Thermodynamics cell potential

The potential of the reaction is given as = (cathodic — anodic reaction) = 0.337 — (—0.440) = +0.777 V. The positive value of the standard cell potential indicates that the reaction is spontaneous as written (see Electrochemical processing). In other words, at thermodynamic equihbrium the concentration of copper ion in the solution is very small. The standard cell potentials are, of course, only guides to be used in practice, as rarely are conditions sufftciendy controlled to be called standard. Other factors may alter the driving force of the reaction, eg, cementation using aluminum metal is usually quite anomalous. Aluminum tends to form a relatively inert oxide coating that can reduce actual cell potential. 

Determining the cell potential requites knowledge of the thermodynamic and transport properties of the system. The analysis of the thermodynamics of electrochemical systems is analogous to that of neutral systems. Eor ionic species, however, the electrochemical potential replaces the chemical potential (1). 

Each reactant and product appears in the Nemst equation raised to its stoichiometric power. Thermodynamic data for cell potentials have been compiled and graphed (3) as a function of pH. Such graphs are known as Pourbaix diagrams, and are valuable for the study of corrosion, electro deposition, and other phenomena in aqueous solutions.Erom the above thermodynamic analysis, the cell potential can be related to the Gibbs energy change... 

Predicting the cell potential requires knowledge of thermodynamic properties and transport processes ia the cell. Conversely, the measurement of cell potentials can be used to determine both thermodynamic and transport properties (4). 

Because some substances may preferentially adsorb onto the surface of the electrode, the composition near the iaterface differs from that ia the bulk solution. If the cell current is 2ero, there is no potential drop from ohmic resistance ia the electrolyte or the electrodes. Yet from the thermodynamic analysis it is seen that there is a measurable cell potential. The question from where this potential arises can be answered by considering the iaterface. 

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... 

Redox potential (thermodynamic derivation). Suppose we take an electrochemical cell represented by Fig. 2.7. We shall now address the question of both the potential values and the equilibrium state that can be finally attained... 

Calculation of the internal cell potential is a very complicated matter because the electrochemistry of all of the species within the protocell would have to be balanced subject to their composition quotient Q, after which the standard free energy would have to be established from tabulations. The transport of Na+ would also change this balance, along with the ionic strength of the solution and the stability of the proteins or prebiotic molecules within the protocell. Such non-equilibrium thermodynamics forms the basis of the protocell metabolism. The construction... 

Before considering different theoretical approaches to determining the free energies and other thermodynamic properties of ionic solvation, it is important to be aware of a problem on the experimental level. There are several methods available for obtaining these quantities for electrolyte solutions, both aqueous and nonaqueous some of these have been described by Conway and Bockris162 and by Padova.163 For example, enthalpies of solvation can be found via thermodynamic cycles, free energies from solubilities or galvanic cell potentials. However the results... 

Reversible cell potentials have been the source of much thermodynamic data on aqueous electrolytes. In recent years, this technique has been extended to nonaqueous solutions and to molten salt systems. Its use for aqueous solutions, relative to other techniques, has decreased. Various ion specific electrodes have been developed in recent years. These are used primarily in analytical chemistry and have not produced much thermodynamic data. 

Consequently, a wealth of information on the energetics of electron transfer for individual redox couples ("half-reactions") can be extracted from measurements of reversible cell potentials and electrochemical rate constant-overpotential relationships, both studied as a function of temperature. Such electrochemical measurements can, therefore, provide information on the contributions of each redox couple to the energetics of the bimolecular homogeneous reactions which is unobtainable from ordinary chemical thermodynamic and kinetic measurements. 

We will now look at the effects of Ej on thermodynamic calculations, and then decide on the various methods that can be used to minimize them. One of the most common reasons for performing a calculation with an electrochemical cell is to determine the concentration or activity of an ion. In order to carry out such a calculation, we would first construct a cell, and then, knowing the potential of the reference electrode, we would determine the half-cell potential, i.e. the electrode potential E of interest, and then apply the Nemst equation. 

