Electrode potentials chemical reaction rate

Reaction rate imaging is unique to SECM and clearly illustrates its chemical imaging capability. By proper choice of solution components to control the tip reaction and the electrochemistry at the substrate/solution interface by varying the electrode potential, differential reaction rates at various surfaces can be probed. For example, the location of enzyme sites in a membrane or organelle, where a particular reaction is catalyzed, can be... 

When ions migrate through a solid electrolyte, they diffuse from this onto the gas-exposed surface of the metal electrode. These ions form a double layer (and hence a potential difference) at the metal/gas interface. I Iowcver, this potential difference (which varies with the electrode potential) in turn changes the work function at the gas/metal interface. The ease of availability of electrons in the bonding of radicals adsorbed from the gas phase onto the electrode increases as the electronic work function of the solid decreases. The chemical reaction rate of the catalyzed reaction depends on the bonding strength of these radicals to the electrode catalyst, which involves electrons from the metal and is therefore dependent on the work function of the metal this itself is a function of the electrode potential. In this way, a dependence of the rate of the chemical reaction upon the potential of the working electrode can be rationalized. 

In a direct electrolysis, the electron is exchanged between the electrode and the substrate, and the rate of the reaction depends on the electrode potential and the rate constant of the heterogeneous electron-transfer reaction. In an indirect electrolysis, the electron is primarily exchanged with a substance (a mediator) that exchanges the electron with the substrate in a chemical reaction, and the rate does not depend on the ability of the substrate to exchange an electron with the electrode. 

The overpotential 77 is required to overcome hindrance of the overall electrode reaction, which is usually composed of the sequence of partial reactions. There are four possible partial reactions and thus four types of rate control charge transfer, diffusion, chemical reaction, and crystallization. Charge-transfer reaction involves transfer of charge carriers, ions or electrons, across the double layer. This transfer occurs between the electrode and an ion, or molecule. The charge-transfer reaction is the only partial reaction directly affected by the electrode potential. Thus, the rate of charge-transfer reaction is determined by the electrode potential. 

By the beginning of the 20th century an independent field of physical chemistry, namely chemical kinetics, had been developed. Temkin treats chemical kinetics as a science dealing with chemical reaction rates and specifies the reaction kinetics as "the dependence of the rate of a given reaction on the substance concentration, temperature and some other parameters, e.g. the electrode potential in electrochemical reactions . Semenov interprets chemical kinetics as a science "not only about the rates but also about the mechanism of chemical reactions [5, p. 9]. 

The analysis is concerned with a one-dimensional model of electrodes in which reaction rates are distributed unevenly due to diffusion as well as a variation in electrode potential.4,5 The treatment of the problem of a simultaneous variation in electrolyte concentration and potential distribution in the electrode is treated in an analogous manner to that of non-isothermal chemical reactions in porous catalysts.16 The results show that several dimensionless groups or numbers control the electrode behavior. Figure 8 shows a back fed porous anode used in the model. 

The diffusion and chemical reaction rates depend only on the concentration of gaseous reactants at the working electrode surface, and by definition, are independent of electrode potential. When concentration overpotentiai dominates the total overpotentiai at the working electrode, a limiting current exists. This limiting current is the maximum current obtained when the electrochemical reaction is completely mass-transfer controlled. ... 

A homogeneous chemical reaction occurring in the gap between the tip and substrate electrodes causes a change in iT, therefore its rate can be determined from SECM measurements. If both heterogeneous processes at the tip and substrate electrodes are rapid (at extreme potentials of both working electrodes) and the chemical reaction (rate constant, kc) is irreversible, the SECM response is a function of a single kinetic parameter K = const X kc/D, and its value can be extracted from IT vs. L dependencies. 

Rate of reactions at the electrode surfaces depends on mass transfer, which mainly influences the current 1. The simplest electrode reactions are those in which the rates of all associated chemical reactions are very rapid compared to those of the mass transfer processes. If an electrode process involves only fast heterogeneous charge transfer and mobile, reversible homogeneous reactions, it implies that (1) the homogeneous reactions are at equilibrium and (2) the surface concentrations of species involved in the faradaic process are related to the electrode potential. The net rate of the electrode reaction, Vrxn, is then governed totally by the rate at which the electroactive species is brought to the surface by mass transfer, v f The reaction rate can be expressed as ... 

Fig. 9. Redox mode detection mechanism for physostigmine. Electrode potentials versus Pd ki and k2 represent chemical reaction rates. Detection system from Hurst and Whelpton (1989), structures from Isaksson and Kissinger (1987)...

In all three reaction equations above, k 10 and k 10 represent the chemical reaction rate constants in the forward and backward directions, respectively, which are independent of the electrode potential. For electrochemical reactions (4-XII) and (4-XIII), the... 

This is our first encounter with the use of simulation to analyze CV results. Through the theory of simulation (Chapters 4-6), a cyclic voltammetric or potential step response can be calculated for any electrochemical mechanism, given the parameters that describe the experiment (scan rate, scan range, electrode area) and the mechanism (reduction potentials, electrode kinetics, chemical reaction kinetics, and diffusion coefficients of all chemical species). The unknown parameters of the electrochemical mechanism can be varied until a simulation is obtained that closely resembles the experimental result. 

