Electrochemical cells overall chemical reaction

We see that the overall chemical reaction that occurs in an electrochemical cell is conveniently described in terms of two types of half-reactions. In one, electrons are lost in the other, they are gained. To distinguish these half-reactions we need two identifying names. 

How is Eq. (14.18), contrived in electrochemical format, i.e., how does it get driven to the right In the fuel cell model, this must occur via two constituent reactions just as the overall chemical reaction in a battery is composed of two electrode reactions. Possible reactions are ... 

Michael Faraday (1791-1867) was the first to realize that electrochemical processes are stoichiometrically associated with straightforward chemical reactions. Consider the cell of Figure 3.1.5. The two electrode reactions of the cell have been represented previously in Equations (2a) and (7). Thus, the spontaneous overall chemical reaction is... 

In the above description of what happens in a cell, an overall reaction has been found by combination of the reactions occurring at the two electrodes. This overall cell reaction is a formal representation in the sense that it does not actually take place in the cell. The only chemical reactions which actually occur are those at the electrodes, but their net effect corresponds in quantitative terms to what would be expected if the overall chemical reaction did actually occur. The observed potential difference or emf is related to the AG for the overall cell reaction. It is this property of electrochemical cells which makes them so usefirl as they allow determination of thermodynamic quantities which are impossible to study directly. 

When two electrochemical redox systems are coupled together, one electrode providing and the other taking up electrons, the net effect will be similar to that indicated in the chemical scheme. Such is the situation observed with electrochemical cells for which there are associated overall chemical reactions. Further, owing to the precision with which electrochemical measurements may be made for such systems, it is often possible to use then to obtain precise thermodynamic data characteristic of reactions occurring within them. 

An electrochemical cell consists of two electrodes as shown in Figure 1.9. This set-up separates an overall chemical reaction into two partial electrochemical reactions one at each electrode with an electronic exchange via the outer circuit. Figure 1.9 shows the Fe/Fe + electrode in combination with the H2/H+ electrode as an example. At the iron surface, Fe dissolution as Fe " is one half reaction, at the platinum electrode hydrogen evolution by H+ reduction is the other half-reaction. Both partial reactions compensate each other electronically by a transfer of electrons in the outer circuit from the Fe/Fe + electrode to the H2/H+ electrode. Both electrodes need an electrolytic contact to form... 

I-. Steady-state operation is achieved by reduction of surface-bound Ru(III) back to Ru(II) through the oxidation of I-. Thus, chemical reactions mediate the generation of current and the overall process is simply the conversion of light to electrical current. It is believed that these cells hold considerable commercial potential, and since their development entails research in synthetic, electrochemical, photochemical, and inorganic chemistry, the commitment of intensive research to the area is understandable. The dynamics of several types of electron-transfer processes are central in the operation of these cells. 

The electrochemical cell with zinc and copper electrodes had an overall potential difference that was positive (+1.10 volts), so the spontaneous chemical reactions produced an electric current. Such a cell is called a voltaic cell. In contrast, electrolytic cells use an externally generated electrical current to produce a chemical reaction that would not otherwise take place. 

The collision between reacting atoms or molecules is an essential prerequisite for a chemical reaction to occur. If the same reaction is carried out electrochemically, however, the molecules of the reactants never meet. In the electrochemical process, the reactants collide with the electronically conductive electrodes rather than directly with each other. The overall electrochemical Redox reaction is effectively split into two half-cell reactions, an oxidation (electron transfer out of the anode) and a reduction (electron transfer into the cathode). 

The reaction of an electrochemical cell always involves a combination of two redox half reactions such that one species oxidises a second species to give the respective redox products. Thus, the overall cell reaction can be expressed by a balanced chemical equation ... 

Students will write balanced chemical half reactions and an overall redox reaction for the reaction that takes place in the electrochemical cell. 

Figure 3.3.3 schematically depicts the basic structure of an electrochemical fuel cell device. Generally, in electrochemical cells the overall chemical redox reaction proceeds via two coupled, yet spatially separated half-cell redox reactions at two separate electrodes. 

The potential profiles in this PEVD system are illustrated in Figure 17. Although there is no driving force due to a difference in the chemical potential of sodium in the current PEVD system, the applied dc potential provides the thermodynamic driving force for the overall cell reaction (62). Consequently, electrical energy is transferred in this particular PEVD system to move Na COj from the anode to the cathode of the solid electrochemical cell by two half-cell electrochemical reactions. In short, this PEVD process can be used to deposit Na CO at the working electrode of a potentiometric CO sensor. 

The two major classes of voltammetric technique 4 Evaluation of reaction mechanisms 6 General concepts of voltammetry 6 Electrodes roles and experimental considerations 8 The overall electrochemical cell experimental considerations 12 Presentation of voltammetric data 14 Faradaic and non-Faradaic currents 15 Electrode processes 17 Electron transfer 22 Homogeneous chemical kinetics 22 Electrochemical and chemical reversibility 25 Cyclic voltammetry 27 A basic description 27 Simple electron-transfer processes 29 Mechanistic examples 35... 

As is standard practice, the above reactions are written as equilibrium reduction reactions and the total chemical change in an electrochemical cell is found by adding the individual anode and cathode reactions (the so-called half-cell reactions). Thus, the overall reaction for the Cu/Zn cell in Figure 26.1 is... 

When a current flows through a heterogeneous system it gives rise to phenomena in the different volumes (mass transport and possible chemical reactions) as well as interfacial phenomena. This section focuses on the overall characteristics (U,I,t) of an electrochemical system in relation with the characteristics of the studied interfaces. The influence of these latter on how that system is described can be found in two different ways. On the one hand they define the conditions at the volume boundaries for concentration and potential profiles, and on the other hand they determine the interfacial contributions which, added to the ohmic drop, give the overall cell voltage . 

This chapter is concerned with measurements of kinetic parameters of heterogeneous electron transfer (ET) processes (i.e., standard heterogeneous rate constant k° and transfer coefficient a) and homogeneous rate constants of coupled chemical reactions. A typical electrochemical process comprises at least three consecutive steps diffusion of the reactant to the electrode surface, heterogeneous ET, and diffusion of the product into the bulk solution. The overall kinetics of such a multi-step process is determined by its slow step whose rate can be measured experimentally. The principles of such measurements can be seen from the simplified equivalence circuit of an electrochemical cell (Figure 15.1). 

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