In contrast to molecular theory, classic thermodynamics deals only with measurable properties of matter in bulk (for example, pressure, temperature, volume, cell potential,... 

For heavily doped n-type semiconductors, the flat band is nearly coincident with the conduction band, while for heavily doped p-type semiconductors the flat band lies very close to the valence band edge. A necessary thermodynamic condition for the photoproduction of hydrogen and oxygen is that the p-type conduction band must be at or above the H7H2 half cell potential, while n-type valence band must lie below the 02/0H half cell potential. 

Tower, Stephen. All About Electrochemistry. Available online. URL http //www.cheml.com/acad/webtext/elchem/. Accessed May 28, 2009. Part of a virtual chemistry textbook, this excellent resource explains the basics of electrochemistry, which is important in understanding how fuel cells work. Discussions include galvanic cells and electrodes, cell potentials and thermodynamics, the Nernst equation and its applications, batteries and fuel cells, electrochemical corrosion, and electrolytic cells and electrolysis. 

A number of detailed thermodynamic comparisons of half-cells containing alkali metal and alkali metal amalgams are available. For example, Cogley and Butler examined cell potentials as a function of amalgam concentration for the cell shown below [22]. 

If it is recalled that on a purely thermodynamic basis, any cell that has a tendency to drive a current through an external load seems capable of being harnessed as an energy-producing device, then the conclusion just reached is serious, for it bears the following implication In the development of an energy-producing device, the variation of cell potential with cell current is as important as, if not more important than,... 

WIn the preelectrodic days, essentially before 1950, the attitude of most workers toward electrochemical cells was such that mainly the thermodynamic and diffusion aspects were important. When the cell potentials decreased as the power drawn from them increased, the causes were sought in special phenomena such as gas layers on the dectrode. The general character of such a decrease, above all its relation to bonding between substrate and reactant and to electrocatalysis (Section 7.11.1). was not realized... 

One application of Eq. 2 is the determination of a reaction free energy—a thermodynamic quantity—from a cell potential, an electrical quantity. Consider the chemical equation for the reaction in the Daniel cell (reaction A) again. For this reaction, n = 2 because 2 mol of electrons migrate from Zn to Cu and we measure E = 1.1 V. It follows that... 

For a better comprehension of the ED processes it is necessary to refresh a few basic concepts and definitions regarding the electrolytic cell and thermodynamic electrode potential, Faraday s laws, current efficiency, ion conduction, diffusivity, and transport numbers in solution. 

An electrolytic cell is essentially composed of a pair of electrodes submerged into an electrolyte for conduction of ions and connected to a direct current (DC) generator via an external conductor to provide for continuity of the circuit. The electrode connected to the positive pole of the DC generator is called anode, while that linked to the negative one, cathode. The current flow in an electrolyte results from the movement of positive and negative ions and is assumed as positive when directed as the positive charges or opposite to the electrons in the external circuit. When the cell is not operating under conditions of standard concentration, the thermodynamic electrode (or cell) potential (ET) can be estimated from the Nernst equation ... 

The evolution of H2 and Cl2 transforms the cathode into a hydrogen electrode and the anode into a chlorine one (Prentice, 1991), thus resulting in an electrochemical cell with a thermodynamic cell potential difference (/id) at 25°C equal to ... 

The key connection between an electrochemical cell potential E and thermodynamics is ... 

Equations (47)-(50) indicate how thermodynamic quantities can be obtained from cell potentials measured under standard conditions. However, standard states are hypothetical states (e.g., infinitely dilute behavior at 1.0 m concentration), which cannot be prepared in the cell. As a result, an extrapolation procedure is used to find 8° from measured cell voltages as a function of concentration. From Eq. (47), we write the dependence of 8 on the concentration of the electrolyte in the form... 

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