ELECTROCHEMISTRY CHAPTER PDF
Electrochemistry is the study of production of electricity from energy released during spontaneous chemical reactions and the use of electrical energy to bring. chapter, we describe electrochemical reactions in more depth and explore some of their applications. In the first three sections, we review redox reactions;. Chapter Electrochemistry. Key topics: Galvanic cells. Nernst equation. Batteries; electrolysis. Balancing Redox Reactions. A redox reaction involves a.
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CLASS XII: CHEMISTRY. CHAPTER 3: ELECTROCHEMISTRY. COMMON MISTAKS BY THE STUDENTS IN EXAMINATION. • REPRESENTATION OF. Electrochemistry a Chem1 Supplement Text. Stephen K. Lower. Simon Fraser University. Contents. 1 Chemistry and electricity. 2. Electroneutrality. Chapter 1. Introduction of Electrochemical. Concepts. • Electrochemistry – concerned with the interrelation of electrical and chemical effects. Reactions involving.
The effect of differences in structure and electronic distribution of different metals are indicated. The space-charge region in semiconductors is then discussed.
Finally some properties of colloids are mentioned, given that they possess an interfacial region very similar to an electrode. The designation 'double layer' reflects the first models developed to describe the region, see Section 3. The basic concept was of an ordering of positive or negative charges at the electrode surface and ordering of the opposite charge and in equal quantity in solution to neutralize the electrode charge. The function of the electrode was only to supply electrons to, or remove electrons from, the interface: More sophisticated models required accurate experimental observations.
The proportionality constant between the applied potential and the charge due to the species ordering in the solution interfacial region is the double layer capacity. The study of the double layer capacity at different applied potentials can be done by various methods. One much used is the impedance technique, which is applicable to any type of electrode, solid or liquid, and is described in Chapter Another method uses electrocapillary measurements.
It was developed for the mercury elec- trode, being only applicable to liquid electrodes, and is based on measurement of surface tension.
The principle of electrocapillary measurements was described more than a century ago by Lippmann9. It is a null-point technique that counterbalances the force of gravity and surface tension, and highly accurate results can be obtained. The experimental system is shown in Fig.
The contact angle is measured with a microscope.
E is called an electrocapillary curve and has the form of Fig. A variation on this method consists in using the dropping mercury electrode10 Section 8. The mass flux, rab is 3. Substituting in 3. E gives a curve of the same form as the electrocapillary curve Fig. The first derivative gives the charge on the interface, and is the Lippmann equation 3. Curve b is obtained by differentiating curve a , and c by differentiation of b , Ez is the point of zero charge.
The potential where this occurs is called the point of zero charge, Ez, and occurs at the maximum in the electroca- piUary curve, see Fig. A second differentiation of the electrocapiUary curve gives the value of the interfacial capacity. There are, however, two definitions of this: This is the derivative of the curve of aM vs. E Fig. From Ref.
Measuring a M for two reasonably different potentials, the value of the calculated capacity is the average value in that zone, assuming that C d varies with E. Double layer models have to explain the shape of these curves. The impedance technique gives values of C d directly.
It consists of the application of a small sinuisoidal perturbation superimposed on a fixed applied potential. The component of the resulting current that is out of phase with the applied signal leads to calculation of the differential capacity of the interface. More details are given in Chapter Besides making the use of solid, and not only liquid, electrodes possible, another advantage is that integration tends to reduce the errors in the experimental measure- ments, whereas differentiation increases them.
Until the s measurements were made almost exclusively at mercury electrodes and models were developed for this electrode.
The fact that mercury is an ideally polarizable liquid in the zone negative to the hydrogen electrode means that its behaviour is often different from solid electrodes mono- crystalline and polycrystalline. These models are, therefore, of a predominantly electrostatic nature. Nevertheless, an important application of electrostatic models is to the interface between two immiscible electrolyte solutions.
This can be viewed as two electrolyte double layers arranged back to back.
