mechanisms are also indicated in Table 13.2 some mechanisms lead exclusively to specific acid or base catalysis, while others lead to general acid or base catalysis. Furthermore, the specific catalysis is associated with the existence of a limiting rate, that is the rate that will not increase indefinitely with the H+, or OH , concentration, but attain a limiting valne eqnal to 2[S]o-... 

Acid catalysis Arrhenius intermediates van t Hoff intermediates... 

F. V. Kekule interpreted benzene to be a cyclic entity in 1865. The concept of carbon as a tetrahedraUy, four-fold coordinated atom was presented independently by J. H. van t Hoff and J. A. Le Bel in 1874 and revolutionized the interpretation of the element s chemical activity (Figure 1.1). Since then, fundamental discoveries on this ubiquitous element multiplied. L. Mond and co-workers published the first metal carbonyls in 1890, and in 1891, E. G. Acheson for the first time achieved artificial graphite via intermediate silicon carbide (carborundum), which itself had been unknown then, too. 

Previously, for 2-methyl pentane cracking we have used Eley-Rideal kinetic rate expressions to describe the inhibition and poisoning effects of species in the feed, as well as intermediate and product species. In order to utilise such kinetic expressions values for the adsorption equilibrium constants are required. A method for estimating adsorption equilibrium constants has been proposed that uses an integrated form of van t Hoff equation. The heats of adsorption have been calculated using proton affinities and heats of condensation. The entropy of adsorption has been calculated using the Sackur-Tetrode expression. 

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