Activated complex theory, dissolution rate

However, we have to reflect on one of our model assumptions (Table 5.1). It is certainly not justified to assume a completely uniform oxide surface. The dissolution is favored at a few localized (active) sites where the reactions have lower activation energy. The overall reaction rate is the sum of the rates of the various types of sites. The reactions occurring at differently active sites are parallel reaction steps occurring at different rates (Table 5.1). In parallel reactions the fast reaction is rate determining. We can assume that the ratio (mol fraction, %a) of active sites to total (active plus less active) sites remains constant during the dissolution that is the active sites are continuously regenerated after AI(III) detachment and thus steady state conditions are maintained, i.e., a mean field rate law can generalize the dissolution rate. The reaction constant k in Eq. (5.9) includes %a, which is a function of the particular material used (see remark 4 in Table 5.1). In the activated complex theory the surface complex is the precursor of the activated complex (Fig. 5.4) and is in local equilibrium with it. The detachment corresponds to the desorption of the activated surface complex. 

Combining concepts of surface coordination chemistry with established models of lattice statistics and activated complex theory, Wieland et al. (8) proposed a general rate expression for the proton-catalyzed dissolution of oxide minerals ... 

For the explanation of hydrolytic dissolution processes currently is widely used activated-complex theory, which is also called transition state theory or absolute reaction rate theory. According to this theory, at hydration and protonation on the surface of the mineral form functional groups X-OH, X-OH and X-0 , which have acid-alkali properties dependent on pH of the solution. However, not the entire specific surface of the mineral participates in dissolution reactions but only its effective portion, which is taken by the... 

Under the activated complex theory, chemical interaction of most minerals with a solution has mostly acid-alkali nature and depends on relative concentrations of complexes X-OH, X-OH and X-O" on their surface. That is why ions H " (HjO" ) and OH" are most active components in water composition, which serves as catalyst or inhibitor. Their relative role is defined by the pH value of the solution. As a rule, this correlation rate of dissolution vs. pH has a trough-like shape (Figure 2.33). At the same temperature, the slowest dissolution rate is usually observed in neutral water (pH = 7 2). As pH decreases or increases, the dissolution rate increases... 

Identification of the surface species taking part in anodic dissolution can be tentatively dealt with in the framework of the absolute reaction rate and activated complex theory [18]. A description of the activated state in metal dissolution is central to the imderstanding of corrosion and passivation. However, the identification of this activated state is difficult. For active metal dissolution the ionization is a very fast process (characteristic time estimated to be less than 10 ps). Following the chemical relaxation technique introduced by Eigen [19,20] for investigating fast homogeneous reactions, so-called scr e potential measiu ements were applied to the determination of the initial potential and of its relaxation time on fresh surfaces exposed to aqueous solution [21]. 

Identification of the surface species taking part in anodic dissolution can be tentatively dealt with in the framework of the absolute reaction rate and activated complex theory [18]. 

Transition-state theory may be useful in testing the dissolution mechanisms presented above. According to TST, for any elementary chemical reaction the reactants should pass through a free-energy maximum, labeled the activated complex , before they are converted to products. It is assumed that the reaction rate-determining step is related to the decomposition of this activated complex ... 

Recent advances in applying transition state theory to geochemical kinetics (SQ, SD have emphasized the interaction of the activated complex with specific surface reaction sites. The rate of reaction is assumed to be a function of the surface reaction site density. A correspondence is also observed between surface dissolution features such as etch pits, and crystallographically controlled extended defect features such as edge and screw dislocations (S2). Based on these lines of evidence, the reactive surface area has been proposed to be proportional to the defect density within minerals... 

There are various concepts about the aluminum silicates dissolution mechanism. Relatively recently a low rate of their dissolution was explained by inner diffuse regime. Currently more substantiated appears hydrolysis with the formation of activated complexes. According to this theory, the dissolution begins with the exchange of alkaline, alkaline-earth and other metals on the mineral surface of H+ ions from the solution (see Figure 2.26). At that, metals in any conditions are removed in certain sequence. In case of the presence of iron and other metals with variable oxidation degree the process may be accompanied with redox reaction. Hydrolysis is a critical reaction in the dissolution of aluminum silicates. It results in the formation on the surface of a very thin layer of activated complexes in Na, K, Ca, Mg, Al and enriched with H+, H O or H O. The composition and thickness of this weakened layer depend on the solution pH. These activated complexes at disruption of weakened bonds with mineral are torn away and pass into solution. For some minerals (quartz, olivine, etc.) the disruption of one inner bond is sufficient, for some others, two and more. The very formation of activated complexes is reversible but their destruction and removal from the mineral are irreversible. 

The theories proposed to explain the formation of passivation film are salt-film mechanism and acceptor mechanism [21]. In the salt-film mechanism, the assumption is that during the active dissolution regime, the concentration of metal ions (in this case, copper) in solution exceeds the solubility limit and this results in the precipitation of a salt film on the surface of copper. The formation of the salt film drives the reaction forward, where copper ions diffuse through the salt film into electrolyte solution and the removal rate is determined by the transport rate of ions away from the surface. As the salt-film thickness increases, the removal rate decreases. In the acceptor mechanism, it is assumed that the metal-ion products remain adsorbed onto the electrode surface until they are complexed by an acceptor species like water or anions. The rate-limiting step is therefore the mass transfer of the acceptor to the surface. Recent studies confirmed that water may act as an acceptor species for dissolving copper ions [22]. 

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