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In reality, however, total immiscibility never occurs and the degree of miscibility increases with the presence of electrolyte, so that corrections to the models need to be introduced. It was only after making measurements with solid electrodes that the concept of the energy associated with the electrode's electronic distribu- tion in the interfacial region was introduced.
This distribution depends on the electrode material as well as on its crystalline structure and exposed crystallographic face. However, it is interesting to see the historical evolution of the models, given that successively more factors that reflect the structure have been introduced.
Helmholtz, Gouy -Chapman, Stern and Grahame Helmholtz Model The first double layer model, due to Helmholtz12, considered the ordering of positive and negative charges in a rigid fashion on the two sides of the interface, giving rise to the designation of double layer or compact layer , the interactions not stretching any further into solution.
This model of the interface is comparable to the classic problem of a parallel-plate capacitor. The other, formed by the ions of opposite charge from solution rigidly linked to the electrode, would pass through the centres of these ions Fig. So xu would be the distance of closest approach of the charges, i. By analogy with a capacitor the capacity would be d,H ' 3. The two principal defects of this model are first that it neglects interactions that occur further from the electrode than the first layer of adsorbed species, and secondly that it does not take into account any dependence on electrolyte concentration.
Gouy-Chapman Model At the beginning of this century Gouy13 and Chapman13 independently developed a double layer model in which they considered that the applied potential and electrolyte concentration both influenced the value of the double layer capacity.
Thus, the double layer would not be compact as in Helmholtz's description but of variable thickness, the ions being free to move Fig. This is called the diffuse double layer. The Poisson equation relates the potential with the charge distribution Combining 3. For an electrode, which is much larger an electrode can be thought of as a giant ion the linear approximation is not valid.
In solving 3. The decay of potential is shown in Fig. The value of C d is easily obtained from 3. The charge density of the diffuse layer is 3. The minimum in the curve is identifiable with the point of zero charge, Ez, and the curve is symmetric around Ez. We remember the approximation that ions are considered as point charges and that, consequently, there is no maximum concentration of ions close to the electrode surface!
The physical explanation of the experimental measurements is that, far from Ez, the electrode exerts a strong attraction towards the ions that are therefore attached rigidly to the surface, all the potential drop being restricted to within the distance corresponding to the first layer of ions compact layer.
Close to Ez there is a diffuse distribution of ions diffuse layer. In mathematical terms this is equivalent to two capacitors in series, with capacities CH representing the rigid compact layer and CGC representing the diffuse layer. The smaller of the two capacities deter- mines the observed behaviour: There are two extreme cases: As in the Gouy-Chapman model, the more concentrated the elec- trolyte the less the importance of the thickness of the diffuse layer and the more rapid the potential drop.
At distance xH there is the transition from the compact to the diffuse layer. The separation plane between the two zones is called the outer Helmholtz plane OHP: Comparison between Figs 3.
Indeed, as already mentioned, mercury, as a liquid, is a special case. Results with other electrolytes and with solid electrodes show a more complicated behaviour.
Grahame Model In spite of the fact that Stern had already distinguished between ions adsorbed on the electrode surface and those in the diffuse layer, it was 11 Grahame who developed a model that is constituted by three regions Fig.
The difference between this and the Stern model is the existence of specific adsorption Section 3. The inner Helmholtz plane IHP passes through the centres of these ions. The outer Helmholtz plane OHP passes through the centres of the solvated and non-specifically adsorbed ions. The diffuse region is outside the OHP. In both the Stern and Grahame models, the potential varies linearly with distance until the OHP and then exponentially in the diffuse layer.
Bockris, Devanathan, and Muller Model More recent models of the double layer have taken into account the physical nature of the interfacial region. In dipolar solvents, such as water, it is clear that an interaction between the electrode and the dipoles must exist. That this is important is reinforced by the fact that solvent concentration is always much higher than solute concentration.
For example, pure water has a concentration of The Bockris, Devanathan, and Miiller model16 recognizes this situation and shows the predominance of solvent molecules near the interface Fig. The solvent dipoles are oriented according to the electrode charge where they form a layer together with the specifically adsorbed ions. Regarding the electrode as a giant ion, the solvent molecules form its first solvation layer; the IHP is the plane that passes through the centre of these dipoles and specifically adsorbed ions.
