Cell reaction, representation

In assembling cells for making thermodynamic measurements, one should try not to combine half-cells in a manner that results in a junction potential. Figure 9.7 is a schematic representation of the Daniell cell, which is one with a junction potential. The half-cell reactions are... 

The cell reaction is the sum of the electrochemical reactions taking place at both electrodes. The cell reaction may be written in two ways which are dependent on the sequence of phases in the graphical scheme of the cell. The representation of the cell reaction should correspond to the flow of positive charge through the cell (in a graphical scheme) from left to right ... 

If the reaction in the cell proceeds to unit extent, then the charge nF corresponding to integral multiples of the Faraday constant is transported through the cell from the left to the right in its graphical representation. Factor n follows from the stoichiometry of the cell reaction (for example n = 2 for reaction c or d). The product nFE is the work expended when the cell reaction proceeds to a unit extent and at thermodynamic equilibrium and is equal to the affinity of this reaction. Thus,... 

At the heart of the SCE is a paste of liquid mercury and mercurous chloride (Hg2Cl2), which has the old-fashioned name calomel . Figure 7.13 depicts a simple representation of the SCE. The half-cell reaction in the SCE is... 

O Use the following shorthand representation to sketch a possible design of the cell. Include as much information as you can. Identify the anode and cathode, and write the half-reactions and the overall cell reaction. 

Half-cell reaction A conceptual representation of electron transfer in which the number of electrons gained by a molecule or atom is indicated. Eor example, the half-cell reduction of Mn to Mn ... 

With this understanding, we can derive the correct expression for Equation (12.47) from Equation (12.46). We emphasize that the cell is a closed system at equilibrium (open-circuit) at constant temperature and with a constant pressure on each phase. We let the cell reaction as determined from inspection of the representation of the cell in accordance with the established conventions be represented as v,B,. Then... 

Suppose now that the potentiometer is so adjusted that its emf counteracts and exceeds that of the Daniell cell. In this event the potentiometer acts as a battery electrons are now forcibly transported from the Cu to the Zn terminal. This is accompanied by the forced release of electrons to the Cu terminal according to the oxidative process Cu - Cu2+ + 2e and by the forced acceptance of electrons at the Zn terminal in the reductive process Zn2+ + 2e - Zn. The net process inside the cell is representable according to the reaction Cu + ZnS0A -CuSO + Zn. The cell operation has now been reversed it should be noted that now the Cu electrode becomes the anode and the Zn electrode, the cathode. 

The anodic or corrosion half-cell reaction has been generalized as M —> Mm+ + me. The previously presented schematic representations of anodic corrosion processes immediately raise three questions ... 

It is useful to establish a more generalized representation for the electrochemical cell reaction as follows ... 

Fig. 2.4 Abbreviated cell representation showing Ece and half-cell reactions...

Fig. 2.5 Abbreviated cell representation showing current flow when the half-cell reactions are coupled...

From the convention relating the cell reaction to the cell representation (Table 2.2), the cell potentials are written as ... 

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. 

Buried metal pipes can be protected from corrosion by connecting them to blocks of metal such as magnesium, called sacrificial anodes, which corrode in preference to the pipe. A cell made from the redox couples Fe/ Fe + and Mg/Mg is a laboratory representation of a sacrificial anode, (a) Draw the cell diagram [not shown in the answers at the end of this book], (b) write the cell half-reactions and the cell reaction and (c) calculate the standard cell voltage. 

It is desirable to evaluate potential catalysts for fuel cell reactions in fuel ceU prototypes under the real conditions of application. The standard approach involves profiling the voltage and power output as a function of the current drawn by a load during operation of a fuel ceU. A schematic representation of the t5 ical features of the voltage/power-current graphs is shown in Fig. 7.4. Fuel cells generally exhibit a... 

For the representation of electrochemical cells, we use a set of symbols that distinguish between the different phases involved. The electrochemical cell of Figure 2.5 corresponds to that of Figure 1.2 under conditions of zero current. In the cell reaction (2.38), equivalent to (1.11), 4 moles of electrons are transferred per mole of oxygen (n = 4). 

A cell diagram is a symbolic representation of an electrochemical cell that indicates the substances entering into the cell reaction, electrode materials, solution concentrations, etc. 

Figure 4.2 Graphical representation of the supetcell structure, with a single (3 x 3) unit cell indicated by dashed lines that is repeated along lattice vectors a, b, and c, as indicated, (a) and (b) are vapor phase and aqueous phase models of the reaction environment, respectively, for an adsorbed CH2OH intermediate with a surface coverage of...

Figure 15.2 Schematic representation of different electrochemical cell types used in studies of electrocatalytic reactions (a) proton exchange membrane single cell, comprising a membrane electrode assembly (b) electrochemical cell with a gas diffusion electrode (c) electrochemical cell with a thin-layer working electrode (d) electrochemical cell with a model nonporous electrode. CE, counter-electrode RE, reference electrode WE, working electrode.

Figure 17.19 A membianeless ethanol/02 enz3fme fuel cell. Alcohol dehydrogenase and aldehyde dehydrogenase catalyze a stepwise oxidation of ethanol to acetaldehyde and then to acetate, passing electrons to the anode via the mediator NAD+/NADH. At the carhon cathode, electrons are passed via the [Ru(2,2 -bipyridyl)3] and biUverdin/bilimbin couples to bilirubin oxidase, which catalyzes O2 reduction to H2O. (a) Schematic representation of the reactions occruring. (b) Power/cmrent response for the ceU operating in buffered solution at pH 7.15, containing 1 mM ethanol and 1 mM NAD. Panel (b) reprinted from Topcagic and Minteer [2006]. Copyright Elsevier, 2006.

Figure 7. Schematic representation of collision cells for reactive MPC dynamics. Each cell contains various numbers of the different species. The species numbers change in the cells as a result of chemical reactions.

The representation of the biological conservation of substrate to cell mass by an overall chemical reaction. The stoichiometric relationships are then used to calculate various rates such as cell mass concentration [83]. 

Figure 17.3. The brain in stress (HPA, hypothalamus-pituitary-adrenal axes). Schematic representation of some of the inflammatory response reactions that may even result in cell death as a response to prolonged inflammatory reactions. (See color insert.)...

The intra-cell processes are common to all PDF codes, and are treated the same in both Eulerian and Lagrangian PDF codes.8 On the other hand, inter-cell processes are treated differently in Eulerian PDF codes due to the discrete representation of space in terms of x . In PDF codes, fractional time stepping is employed to account for each process separately. Methods for treating chemical reactions and mixing are described in Section 6.9. Thus we will focus here on the treatment of inter-cell processes in Eulerian PDF codes. 

Figure 9. Schematic representation of the acrylic chambers used for treatment of P815 cells with DC. Chambers are connected in series by filter-paper bridges, and fitted with platinum electrodes in their extremities. In this system, cell suspensions can be exposed directly to the anodic reactions (AC) or cathodic reactions (CC) or to electric current without contact with the electrodes, in the intermediary chamber (IC). Internal volume 3 cm3. After Veiga et al.62...

The reduction half-reaction does not include a solid conductor of electrons, so an inert platinum electrode is used in this half-cell. The platinum electrode is chemically unchanged, so it does not appear in the chemical equation or half-reactions. However, it is included in the shorthand representation of the cell. 

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