As stated in Sect. 7.2.1 of this chapter, for electrodes of the first kind, the metal ions should not react with the ILs. Basile et al. have noted that the presence of water results in the formation of silver nanoparticles for solutions of Ag" ions dissolved into [Cqmpyr] [TFSI] [31]. These authors postulate that this occurs via chelation of Ag" by the [TFSI] anion and a subsequent disproportionation reaction in the presence of water to form a Ag -[TFSI] complex and Ag° [31]. Thus, the choice of a reference electrode based on AglAgOTfl[C4mpyr][TFSI] may be problematic. The authors did note that over a short period of time, when chemical reaction rates were low, the FcIFc" reversible potential was close to that reported by Snook et al. [27, 31]. According to Snook et al. [27] the AglAgOTfl[C4mpyr][TFSI] reference electrode is only stable for 3 weeks before the solution needs to be replaced and the electrode remade. Therefore, if the AglAg" couple is to be used in other ILs, then any potential chemical reactions of Ag irais within this IL should be explored prior to use, as well as the stability of the reference electrode over time. 

To understand the danger in proposing which reactions takes place to provide the required ES current on the basis of the standard electrode potentials alone, one must consider that several other factors influence the actual rate of a specific reaction. Assuming that mass transport is not the limiting factor, other processes such as the electron transfer rate at the electrode surface, chemical reactions preceding the electron transfer, or chemical... 

The first work on measuring the rate constant of the protolytic reaction by the polarographic method was published in 1947 (R. Brdiehka). Later several other electrochemical methods were developed for measuring rates of fest ion reactions. For the electrochemical determination of the reaction rate constant, it is necessary for the chemical equilibrium to exist in the system and for at least one of the reactants to participate in the electrode process. The reaction rate of electron transfer on the electrode increases exponentially with an increase in its potential E when E > Eg, where Egq is the equilibrium oxidation or reduction potential of the reactant on the electrode. The current strength is... 

At low currents, the rate of change of die electrode potential with current is associated with the limiting rate of electron transfer across the phase boundary between the electronically conducting electrode and the ionically conducting solution, and is temied the electron transfer overpotential. The electron transfer rate at a given overpotential has been found to depend on the nature of the species participating in the reaction, and the properties of the electrolyte and the electrode itself (such as, for example, the chemical nature of the metal). 

Activation Processes. To be useful ia battery appHcations reactions must occur at a reasonable rate. The rate or abiUty of battery electrodes to produce current is determiaed by the kinetic processes of electrode operations, not by thermodynamics, which describes the characteristics of reactions at equihbrium when the forward and reverse reaction rates are equal. Electrochemical reaction kinetics (31—35) foUow the same general considerations as those of bulk chemical reactions. Two differences are a potential drop that exists between the electrode and the solution because of the electrical double layer at the electrode iaterface and the reaction that occurs at iaterfaces that are two-dimensional rather than ia the three-dimensional bulk. 

The electrode current depends on the rates of the coupled reactions, but by suitable adjustment of the electrode potential (into the diffusion current region for the electrode reaction) the rate of the reduction reaction can be made so fast that the current depends only on the rate of the prior chemical reaction. The dependence of the observed current on the presence of the chemical reaction is a measure of the rate. 

Additional information on the rates of these (and other) coupled chemical reactions can be achieved by changing the scan rate (i.e., adjusting the experimental time scale). In particular, the scan rate controls the tune spent between the switching potential and the peak potential (during which the chemical reaction occurs). Hence, as illustrated in Figure 2-6, i is the ratio of the rate constant (of the chemical step) to die scan rate, which controls the peak ratio. Most useful information is obtained when the reaction time lies within the experimental tune scale. For scan rates between 0.02 and 200 V s-1 (common with conventional electrodes), the accessible... 

The principles outlined above are, of course, important in electro-synthetic reactions. The pH of the electrolysis medium, however, also affects the occurrence and rate of proton transfers which follow the primary electron transfer and hence determine the stability of electrode intermediates to chemical reactions of further oxidation or reduction. These factors are well illustrated by the reduction at a mercury cathode of aryl alkyl ketones (Zuman et al., 1968). In acidic solution the ketone is protonated and reduces readily to a radical which may be reduced further only at more negative potentials. 

Electrochemical reactions differ fundamentally from chemical reactions in that the kinetic parameters are not constant (i.e., they are not rate constants ) but depend on the electrode potential. In the typical case this dependence is described by Eq. (6.33). This dependence has an important consequence At given arbitrary values of the concentrations d c, an equilibrium potential Eq exists in the case of electrochemical reactions which is the potential at which substances A and D are in equilibrium with each other. At this point (Eq) the intermediate B is in common equilibrium with substances A and D. For this equilibrium concentration we obtain from Eqs. (13.9) and (13.11),... 

This last equation contains the two essential activation terms met in electrocatalysis an exponential function of the electrode potential E and an exponential function of the chemical activation energy AGj (defined as the activation energy at the standard equilibrium potential). By modifying the nature and structure of the electrode material (the catalyst), one may decrease AGq, thus increasing jo, as a result of the catalytic properties of the electrode. This leads to an increase in the reaction rate j. 

The basic relationships of electrochemical kinetics are identical with those of chemical kinetics. Electrochemical kinetics involves an additional parameter, the electrode potential, on which the rate of the electrode reaction depends. The rate of the electrode process is proportional to the current density at the studied electrode. As it is assumed that electrode reactions are, in general, reversible, i.e. that both the anodic and the opposite cathodic processes occur simultaneously at a given electrode, the current density depends on the rate of the oxidation (anodic) process, ua, and of the reduction (cathodic) process, vc, according to the relationship... 

Consider the relatively simple case where the chemical reactions of formation and dissociation of the complexes are sufficiently fast so that equilibrium among the complexes, free ions and complexing agent is maintained everywhere in the electrolyte during the electrolysis. The rate of deposition of the metal is then determined by the electrode reaction of the complex MX/, for which the product kc,MXicMX is, at the given potential, the largest for all the complexes. 

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