In a similar fashion, OHP refers to adsorption of solvated ions that could be identified with a second solvation layer. Outside this comes the diffuse layer. Note that the actual profile of electrostatic potential variation with distance Fig. These authors also defined a shear plane, not necessarily coincident with the outer Helmhoitz plane, which is extremely important in electrokinetic effects Section 3. The shear plane limits the zone where the rigid holding of ions owing to the electrode charge ceases to operate.
The potential of this plane is called the zeta or electrokinetic potential, f. The models presented above give emphasis to electros- tatic considerations. NHE Fig. For example, there is a difference between sp metals and transition metals Fig.
Since the first model of this kind proposed by Damaskin and Frumkin17, and based on these principles, there has been a gradual evolution in the models, reviewed by Trasatti18 and more recently by Parsons The break in the structure of the solid causes a potential difference that begins within the solid—the surface potential Fig.
The interfacial region of a metal up to the IHP has been considered as an electronic molecular capacitor, and this model has explained many experimental results with success Another important model is the jellium model21 Fig. From an experimental point of view, the development of in situ infrared and Raman spectroscopic techniques Chapter 12 to observe the structure, and the calculation of the bond strength at the electrode surface can better elucidate the organization of the double layer.
Some of these ideas are developed in Sections 3. These ions can have the same charge or the opposite charge to the electrode. Bonds formed with the electrode in this way are stronger than for solvated ions. The idea of the existence of specific adsorption appeared as an explanation for the fact that electrocapillary curves at mercury electrodes are different for different electrolytes at the same concentration Fig.
For sodium and potassium halides in water the differences arise at potentials positive of Ez, which suggests an interaction with the anions. As the effect is larger the smaller the ionic radius of the anion, the idea of specific adsorption with partial or total loss of hydration arose.
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The degree of specific adsorption should vary with electrolyte con- centration, just as there should be a change in the point of zero charge due to specific adsorption of charges. For anion adsorption, and constant charge density, the point of zero charge moves in the negative direction in order to counterbalance adsorption.
For cations, Ez moves in the positive direction, assuming constant charge density. Experimentally it is observed that specific adsorption occurs more with anions than with cations. This is in agreement with chemical models of the interfacial region. Since, according to the free electron model, a metallic lattice can be considered as a cation lattice in a sea of electrons in free movement, it is logical to expect a greater attraction for anions in solution. The degree of adsorption depends on electrolyte concentration.
The degree of coverage of a surface by specific adsorption of ions can be described by monolayer adsorption isotherms Fig. Three types of isotherm are generally considered: It is assumed that there is no interaction between adsorbed species, that the surface is smooth, and that eventually surface saturation occurs.
This considers interactions in a different way: A positive value of g implies attractive interaction and negative g repulsive interaction. Additionally, comparison of 3.
When electroactive species are adsorbed, reagents or products of electrode reactions or both, a significant change in voltam- metric response can occur. Adsorption of non-electroactive species can inhibit the electrode reaction. These processes depend on electrode material as well as on solution composition.
A solid electrode has a well-defined structure, probably polycrystalline and in some cases monocrystalline. In a solid metallic electrode conduction is predominantly electronic owing to the free movement of valence elec- trons, the energy of the electrons that traverse the interface being that of the Fermi level, EF Section 3. The Fermi energy is the electrochemical potential of the electrons in the metal electrode.
By substitution in 3. By convention, for a metal, it is said that only electrons with energies within kBT of EF can be transferred. In the case of semiconductors 3. The interfacial structure of a solid electrode depends on various factors. The interatomic distance varies with the exposed crystallographic face and with the interaction energy; between the crystallites in a polycrystalline material there are breaks in the structure and one- dimensional and two-dimensional defects, such as screw dislocations, etc.
