Thermodynamic Information from Electrochemistry

Thermodynamic information about many chemical reactions that occur outside of electrochemical cells can be obtained from electrochemical data. For a general reaction written in the form of Eq. (8.2-18), the analogue of Eq. (8.2-8) is 

The Gibbs energy change in this equation is the same as for the reaction outside of the cell. The equilibrium constant for the reaction can be calculated using the relations shown in Eq. (7.1-20) and Eq. (8.5-2) 

Find the value of the equilibrium constant for the reaction of Exercise 8.3 at 298.15 K. 

We can write an expression for the entropy change of a reaction outside of an electrochemical cell (we now omit the subscript chem )  

For the standard-state reaction, this equation becomes 

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The wish to determine thermodynamic data of electrochemical reactions and of the involved compounds is one of the most important motivations to perform electrochemical measurements. After calorimetry, electrochemistry is the second most important tool to determine thermodynamic data. Although ab initio quantum chemical calculations can be used for the calculation of thermodynamic data of small molecules, the day is not yet foreseeable when electrochemical experiments will be replaced by such calculations. In this chapter we provide the essential information as to what thermodynamic information can be extracted from electrochemical experiments and what the necessary prerequisites are to do so. 

EC-NMR has made considerable progress during the past few years. It is now possible to investigate in detail metal-liquid interfaces under potential control, to deduce electronic properties of electrodes (platinum) and of adsorbates (CO), and to study the surface diffusion of adsorbates. The method can also provide information on the dispersion of commercial carbon-supported platinum fuel cell electrocatalysts and on electrochem-ically generated sintering effects. Such progress has opened up many new research opportunities since we are now in the position to harness the wealth of electronic, Sp-LDOS as well as dynamic and thermodynamic information that can be obtained from NMR experiments. As such, it is to be expected that EC-NMR will continue to thrive and may eventually become a major characterization technique in the field of interfacial electrochemistry. 

Practically in every general chemistry textbook, one can find a table presenting the Standard (Reduction) Potentials in aqueous solution at 25 °C, sometimes in two parts, indicating the reaction condition acidic solution and basic solution. In most cases, there is another table titled Standard Chemical Thermodynamic Properties (or Selected Thermodynamic Values). The former table is referred to in a chapter devoted to Electrochemistry (or Oxidation - Reduction Reactions), while a reference to the latter one can be found in a chapter dealing with Chemical Thermodynamics (or Chemical Equilibria). It is seldom indicated that the two types of tables contain redundant information since the standard potential values of a cell reaction ( n) can be calculated from the standard molar free (Gibbs) energy change (AG" for the same reaction with a simple relationship... 

In all of these systems, certain aspects of the reactions can be uniquely related to the properties of a surface. Surface properties may include those representative of the bulk material, ones unique to the interface because of the abrupt change in density of the material, or properties arising from the two-dimensional nature of the surface. In this article, the structural, thermodynamic, electrical, optical, and dynamic properties of solid surfaces are discussed in instances where properties are different from those of the bulk material. Predominantly, this discussion focuses on metal surfaces and their interaction with gas-phase atoms and molecules. The majority of fundamental knowledge of molecular-level surface properties has been derived from such low surface area systems. The solid-gas interface of high surface area materials has received much attention in the context of separation science, however, will not be discussed in detail here. The solid-liquid interface has primarily been treated from an electrochemical perspective and is discussed elsewhere see Electrochemistry Applications in Inorganic Chemistry). The surface properties of liquids (liquid-gas interface) are largely unexplored on the molecular level experimental techniques for their study have begun only recently to be developed. The information presented here is a summary of concepts a more complete description can be found in one of several texts which discuss surface properties in more detail. ... 

The text largely contains fundamental material and focuses on understanding the basic principles rather than learning factual information. Since it is impossible to include all branches of surface science in such an introductory book because of its wide and multidisciplinary scope, a specific and narrow topic, the interfacial interactions between solids and liquids, has been chosen for this book. For this reason, the ionic interactions, charged polymers, electrochemistry, electrokinetics and the colloid and particulate sciences cannot be included. Some fundamental physical chemistry subjects such as basic thermodynamics are covered, and many equations are derived from these basic concepts throughout the book in order to show the links between applied surface equations and the fundamental concepts. This is lacking in most textbooks and applied books in surface chemistry, and for this reason, this book can be used as a textbook for a course of 14-15 weeks. 

Electroanalytical techniques are traditionally associated with studies of solutions however, direct studies of the electrochemistry of solid materials are very tempting because they can give access to a wealth of information, ranging from elemental composition to thermodynamic and kinetic data, from structure-reactivity relations to new synthetic routes. 

Differential values of Y can be also useful for interpretation of electrosorption valency in terms of sulfate vesus bi-suUate adsorption, as one of mostly untapped resources of platinum electrochemistry. However the main progress is expected from combination of precise electrochemistry, thermodynamic analysis, and independent physical information supported by an appropriate theoretical basis, with necessary links provided by computational community. The final Section contains some brief notes in this respect. 

His three early Leipzig papers (5-7) represent a synthesis of concepts that he was well qualified to make. Working in Ostwald s laboratory, he must have absorbed some of the mass of electrochemical information which appeared a few years later in Ostwald s two-volume work on the history and theory of electrochemistry (H). He was thoroughly familiar with the second-law thermodynamics of Thomson and Clausius, and with the more recent pronouncements of van t Hoff and Helmholtz. Nernst was also imbued with the atomism of Dalton and Boltzmann, in v hich respect he differed from Ostwald and Helmholtz, and he had accepted Arrhenius s recently published (12,13) hypothesis of the complete dissociation of strong electrolyses in solution. However, his conductance work in Kohlrausch s laboratory had given him a lively appreciation of the effects of incomplete ionization of weak electrolytes. 

Electrochemistry can provide both thermodynamic and kinetic information on a range of chemical processes driven by electron transfer. However, electrochemistry can rarely unequivocally identify electroactive species the molecular identity of a new electrogenerated material is typically inferred from the measured physical properties of a known standard system. In addition, electrochemistry provides only limited and indirect information on structural changes accompanying redox events. 

We close this section with a reminder of a fnndamental issue in electrochemistry Not all the quantities in Equations 13.8 throngh 13.13 are accessible to measurement by electrochemical or thermodynamic methods. Only the electrochemical potential ( i ), the work function (W ) or equivalently the real potential (a ) and the Volta potential ( / ) are. Equations 13.9, 13.11, and 13.13 are therefore formal resolutions. It is not possible to assign actual values to the separate terms, the chemical potential ( t ), the Galvani potential (cp ), nor the surface potential (x ), without making extrathermodynamic assumptions. These quantities must therefore be considered unphysical, at least from the point of view of thermodynamics. This statement, which is called the Gibbs-Guggenheim Principle in [42], is often met with disbelief from theoretical and computational chemists, particularly in the case of the chemical potential (Equation 13.10). The standard chemical potential is essentially the (absolute) solvation free energy AjG of species i. One would hope that a molecular simulation contains all information needed to compute AjG . Indeed, there seems to be a way around this thermodynamic verdict for computation and also mass spectroscopic. This continues to be, however, hazardous territory, particularly for DFT calculations in periodic systems. ... 

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