Thermodynamics electrochemical cell reactions

Thermodynamic calculations are always based on an electrochemical cell reaction, and the derived voltage means the voltage difference between two electrodes. The voltage difference between the electrode and the electrolyte, the absolute potential , cannot exactly be measured, since potential differences can only be measured between two electronic conductors (2). Single electrode potential always means the cell voltage between this electrode and a reference electrode. To get a basis for the electrode-potential scale, the zero point was arbitrarily equated with the potential of the standard hydrogen electrode (SHE), a hydrogen electrode under specihed conditions at 25 °C (cf. Ref. 3). 

Measurement of E° and Activities Electrochemical cells can be constructed to measure E° and thermodynamic properties such as K, AG, AH, AS, A V, and ACP for a reaction. Consider as an example the cell shown schematically in Figure 9.4.x The cathode consists of an Ag metal rod coated with AgCl(s). The anode is a Pt metal rod around which H2(g) is bubbled. The two electrodes are... 

We can use the electrochemical series to predict the thermodynamic tendency for a reaction to take place under standard conditions. A cell reaction that is spontaneous under standard conditions (that is, has K > 1) has AG° < 0 and therefore the corresponding cell has E° > 0. The standard emf is positive when ER° > Et that is, when the standard potential for the reduction half-reaction is more positive than that for the oxidation half-reaction. 

Electrochemically, a spontaneous reaction generates a positive cell potential, Scell Thermodynamically, a spontaneous reaction has a negative change in free energy, AG. Thus, a reaction that has a negative change in free... 

The foregoing considerations are based on the concepts of reversible thermodynamics the electrochemical cells are considered to be operating reversibly, which means in effect that no net current is drawn. Real cell EMFs, however, can differ substantially from the predictions of the Nernst equation because of electrochemical kinetic factors that emerge when a nonnegligible current is drawn. An electrical current represents electrons transferred per unit of time, that is, it is proportional to the extent of electrochemical reaction per unit of time, or reaction rate. The major factors that can influence the cell EMF through the current drawn are... 

Thermodynamics of a solid-state reaction. The following electrochemical cell is reversible at 1 000 K in an atmosphere of flowing 02(g) 16... 

The thermal energy generated or absorbed by an electrochemical cell is determined first by the thermodynamic parameters of the cell reaction, and second by the overvoltages and efficiencies of the electrode processes and by the internal resistance of the cell system. While the former are generally relatively simple functions of the state of charge and temperature, the latter are dependent on many variables, including the cell history. 

Potentiometric measurements are based on the Nernst equation, which was developed from thermodynamic relationships and is therefore valid only under equilibrium (read thermodynamic) conditions. As mentioned above, the Nernst equation relates potential to the concentration of electroactive species. For electroanalytical purposes, it is most appropriate to consider the redox process that occurs at a single electrode, although two electrodes are always essential for an electrochemical cell. However, by considering each electrode individually, the two-electrode processes are easily combined to obtain the entire cell process. Half reactions of electrode processes should be written in a consistent manner. Here, they are always written as reduction processes, with the oxidised species, O, reduced by n electrons to give a reduced species, R ... 

We now look at some examples of redox reactions involving simple cations in aqueous solution. Electrochemical terminology will often be encountered, since e.m.f. measurements on electrochemical cells are important sources of thermodynamic data in this area. For example, the reduction potential ° for the half-reaction ... 

Factors Involved in Galvanic Corrosion. Emf series and practical nobility of metals and metalloids. The emf. series is a list of half-cell potentials proportional to the free energy changes of the corresponding reversible half-cell reactions for standard state of unit activity with respect to the standard hydrogen electrode (SHE). This is also known as Nernst scale of solution potentials since it allows to classification of the metals in order of nobility according to the value of the equilibrium potential of their reaction of dissolution in the standard state (1 g ion/1). This thermodynamic nobility can differ from practical nobility due to the formation of a passive layer and electrochemical kinetics. 

Reversibility — This concept is used in several ways. We may speak of chemical reversibility when the same reaction (e.g., -> cell reaction) can take place in both directions. Thermodynamic reversibility means that an infinitesimal reversal of a driving force causes the process to reverse its direction. The reaction proceeds through a series of equilibrium states, however, such a path would require an infinite length of time. The electrochemical reversibility is a practical concept. In short, it means that the -> Nernst equation can be applied also when the actual electrode potential (E) is higher (anodic reaction) or lower (cathodic reaction) than the - equilibrium potential (Ee), E > Ee. Therefore, such a process is called a reversible or nernstian reaction (reversible or nerns-tian system, behavior). It is the case when the - activation energy is small, consequently the -> standard rate constants (ks) and the -> exchange current density (jo) are high. 

One of the principal reasons for failure due to reaction with the service environment is the relatively complex nature of the reactions involved. Yet, in spite of all the complex corrosion jargon, whether a metal corrodes depends on the simple electrochemical cell set up by the environment. This might give the erroneous impression that it is possible to calculate such things as the corrosion rate of a car fender in the spring mush of salted city streets. Dr. M. Pourbaix has done some excellent work in the application of thermodynamics to corrosion, but this cannot yet be applied directly to the average complex situation. 

Intercalation reactions of the dichalcogenides with alkali metals are redox reactions in which the host lattice is reduced by electron transfer from the alkali metal. Lithium and sodium intercalation reactions, for example, have been studied using cells of the type Li/LiC104-dioxolane/MX2 andNa/Nal-propylene carbonate/MX2. The reactions proceed spontaneously to form the intercalation compound if the cell is short circuited alternatively, a reverse potential can be apphed to control the composition of the final product. Apart from their application in synthesis, such electrochemical cells can be used to obtain detailed thermodynamic information and to establish phase relations by measuring the dependence of the equilibrium cell voltage on composition (see Figure 4). 

This reaction does not have to be a thermodynamically favorable one, since an external dc electric potential is applied via the solid electrochemical cell in the PEVD system to drive the reaction in the desired direction. Thus, the activity of (A) at the reaction site is controlled by the applied dc electric potential. 

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. 

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