Adsorption of species can be facilitated or made more difficult, and at the macroscopic level we observe the average behaviour. Recently the structure of the double layer associated with the interface of gold and platinum monocrystals with solution has been investigated A clear difference between crystallographic faces is noted, manifested in the values of differential capacity and in evidence of adsorption in voltammograms.
Cyclic voltammograms suggest that there is a reor- ganization on the metal surface to give the equivalent of a surface layer of low Miller index, the identity of this face depending on the applied potential These studies are still in their early stages and concrete conclusions concerning a possible restructuring cannot yet be stated. Electrode a Electrode b Fig. The effects of the crystallographic face and the difference between metals are evidence of the incorrectness of the classical representations of the interface with all the potential decay within the solution Fig.
In fact a discontinuity is physically improbable and experimental evidence mentioned above confirms that it is incorrect, the schematic repre- sentation of Fig. This corresponds to the 'chemical' models Section 3.
As is well known, in a semiconductor there is a separation between the occupied valence band and the unoccupied conduction band.
By convention, if the separation is greater than 3 eV the solid is called an insulator for example diamond 5. The valence band is totally filled and the conduction band empty. Conduction occurs via promotion of electrons from Ev to Ec, the conductivity increasing with increase in temperature, a Definition of energy levels; b Variation of density of available states with energy.
For this reason it is useful to speak not only of electron movement but also of hole movement. Conduction occurs by movement of electrons in the conduc- tion band or of holes lack of electrons in the valence band. In an intrinsic semiconductor electron promotion to the conduction band occurs through thermal or photon excitation.
The Fermi energy is in the middle of the bandgap, i. Other electronic levels surface states can exist on the semiconductor surface due to adsorbed species or surface reorganization. These states can facilitate electron transfer between electrode and solution. If the semiconductor is an ionic solid, then electrical conduction can be electronic and ionic, the latter being due to the existence of defects within the crystal that can undergo movement, especially Frenkel defects an ion vacancy balanced by an interstitial ion of the same type and Schottky defects cation and anion vacancies with ion migration to the surface.
This will be discussed further in Chapter 13, as ionic crystals are the sensing components of an important class of ion selective electrodes. Bandgap energy, Eg, and cor- responding wavelength Abg important for photo-excitation of some semiconductors of electrochemical interest Owing to difficulties in electron movement in semiconductors, when a steady state has been achieved almost all the applied potential appears within the semiconductor, creating a region of potential variation close to the surface called the space-charge region Fig.
In fact intrinsic semiconductors, which are necessarily pure crystals, are Solution Space-change region Surface Fig. It is more common to use doped semiconductors, the doping normally being introduced externally. In an n-type semiconductor doping is obtained by introducing an atom into the lattice that has approximately the same size as the substrate atoms but with more electrons, e. The energy of these electrons is slightly less than Ec; electronic conduction can be by thermal excitation from the impurity band to the conduction band Fig.
By doping it is possible to change a solid that under normal conditions would be an insulator, owing to the larger bandgap, into a semiconductor. In a semiconductor electrode, almost all the potential variation in the interfacial region occurs in the space-charge region.
This is due to the fact that the values for the space-charge capacity, Csc, are from 0. The theory of the space-charge region was developed by Schottky26, Mott27, Davydov28, and more completely by Brittain and Garrett We now describe the effect of applied potential to an n-type semicon- ductor in its space-charge region. The mode of electron conduc- tion is indicated by the arrows.
According to the potential applied we create various types of region, as shown in Fig. At this point a difficulty in the nomenclature used in semiconductor electrochemistry should be noted: To attempt to avoid confusion, the symbol U is used within this area of electrochemistry for potential V.
The rest of the section follows this convention. The most important situations that we should stress are: An inversion layer is formed, so called because the n-type semiconductor is converted into a p-type semiconductor at the surface. Adsorbates can facilitate this process. To have passage of current it is necessary that EF is within the conduction or within the valence band in the space-charge region, i.
There is an analogy with the Schottky diode. Another important aspect refers to adsorbates. These have their own associated energy levels, known as surface states, and can aid electron transfer if there is superposition of the conduction band and that corresponding to the surface state Fig. A better understanding of the energy distributions of the solution species in Fig.
In the absence of the surface state, there would be no reaction. Due to the great extension of the space-charge region, almost all the potential drop occurs across it. The presence of adsorbates modifies C s c and is manifested in the non-linearity of the plots. Knowing the values of the bandgap energy, Eg Table 3.
Semiconductors are extremely important in photoelectrochemistry, where the energy necessary to jump from the valence to the conduction band is supplied by visible light. These aspects are developed in Chapter All combinations of gases, liquids, and solids are possible except for a gas dispersed in a gas. The solid particles are charged, which causes repulsion between the particles and gives temporal stability to the colloidal system.
Recently there has been increasing interest in colloids because of their possible use as electrodes for electrolysis, each particle acting as anode and cathode at the same time. Their particular advantage is the large surface area exposed to solution in relation to their solid volume. Therefore, the study of colloids can also lead to a better knowledge of the double layer region, especially for ionic solids and semiconductors.
A very useful type of phenomenon in the study of colloidal particles is the electrokinetic phenomenon that results from the movement of a solid phase with surface charge relative to an electrolyte-containing liquid phase. An applied electric field induces movement or, conversely, movement induces an electric field.
The phenomena can be divided into two types: These effects are nor- mally studied in fine capillaries in order to maximize the ratio of the solid surface area to the liquid volume. These four manifestations of the electrokinetic effect are summarized in Table 3. Table 3. The shear plane can therefore be associated roughly with the outer Helmholtz plane, an approximation often made.
This is, in general, not equal to the point of zero charge, as the value of the latter is affected by the presence of specifically adsorbed species Section 3. We now consider briefly the four effects that are described in Table 3. Electrophoresis In electrophoresis the solid moves in a liquid phase due to the application of an electric field.
The forces acting on the particles are similar to those that act on solvated ions: This depends on particle size and double layer thickness. All other situations lead to intermediate numerical factors.
Measurements of electrophoretic mobility, using 3. As can be inferred, electrophoretic mobility depends on solution ionic strength since double layer thickness decreases with increasing electrolyte concentration. It also depends on the surface charge of the particles. If this charge varies in colloidal particles of similar dimensions then electrophoresis provides a basis for their separation.
An example of this is in proteins, where the surface charge varies with pH in a different way according to the protein identity. Sedimentation potential Colloidal particles are affected by the force of gravity, either natural or through centrifugation.
Sedimentation of the particles often gives rise to an electric field. This occurs because the particles move, whilst leaving some of their ionic atmosphere behind. These potentials are usually difficult to measure, and are an unwanted side effect in ultracentrifuga- tion, where they are minimized by adding a large concentration of inert electrolyte.
Electroosmosis In electroosmosis, the stationary and mobile phases are exchanged in relation to electrophoresis. As measurement of the rate of movement of a liquid through a capillary is difficult, the force that it exerts is measured, i.
The volume flow of liquid, Vf, is veoAy where A is the cross-sectional area of the capillary. Limitations in the calculation of the zeta potential Quantitative measurements of electrokinetic phenomena permit the calculation of the zeta potential by use of the appropriate equations. However, in the deduction of the equations approximations are made: Corrections to compensate for these approximations have been introduced, as well as consideration of non-spherical particles and particles of dimensions comparable to the diffuse layer thickness.
This should be consulted in the specialized literature. Grahame, Ann. Parsons, Modern aspects of chemistry, Butterworths, London, Vol. Conway, pp. Parsons, Advances in Electrochemistry and Electrochemical Engineering, ed. Tobias, Wiley, New York, Vol.
Silva ed. Conway and J. O'M, Bockris, pp. Morrison, Electrochemistry at semiconductor and oxidised metal electrodes. Plenum, New York, Uosaki and H. White, J. Hamnett, in Comprehensive chemical kinetics, ed. Compton, Elsevier, Amsterdam, Vol. References 69 9. Lippmann, CompL Rend. Heyrovsky, Chem. Listy, , 16, Physik, , 89, ; , 7, Gouy, Compt. Chapman, Phil. Stern, Z. Bockris, M.
Devanathan, and K. Muller, Proc. Soc, , A, Damaskin and A. Frumkin, Electrochim, Acta, , 19, ; B. Damaskin, U. Palm, and M. Trasatti, in Ref. Parsons, Chem. Martynov and R. Salem, Electrical double layer at a metal-dilute electrolyte solution interface, Lecture Notes in Chemistry 33, Springer-Verlag, Berlin, Conway, R.
White, and J. Cervino, W.
Triaca, and A. Schottky, Zeit, fur Physik, , , Mott, Proc. Physics USSR, , 1, Garrett, Phys. Kitahara and A. Watanabe, Electrical phenomena at interfaces, Dekker, New York, Other more complex cases are also referred to. Comparison with electron transfer reactions in homogeneous solution are made. In a system involving reagents and products at equilibrium, the rates of the reactions in each direction are equal.
Equilibrium can thus be seen as a limiting case, and any kinetic model must give the correct equilibrium expression. For reactions at an electrode, half-reactionsy the equilibrium expression is the Nernst equation. Diffusion of the species to where the reaction occurs described by a mass transfer coefficient kd - see Chapter 5. Alterations in the distances between the central ion and the ligands HT 14 s. Electron transfer HT 1 6 s. Relaxation in the inverse sense.
Steps can be seen as a type of pre-equilibrium before the electron transfer. During the electron transfer itself all positions of the atoms are frozen, obeying the Franck-Condon principle adiabatic process. In the equations for energy changes a factor of 2 relative to electrode reactions appears, since whole reactions rather than half- reactions are being considered. Theoretical and experimental com- parisons between electrode reactions and redox reactions in solution have 3 been made with satisfactory results.
The reorientation and rearrangement causes the separation between the energy levels to be different in the activated complex than in the initial state. This level is the Fermi level, EF - electrons are always transferred to and from this level. The situation is shown schematically in Fig. What is, then, the energy profile describing electron transfer? In a similar fashion to the description of the kinetics of homogeneous reactions, in the development of a model for electron transfer parabolic energy profiles have been used for reagents and products.
Nevertheless, the region where the profiles intersect is of paramount interest since this corresponds to the activated complex: Figure 4. The potential applied to the electrode alters the highest occupied electronic energy level, EF, facilitating a reduction or b oxidation.
So for a reduction we can write 4. Values of aa and ac can vary between 0 and 1, but for metals are around 0. A value of 0. Substituting 4. On changing the potential applied to the electrode, we influence ka and kc in an exponential fashion. The electrode is thus a powerful catalyst. When all the species that reach it are oxidized or reduced the current cannot increase further. If there are no effects from migration, diffusion limits the transport of electroactive species close to the electrode; the maximum current is known as the diffusion-limited current Section 5.
Whatever the value of the standard rate constant, k0, if the applied potential is sufficiently positive oxidation or sufficiently negative reduction the maximum current will always be reached. As indicated, for metals the activation barrier Fig. These situations occur with semiconductor electrodes, since the externally applied voltage appears as a potential difference almost totally across the semiconductor space charge layer.
In many cases electrode processes involving the transfer of more than one electron take place in consecutive steps.
The symmetry of the activation barrier referred to above relates to the rate-determining step. Thus extreme care must be Reaction Reaction Reaction coordinate coordinate coordinate a b c Fig.
Finally, since the anodic and cathodic reactions do not occur at the same potential, the mechanism for oxidation may not be the opposite of reduction. This occurs when there is multiple step electron transfer, possibly with intermediate chemical steps.
Rewriting 4. Exactly the same result is obtained by following identical reasoning, using the anodic instead of the cathodic reaction in 4. Any theory must be realistic and take into account the reorientation of the ionic atmosphere in mathematical terms. There have been many contributions in this area, especially by Marcus, Hush, Levich, Dog- nadze, and others5"9. The theories have been of a classical or quantum- mechanical nature, the latter being more difficult to develop but more correct.
It is fundamental that the theories permit quantitative com- parison between rates of electron transfer in electrodes and in homoge- neous solution.
We illustrate the results obtained in the approximate model of Marcus, remembering that the activation barrier results predominantly from solvation changes.
The energy profile can be represented by a parabola. For the intersection of the two parabolas, assumed to be identical in form, one obtains, after a little algebraic manipulation, 4. It is an example of a linear free energy relationship a kinetic parameter, In A: So, for very fast reactions, the theory predicts a variation of a with potential. There is some evidence that this occurs, but given the multistep nature of any electrode reaction no definitive conclusions can be taken, and mechanisms can be elaborated which have constant charge transfer coefficients.
Indeed the fact that the enthalpic and entropic parts of the coefficients have different temperature dependences leads to the question as to what is the real significance of the charge transfer coefficient, a topic currently under discussion9. Another aspect affecting electron transfer that has become more important with the increasing use of semiconductor electrodes 10 " 13 in, for example, solar energy conversion, but is also valid for metal electrodes, should be mentioned.
The Fermi energy is the electrochemical potential of the electrons in the electrode, see Chapter 3.
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The density of states is shown schematically in Fig. Overlap between EF and the distribution for Eo shows that oxidized species can be reduced. In order to relate Eredox, EF, and electrode potentials it is important to utilize the same reference state, namely vacuum In relation to vacuum the energy of the standard hydrogen electrode is —4. It therefore seems logical, when describing the mechanism of an electrode reaction, to speak of an energy associated with the redox couple corresponding to that of the electrons in the solution species that are transferred, and equal to the Fermi energy in the actual electron transfer step after solvent reorganization, etc.
X reflects the break in the structure of the solid and consequent variations in electronic distribution Fig. Energy corresponding to Volta potential Solution Fig. A measurement of potential gives values of electrode potentials and never redox potentials. The crucial point is that the difference of potential available to effect electrode reactions and surmount activation barriers is not simply the difference between the Galvani potential i. On the side of the solid it is the Volta potential and on the side of the solution it is the potential at the inner Helmholtz plane, where species have to reach to in order for electron transfer to be possible.
Corrections to rate constants for the latter are commonly carried out using the Gouy-Chapman model of the electrolyte double layer and will be described in Section 6.
Marcus, J. Butler, Trans. Faraday Soc, , 19, and ; T. Erdey-Gruz and M. Volmer, Z. Marcus, Ann. Levich, Advances in electrochemistry and electrochemical engineering, ed. Dogonadze, Reactions of molecules at electrodes, ed.
Kuznetsov, Faraday Disc. Soc, , 74, Bockris, pp. Tools Get online access For authors. Email or Customer ID. Forgot password? Old Password. New Password. Your password has been changed. Returning user.
Request Username Can't sign in? Forgot your username? Enter your email address below and we will send you your username. Thus this paper will not repeat the historical aspects of CO2 reduction. This chapter will review recent progress, putting emphasis on basic problems and particularly on electrocatalytic aspects. This process is experimental and the keywords may be updated as the learning algorithm improves.
This is a preview of subscription content, log in to check access. Preview Unable to display preview. Download preview PDF.The mass transfer coefficient kd describes the rate of diffusion within the diffusion layer and kc and k. Schlichting, Boundary layer theory, Pergamon Press, London, This classification is useful mainly for electrodes of the first and second types. Experimental arrangement for measurements at equilibrium.
As this reaction continues, the half-cell with the metal-A electrode develops a positively charged solution because the metal-A cations dissolve into it , while the other half-cell develops a negatively charged solution because the metal-B cations precipitate out of it, leaving behind the anions ; unabated, this imbalance in charge would stop the reaction.
Measurement of these velocities gives information about the structure of the solution. The contact angle is measured with a microscope. Chapter 10 - Haloalkanes and Haloarenes.
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